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And of course, Betelgeuse.

December

NASA's Fermi Gamma-ray Space Telescope has discovered a faint but sprawling glow of high-energy light around a nearby pulsar. If visible to the human eye, this gamma-ray "halo" would appear about 40 times bigger in the sky than a full Moon. This structure may provide the solution to a long-standing mystery about the amount of antimatter in our neighborhood. "Our analysis suggests that this same pulsar could be responsible for a decade-long puzzle about why one type of cosmic particle is unusually abundant near Earth," said Mattia Di Mauro, an astrophysicist at the Catholic University of America in Washington and NASA's Goddard Space Flight Center in Greenbelt, Maryland. "These are positrons, the antimatter version of electrons, coming from somewhere beyond the solar system." A neutron star is the crushed core left behind when a star much more massive than the Sun runs out of fuel, collapses under its own weight and explodes as a supernova. We see some neutron stars as pulsars, rapidly spinning objects emitting beams of light that, much like a lighthouse, regularly sweep across our line of sight. Geminga (pronounced geh-MING-ga), discovered in 1972 by NASA's Small Astronomy Satellite 2, is among the brightest pulsars in gamma rays. It is located about 800 light-years away in the constellation Gemini. Geminga's name is both a play on the phrase "Gemini gamma-ray source" and the expression "it's not there"— referring to astronomers' inability to find the object at other energies—in the dialect of Milan, Italy. Geminga was finally identified in March 1991, when flickering X-rays picked up by Germany's ROSAT mission revealed the source to be a pulsar spinning 4.2 times a second. A pulsar naturally surrounds itself with a cloud of electrons and positrons. This is because the neutron star's intense magnetic field pulls the particles from the pulsar's surface and accelerates them to nearly the speed of light. Electrons and positrons are among the speedy particles known as cosmic rays, which originate beyond the solar system. Because cosmic ray particles carry an electrical charge, their paths become scrambled when they encounter magnetic fields on their journey to Earth. This means astronomers cannot directly track them back to their sources. For the past decade, cosmic ray measurements by Fermi, NASA's Alpha Magnetic Spectrometer (AMS-02) aboard the International Space Station, and other space experiments near Earth have seen more positrons at high energies than scientists expected. Nearby pulsars like Geminga were prime suspects. Then, in 2017, scientists with the High-Altitude Water Cherenkov Gamma-ray Observatory (HAWC) near Puebla, Mexico, confirmed earlier ground-based detections of a small gamma-ray halo around Geminga. They observed this structure at energies from 5 to 40 trillion electron volts—light with trillions of times more energy than our eyes can see. Scientists think this emission arises when accelerated electrons and positrons collide with nearby starlight. The collision boosts the light up to much higher energies. Based on the size of the halo, the HAWC team concluded that Geminga positrons at these energies only rarely reach Earth. If true, it would mean that the observed positron excess must have a more exotic explanation. But interest in a pulsar origin continued, and Geminga was front and center. Di Mauro led an analysis of a decade of Geminga gamma-ray data acquired by Fermi's Large Area Telescope (LAT), which observes lower-energy light than HAWC. Particles traveling near light speed can interact with starlight and boost it to gamma-ray energies. This animation shows the process, known as inverse Compton scattering. When light ranging from microwave to ultraviolet wavelengths collides with a fast-moving particle, the interaction boosts it to gamma rays, the most energetic form of light. "To study the halo, we had to subtract out all other sourcesf gamma rays, including diffuse light produced by cosmic ray collisions with interstellar gas clouds," said co-author Silvia Manconi, a postdoctoral researcher at RWTH Aachen University in Germany. "We explored the data using 10 different models of interstellar emission." What remained when these sources were removed was a vast, oblong glow spanning some 20 degrees in the sky at an energy of 10 billion electron volts (GeV). That's similar to the size of the famous Big Dipper star pattern—and the halo is even bigger at lower energies. "Lower-energy particles travel much farther from the pulsar before they run into starlight, transfer part of their energy to it, and boost the light to gamma rays. This is why the gamma-ray emission covers a larger area at lower energies ," explained co-author Fiorenza Donato at the Italian National Institute of Nuclear Physics and the University of Turin. "Also, Geminga's halo is elongated partly because of the pulsar's motion through space." The team determined that the Fermi LAT data were compatible with the earlier HAWC observations. Geminga alone could be responsible for as much as 20% of the high-energy positrons seen by the AMS-02 experiment. Extrapolating this to the cumulative emission from all pulsars in our galaxy, the scientists say it's clear that pulsars remain the best explanation for the positron excess.

Venus

October

These highlands were thought to be formed of granitic rock, like Earth's continents, which required oceans of water to form. Scientists at the Lunar and Planetary Institute (LPI), including undergraduate student intern Frank Wroblewski from Northland College, find that a volcanic flow on Venus' Ovda Regio highlands plateau is composed of basaltic lava, calling into question the idea that the planet might once have been Earth-like with an ancient ocean of liquid water. The LPI team re-mapped the Ovda Fluctus lava flow using radar data. They discovered that the flow is not granitic as was expected from its location, but is more likely made up of basalt rock which can form with or without water. The result has potentially significant implications for the evolutionary history of Venus

September

Dr. Way and his colleague, Anthony Del Genio, have created a series of five simulations assuming different levels of water coverage. In all five scenarios, they found that Venus was able to maintain stable temperatures between a maximum of about 50 degrees Celsius and a minimum of about 20 degrees Celsius for around three billion years. A temperate climate might even have been maintained on Venus today had there not been a series of events that caused a release, or 'outgassing', of carbon dioxide stored in the rocks of the planet approximately 700-750 million years ago. "Our hypothesis is that Venus may have had a stable climate for billions of years. It is possible that the near-global resurfacing event is responsible for its transformation from an Earth-like climate to the hellish hot-house we see today," said Way. Three of the five scenarios studied by Way and Del Genio assumed the topography of Venus as we see it today and considered a deep ocean averaging 310 metres, a shallow layer of water averaging 10 metres and a small amount of water locked in the soil. For comparison, they also included a scenario with Earth's topography and a 310-metre ocean and, finally, a world completely covered by an ocean of 158 metres depth. To simulate the environmental conditions at 4.2 billion years ago, 715 million years ago and today, the researchers adapted a 3-D general circulation model to account for the increase in solar radiation as our Sun has warmed up over its lifetime, as well as for changing atmospheric compositions. Although many researchers believe that Venus is beyond the inner boundary of our Solar System's habitable zone and is too close to the Sun to support liquid water, the new study suggests that this might not be the case.

August

"The difference between Earth and Venus is that on Earth most of the energy from the sun is absorbed at ground level while on Venus most of the heat is deposited in the clouds," explains Sanjay Limaye, a University of Wisconsin–Madison planetary scientist and a co-author of the new study. What is curious about Venus' clouds—other than that they are unlike anything on Earth—is that in those clouds are mysterious dark patches, dubbed "unknown absorbers" by scientists as the tiny particles that make up the patches soak up most of the ultraviolet and some of the visible light from the sun and thus affect the planet's albedo and energy budget.The patches were first observed by ground-based telescopes more than a century ago. They ebb and flow over time, changing their distributions and contrasts. "The particles that make up the dark splotches, have been suggested to be ferric chloride, allotropes of sulfur, disulfur dioxide and so on, but none of these, so far, are able to satisfactorily explain their formation and absorption properties," explains Yeon Joo Lee, the senior author of the new report. On the other hand, Limaye notes observations that the particles are about the same size and have the same light-absorbing properties as microorganisms found in Earth's atmosphere, and scientists, beginning with the noted biophysicist Harold Morowitz and astronomer Carl Sagan, have long speculated about the possibility that the shadowy patches in the clouds of Venus are, in fact, microscopic life. Whatever their composition, Venus' "unknown absorbers," according to the new measurements of the planet's albedo, a feat led by Lee of the Technical University of Berlin, are having an effect on the planet's weather. Lee and her colleagues, including Limaye, studied changes in Venus' albedo using more than a decade of ultraviolet observations of the planet from instruments aboard the planetary probes Venus Express, Akatsuki and Messenger as well as the Hubble Space Telescope. Between 2006 and 2017, Venus' albedo, the measure of ultraviolet light reflected back to space, halved before beginning to rebound. Those changes to the planet's albedo sparked big variations in the amount of solar energy absorbed by the clouds and, consequently, the circulation of Venus' atmosphere. In particular, the albedo changes help explain variations in the vigorous activity of the planet's upper atmosphere, which exhibits what scientists call "super-rotation," a phenomenon driven by winds exceeding 200 miles per hour. Takeshi Horinouchi of Japan's Hokkaido University, also a co-author of the new Astronomical Journal report and an expert on Venusian weather, says the new results of the changes in the planet's albedo provide a link between solar heating and the powerful gusts that underpin the dynamics of the planet's upper atmosphere."What really struck me about this paper is that it shows that Venus' climate has decadal-long climate variations, just like the Earth," says Venus expert Mark Bullock of the Southwest Research Institute and who was not involved in the new study. "Even more amazing, the strength of the climate oscillation on Venus is much greater than Earth's long-term variations." "That is a striking result," Limaye adds. "It suggests that something is changing. We can see the change in brightness. If the albedo is changing, something is driving those changes. The question is, what is the cause?" The ebb and flow of the mysterious dark splotches at the tops of Venus' clouds, the "unknown absorbers," are near the top of the list of suspects and could, in fact, be playing a major role in those changes, say Lee and Limaye. Haze above the clouds and composed of smaller particles, they add, can make Venus appear even brighter. Weather and climate, be it on Earth or on Venus, are driven by solar radiation, including the ultraviolet radiation that we can't see. And clouds and their changing ability to reflect light have a huge influence.

May

A new study by Mattias Green at Bangor University's School of Ocean Sciences and colleagues at NASA and University of Washington have quantified this braking effect on ancient Venus. They show that the tides in a Venusian ocean would have been large enough to slow the rotation rate of Venus by tens of Earth days per million years if Venus were spinning more like Earth does today. This suggests that the tidal brake could have slowed down Venus to its current rotation state in 10-50 million years, and therefore taking it away from being habitable in a short time frame.

April

Now, researchers have used infrared images to spy into the middle layer of Venus's clouds and they have found some unexpected surprises.

The new research, published in the AGU journal Geophysical Research Letters, finds this middle layer of clouds shows a wide variety of cloud patterns that change over time and are very different from the upper layer of Venus's clouds, which are usually studied with ultraviolet images. The study also found changes in the albedo of the middle clouds, or how much sunlight they are reflecting back to space, which could indicate the presence of water, methane or other compounds absorbing solar radiation.

The motions of the middle clouds, combined with previous observations, allowed researchers to reconstruct a picture of the winds on Venus over 10 years, showing the super-fast winds in the planet's middle clouds are fastest at the equator and, like the upper clouds, change speed over time.

The new images taken by Akatsuki show the middle layer of clouds change over time and are also very different than Venus's upper cloud layer, which sit at a height of about 70 kilometers. Sometimes, the images show a slightly darker band of clouds invaded by bright clouds that at times exhibit swirl shapes or look mottled. These observations are suggestive of convection, the vertical movement of heat and moisture in the atmosphere. On Earth, convection can cause thunderstorms. At other times, the images showed clouds that are less turbulent and appear homogenously bright or featureless, with multiple stripes.

From April to May of 2016, Venus's northern hemisphere became periodically darkened every four to five days. Scientists had not previously observed this difference between the hemispheres and the cause is yet to be determined, according to the new study. The images also showed other rare cloud features, including a hook-like dark filament extending more than 7,300 kilometers in the northern hemisphere in May and October of 2016.

Akatsuki also saw unexpected high contrasts in the cloud albedo. The new study suggests there could be compounds in the cloud layer able to absorb at the infrared wavelength or, alternatively, there could changes in the thickness of the clouds.

Since most of the solar energy is absorbed in the cloud layers and the fastest super-rotating winds also occur there, studying several layers of the clouds is critical to understanding the winds, according to Peralta. Scientists suspect changes in Venus's clouds and their albedo could be linked to the planet's super-rotation, and how the wind's momentum and energy is transported.

exomoons

May

Two teams working independently have looked at the possibility of an exomoon circling the exoplanet Kepler-1625b, which orbits the star Kepler-1625. They report little to no evidence supporting its existence. One team, led by Laura Kreidberg, has written a paper describing their work, which is posted on the arXiv preprint server. Another team led by René Heller published a paper in the journal Astronomy Astrophysics. The team that announced possible evidence of the exomoon last year, led by Alex Teachey, has written another paper in response to the findings by the new researchers that is available on arXiv.

In their reply, Teachey and his team took another look at their earlier results and suggest they warrant further observations of Kepler-1625b for more evidence. They also addressed the findings of Kreidberg's team and noted that the technique used by the group could have resulted in some data being erased

June

An international study led by the Monash School of Physics and Astronomy has discovered the first observational evidence for the existence of circumplanetary discs. "We think the large moons of Jupiter and other gas giants were born in such a disc, so our work helps to explain how planets in our solar system formed," he said. Researchers using ALMA (Atacama Large Millimeter/submillimeter Array) found a small dust concentration in the disk around TW Hydrae, the nearest young star. It is highly possible that a planet is growing or about to be formed in this concentration. This is the first time that the exact place where cold materials are forming the seed of a planet has been pinpointed in the disk around a young star.

July

Using Earth's most powerful array of radio telescopes, astronomers have made the first observations of a circumplanetary disk of gas and dust like the one that is believed to have birthed the moons of Jupiter. The find, reported online today in Astrophysical Journal Letters, adds to the intriguing story of planet PDS 70 c, a still-forming gas giant about 370 light years from Earth that was first revealed last month in visible light images. Using the massive 66-antenna Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, Rice University astronomer Andrea Isella and colleagues collected millimeter wave radio signals that revealed the presence of dust grains throughout the star system where PDS 70 c and its sister planet, PDS 70 b, are still forming. "Planets form from disks of gas and dust around newly forming stars, and if a planet is large enough, it can form its own disk as it gathers material in its orbit around the star," Isella said. "Jupiter and its moons are a little planetary system within our solar system, for example, and it's believed Jupiter's moons formed from a circumplanetary disk when Jupiter was very young." But most models of planet formation show that circumplanetary disks disappear within about 10 million years, which means circumplanetary disks haven't existed in our solar system for more than 4 billion years. To look for them elsewhere and gather observational evidence to test theories of planet formation, Isella and colleagues search for very young star systems where they can directly observe disks and the planets still forming inside them. In the new study, Isella and colleagues analyze

Researchers led by Mario Sucerquia, from the Universidad de Antioquia, Colombia, and Jaime Alvarado-Montes from Australia's Macquarie University, modelled the likely behaviour of giant exomoons predicted to form around massive planets—and discovered that they would be expelled and sent packing. Roughly 50 percent of these ejected moons would survive both the immediate expulsion and avoid any subsequent collision with the planet or the star, ending up as quasi-planets travelling around the host star, but in eccentric "Pluto-like" orbits. These rogue moons—dubbed "ploonets" by Sucerquia, Alvarado-Montes and colleagues—could potentially explain several puzzling phenomena, not the least of which is why astronomers have so far confirmed the existence of at least 4098 exoplanets, but not a single exomoon. Most of the planets discovered thus far are of a type known as Hot Jupiters, a fact that reflects mainly the limits of current detection technology. Previous research indicates that at least some of these should be orbited by large moons. Their absence, the researchers write in a paper soon to be published in the journal Monthly Notices of the Royal Astronomical Society, could be explained by a scenario in which the angular momentum between the two bodies results in the moon escaping the gravitational pull of its parent. "These moons would become planetary embryos, or even fully-fledged planets, with highly eccentric orbits of their own," explains Alvarado-Montes. While conceding that ploonets remain hypothetical, the researchers say their existence would offer a possible explanation for several challenging results produced by NASA's now-retired Kepler space telescope.

August

orbit a planet 550 light-years away from us. This is suggested by an international team of researchers led by the University of Bern on the basis of theoretical predictions matching observations. The "exo-Io" would appear to be an extreme version of Jupiter's moon Io. Jupiter's moon Io is the most volcanically active body in our solar system. Today, there are indications that an active moon outside our solar system, an exo-Io, could be hidden at the exoplanet system WASP-49b. Apurva Oza, postdoctoral fellow at the Physics Insitute of the University of Bern and associate of the NCCR PlanetS, "a place where Jedis go to die, Sodium gas was detected at the WASP 49-b at an anomalously high-altitude. "The neutral sodium gas is so far away from the planet that it is unlikely to be emitted solely by a planetary wind," says Oza. Observations of Jupiter and Io in our solar system, by the international team, along with mass loss calculations show that an exo-Io could be a very plausible source of sodium at WASP 49-b. "The sodium is right where it should be" says the astrophysicist. "The enormous tidal forces in such a system are the key to everything," explains the astrophysicist. The energy released by the tides to the planet and its moon keeps the moon's orbit stable, simultaneously heating it up and making it volcanically active. In their work, the researchers were able to show that a small rocky moon can eject more sodium and potassium into space through this extreme volcanism than a large gas planet, especially at high altitudes.

September

"The exomoon is like a comet of ice that is evaporating and spewing off these rocks into space," said Metzger, associate professor of astrophysics at Columbia University and principal investigator on the study. "Eventually the exomoon will completely evaporate, but it will take millions of years for the moon to be melted and consumed by the star. We're so lucky to see this evaporation event happen." The Columbia team suggests that Tabby's Star abducted an exomoon from a now long-gone, nearby planet and pulled it into orbit around itself, where it has been getting torn apart by stronger stellar radiation than existed in its former orbit. Chunks of the exomoon's dusty outer layers of ice, gas, and carbonaceous rock have been able to withstand the radiation blow-out pressure that ejects smaller-grain dust clouds, and the volatile, large-grain material has inherited the exomoon's new orbit around Tabby's Star, where it forms a disk that persistently blocks the star's light.

October

All planets, large and small, start by gathering together asteroid-sized bodies to make a rocky core. At this early stage in the evolution of a planetary system, the rocky cores are still surrounded by a gaseous disk left over from the formation of the parent star. If a core can grow fast enough to reach a mass equivalent to 10 Earths, then it will have the gravitational strength to pull gas in from the surrounding space and grow to the massive size of Jupiter and Saturn. However, this gaseous accumulation is short-lived, as the star is draining away most of the gas in the disk, the dust and gas surrounding a newly formed star. If there are two cores growing in close proximity, then they compete to capture rock and gas. If one core gets slightly larger, it gains an advantage and can capture the bulk of the gas in the neighborhood for itself. This leaves the second body without any further gas to capture. The increased gravitational pull of its neighbor drags the smaller body into the role of a satellite, albeit a very large one. The former planet is left as a super-sized moon, orbiting the planet that beat it out in the race to capture gas. In a new model I developed, I discuss how such a massive exomoon forms through a different process, wherein it is really a captured planet.

Moon

June

A mysterious large mass of material has been discovered beneath the largest crater in our solar system—the Moon's South Pole-Aitken basin—and may contain metal from the asteroid that crashed into the Moon and formed the crater, according to a Baylor University study. "Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground. That's roughly how much unexpected mass we detected," said lead author Peter B. James. The crater itself is oval-shaped, as wide as 2,000 kilometers—roughly the distance between Waco, Texas, and Washington, D.C.—and several miles deep. Despite its size, it cannot be seen from Earth because it is on the far side of the Moon. The study—"Deep Structure of the Lunar South Pole-Aitken Basin"—is published in the journal Geophysical Research Letters. National Aeronautics and Space Administration (NASA) Gravity Recovery and Interior Laboratory (GRAIL) mission. "When we combined that with lunar topography data from the Lunar Reconnaissance Orbiter, we discovered the unexpectedly large amount of mass hundreds of miles underneath the South Pole-Aitken basin," James said. "One of the explanations of this extra mass is that the metal from the asteroid that formed this crater is still embedded in the Moon's mantle." The dense mass—"whatever it is, wherever it came from"—is weighing the basin floor downward by more than half a mile,

July

Prior researchers using data from the Arecibo Observatory and also NASA's MESSENGER spacecraft found evidence of ice on Mercury. As part of this new effort, the researchers studied depth/diameter ratios of 2,000 craters on the planet using Mercury Laser Altimeter data. In so doing, they found that permanently shadowed craters became less shallow in higher latitudes—an indication of ice. Back in 2009, as part of the LCROSS mission, researchers allowed an empty stage of the Lunar Reconnaissance Orbiter (LRO) launch vehicle to crash into the floor of a crater close to the moon's south pole. Testing of the debris cloud by sensors aboard the Shepherding Spacecraft, showed evidence of water and ice, along with other material. The researchers with this new effort believed it was likely that there was more ice on the moon than was shown during the LCROSS impact study—likely existing in shadowed craters similar to those that had been seen on Mercury. To find out, they carried out a parallel crater study, similar to the one they had conducted for Mercury. In this case, they studied 12,000 craters on the moon using data from the LRO. They report that they found "a similar morphological trend" in craters on the south side of the Moon, near the pole. They suggest this indicates that such craters likely harbor thick ice deposits along with other materials similar to those that are believed to exist on Mercury. The researchers suggest that if this is indeed the case, then there could be up to 100 million metric tons of ice in such craters, which they note is double the amount of previous estimates based on data from the LCROSS impact study. The researchers conclude by suggesting that future Moon missions include the use of probes that can be used to study the shaded craters to confirm their suspicions.

As a result of the permanent darkness, NASA's Lunar Reconnaissance Orbiter (LRO) has measured the coldest temperatures in the solar system inside these craters, which have become known as perfect environments for preserving material like water for eons. Or so we thought. It turns out that despite temperature that dips to -388 degrees Fahrenheit (-233 Celsius) and can presumably keep frost locked in soil virtually forever, water is slowly escaping the topmost, super thin layer (thinner than the width of a red blood cell) of the Moon's surface. NASA scientists reported this finding recently in paper in the journal Geophysical Research Letters. "People think of some areas in these polar craters as trapping water and that's it," said William M. Farrell, a plasma physicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who led the lunar frost research. "But there are solar wind particles and meteoroids hitting the surface, and they can drive reactions that typically occur at warmer surface temperatures. That's something that's not been emphasized." Unlike Earth, with its plush atmosphere, the Moon has no atmosphere to protect its surface. So when the Sun sprays charged particles known as the solar wind into the solar system, some of them bombard the Moon's surface and kick up water molecules that bounce around to new locations. Likewise, wayward meteoroids constantly smash into the surface and uproot soil mingled with frozen bits of water. Meteoroids can hurtle these soil particles—which are many times smaller than the width of a human hair—as far as 19 miles (30 kilometers) away from the impact site, depending on the size of the meteoroid. The particles can travel so far because the Moon has low gravity and no air to slow things down: "So every time you have one of these impacts, a very thin layer of ice grains is spread across the surface, exposed to the heat of the Sun and to the space environment, and eventually sublimated or lost to other environmental processes," said Dana Hurley, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.

In the woods

August

An Israeli spacecraft called Beresheet almost made it to the moon in April. It took a selfie with the lunar surface in the background, but then lost contact with Earth and presumably crashed onto the lunar surface. Now it's been revealed that the mission was carrying a cargo of dehydrated microscopic lifeforms known as tardigrades. Beresheet was the first stage of a privately-funded initiative to transfer living DNA to the moon. The project is designed to act as Noah's Ark Mark II, providing a repository from which plants and animals could be regenerated to repopulate the Earth should a catastrophe akin to a flood of biblical proportions overtake the planet. Whether the project is far-sighted or foolish, what has roused interest is the fact that, as a result of the crash, the tardigrades may now be scattered across the lunar surface. They are hardy creatures and could probably survive on the moon for a long time. Is this a matter of concern? I believe so, but possibly not for the reasons you might think. A new study in the AGU journal Geophysical Research Letters finds Io's most powerful, persistent volcano, Loki Patera, brightens on a similar timescale to slight perturbations in Io's orbit caused by Jupiter's other moons, which repeat on an approximately 500-Earth-day cycle. The new results are surprising because volcanoes on Io do not erupt in time with larger fluctuations in the far greater stresses inflicted on Io during the moon's 1.77-Earth-day orbit around Jupiter. Internal friction from these stresses generates the heat that powers Io's volcanoes, but fluctuating stresses during the short period of Io's orbit don't appear to squeeze magma to the surface. The new study, which analyzed 271 nights of observations of Loki Patera from Hawaii's Keck and Gemini North telescopes from 2013–2018, and more sporadic observations dating back to 1987, suggests the gentler, 500-day cycle may act on the right timescale to move magma and bring it to the surface of the moon in an eruption.

Messier 87

Event Horizon telescope

April

our petabytes (4 million billion bytes)—was contained in a mountain of computer hard drives weighing several hundred pounds that had to be physically transported to the Haystack Observatory in Westford, Massachusetts,

The most important initial take-home is that Einstein was right. Again. His general theory of relativity has passed two serious tests from the universe's most extreme conditions in the last few years. Here, Einstein's theory predicted the observations from M87 with unerring accuracy,

The Milky Way's black hole was too challenging to image accurately this time round due to rapid variability in light output. Hopefully, more telescopes will be added to the EHT's array soon

more than 200 scientists and engineers who linked some of the world's most capable radio telescopes

6.5 billion times that of our Sun

eight major radio observatories on four continents, to work together as a virtual, Earth-sized telescope.

a ring of light that appears lopsided—brighter on one side than the other. Relativity predicts that the immense gravitational field will cause light to bend around the black hole, forming a bright ring around its silhouette, and will also cause the surrounding material to orbit around the object at close to light speed. The bright, lopsided ring in the new images offers visual confirmation of these effects: The material headed toward our vantage point as it rotates around appears brighter than the other side.

"Observing the black hole with the Event Horizon Telescope is a bit like listening to a song being played on a piano with over half of its keys broken."

The approach led to numerous gaps that could be filled with infinite possibilities consistent with the data.

"But just as your brain may still be able to recognize a song being played on a broken piano if there are enough functioning keys, we can design algorithms to intelligently fill in the EHT's missing information to reveal the underlying black hole image," she concluded.

The idea is to place two or three satellites in circular orbit around the Earth to observe black holes. The concept goes by the name Event Horizon Imager (EHI)

Mars

November

In June, NASA's Curiosity rover reported the highest burst of methane recorded yet, but neither ESA's Mars Express nor the ExoMars Trace Gas Orbiter recorded any signs of the illusive gas, despite flying over the same location at a similar time. Methane is of such fascination because on Earth a large proportion is generated by living things. It is known that methane has a lifetime of several hundred years before it is broken down by the Sun's radiation, so the fact that it is detected on Mars suggests it has been released into the atmosphere recently—even if the gas itself was generated billions of years ago. The methane mystery on Mars has had many twists and turns in recent years with unexpected detections and non-detections alike. Earlier this year it was reported that ESA's Mars Express had detected a signature that matched one of Curiosity's detections from within Gale Crater. A recent spike by Curiosity, measured on 19 June 2019, and the highest yet at 21 ppbv, adds to the mystery because preliminary analysis suggest that Mars Express did not detect any on this occasion. (For comparison, the concentration of methane in Earth's atmosphere is around 1800 ppbv, meaning that for every billion molecules in a given volume, 1800 are methane.) The Mars Express measurements were taken in the martian daytime about five hours after Curiosity's nighttime measurements; data collected by Mars Express over one day before also did not reveal any signatures. Meanwhile Curiosity's readings had returned to background levels when further measurements were taken in the following days. The Mars Express measurement nique allowing data to be inferred right down to the martian surface with its limit of detection around 2 ppbvThe ESA-Roscosmos Trace Gas Orbiter (TGO), the most sensitive detector for trace gases at Mars, also did not detect any methane while flying nearby within a few days before and after Curiosity's detection. In general, TGO is capable of measuring at parts per trillion levels and accessing down to about 3-kilometer altitude, but this can depend on how dusty the atmosphere is. When measurements were taken at low latitudes on 21 June 2019, the atmosphere was dusty and cloudy, resulting in measurements accessing 20-15 km above the surface with an upper limit of 0.07 ppbv.The global lack of methane recorded by TGO is adding to the overall mystery, and corroborating the results of the different instruments is keeping all teams busy."Taking the results together suggests that the latest spike measured by Curiosity was very short lived—less than one martian day—and likely local," says Marco Giuranna, principal investigator for the Planetary Fourier Spectrometer onboard Mars Express that is used to detect methane."Curiosity measured the methane at night, and if it was released at that time, we would expect it to have been trapped near the surface until sunrise before getting rapidly mixed and transported away. As a result, there would be no chance for it to be detected by Mars Express or TGO."By comparison, the spike we co-measured in 2013 must have been of a longer duration or more intense at its source—which we believe was outside Gale Crater—such that it could be detected by our instrument on Mars Express as well."

For the first time in the history of space exploration, scientists have measured the seasonal changes in the gases that fill the air directly above the surface of Gale Crater on Mars. As a result, they noticed something baffling: oxygen, the gas many Earth creatures use to breathe, behaves in a way that so far scientists cannot explain through any known chemical processes. Over the course of three Mars years (or nearly six Earth years) an instrument in the Sample Analysis at Mars (SAM) portable chemistry lab inside the belly of NASA's Curiosity rover inhaled the air of Gale Crater and analyzed its composition. The results SAM spit out confirmed the makeup of the Martian atmosphere at the surface: 95% by volume of carbon dioxide (CO2), 2.6% molecular nitrogen (N2), 1.9% argon (Ar), 0.16% molecular oxygen (O2), and 0.06% carbon monoxide (CO). They also revealed how the molecules in the Martian air mix and circulate with the changes in air pressure throughout the year. These changes are caused when CO2 gas freezes over the poles in the winter, thereby lowering the air pressure across the planet following redistribution of air to maintain pressure equilibrium. When CO2 evaporates in the spring and summer and mixes across Mars, it raises the air pressure. Within this environment, scientists found that nitrogen and argon follow a predictable seasonal pattern, waxing and waning in concentration in Gale Crater throughout the year relative to how much CO2 is in the air. They expected oxygen to do the same. But it didn't. Instead, the amount of the gas in the air rose throughout spring and summer by as much as 30%, and then dropped back to levels predicted by known chemistry in fall. This pattern repeated each spring, though the amount of oxygen added to the atmosphere varied, implying that something was producing it and then taking it away. "The first time we saw that, it was just mind boggling," said Sushil Atreya, professor of climate and space sciences at the University of Michigan in Ann Arbor. Atreya is a co-author of a paper on this topic published on November 12 in the Journal of Geophysical Research: Planets. A sunset at the Viking Lander 1 site, 1976. Credit: NASA/JPL As soon as scientists discovered the oxygen enigma, Mars experts set to work trying to explain it. They first double- and triple-checked the accuracy of the SAM instrument they used to measure the gases: the Quadrupole Mass Spectrometer. The instrument was fine. They considered the possibility that CO2 or water (H2O) molecules could have released oxygen when they broke apart in the atmosphere, leading to the short-lived rise. But it would take five times more water above Mars to produce the extra oxygen, and CO2 breaks up too slowly to generate it over such a short time. What about the oxygen decrease? Could solar radiation have broken up oxygen molecules into two atoms that blew away into space? No, scientists concluded, since it would take at least 10 years for the oxygen to disappear through this process.

June

A team working with NASA to study data the Mars Curiosity Rover has found high levels of methane at a site on the Red Planet. The existence of methane is, of course, a possible sign of life, since it is produced in abundance by microorganisms here on Earth. The methane readings were reportedly three times as high as the previous record on Mars—enough for officials at NASA to change the schedule for the rover over the weekend. They had the rolling probe take more readings at the same site, which, the JPL reports, should be ready for release as early as today. If the readings are due to some form of life, NASA scientists expect it would be in the form of microbes living just under the surface. But a follow-up experiment this weekend found the methane had returned to background levels, NASA said Tuesday, suggesting the temporary spike was caused by one of a number of transient plumes observed by Curiosity in the past. Scientists have tracked a seasonal rise and fall in background methane levels but haven't been able to establish a pattern for the transient plumes.

Ultima Thule/ KB

September

29P/Schwassmann-Wachmann 1 (SW1), a mid-sized centaur in a nearly circular orbit just beyond Jupiter. SW1 has long puzzled astronomers with its high activity and frequent explosive outbursts that occur at a distance from the sun where ice should not effectively vaporize. "More than one in five centaurs that we tracked were found to enter an orbit similar to that of SW1 at some point in their lifetime," said Maria Womack, a Florida Space Institute scientist and co-author of the study. "Rather than being a peculiar outlier, SW1 is a centaur caught in the act of dynamically evolving into a JFC." In addition to the commonplace nature of SW1's orbit, the simulations lead to an even more surprising discovery, Womack says. "Centaurs passing through this region are the source of more than two thirds of all JFCs, making this the primary gateway through which these comets are produced," says Womack. The Gateway region does not hold resident objects for long, with most centaurs becoming JFCs within a few thousand years. This is a short portion of any solar system object's lifetime, which can span millions and sometimes billions of years. The presence of the gateway provides a long sought-after means of identifying the centaurs on an imminent trajectory toward the inner solar system. SW1 is currently the largest and most active of the handful of objects discovered in this gateway region, which makes it a "prime candidate to advance our knowledge of the orbital and physical transitions that shape the comet population we see today," Sarid says.

What if I apply our existing chemical models to comets?", Eistrup thought during his Ph.D. at Leiden University. In the research team at Leiden Observatory, which included Kavli Prize winner Ewine van Dishoeck, he developed models to predict the chemical composition of protoplanetary discs—flat discs of gas and dust encompassing young stars. Understanding these discs can give insight into how stars and planets form. Conveniently, these Leiden models turned out to be of help in learning about comets and their origins. "I thought it would be interesting to compare our chemical models with published data on comets," says the astronomer. "Luckily, I had the help of Ewine. We did some statistics to pin down if there was a special time or place in our young solar system, where our chemical models meet the data on comets." This happened to be the case, and to a surprising extent. Where the researchers hoped for a number of comets sharing similarities, it turned out that all fourteen comets showed the same trend. "There was a single model that fitted each comet best, thereby indicating that they share their origin." The model suggests a zone around the Sun, inside the range where carbon monoxide becomes ice—relatively far away from the nucleus of the young Sun. "At these locations, the temperature varies from 21 to 28 Kelvin, which is around minus 250 degrees Celsius. That's very cold, so cold that almost all the molecules we know are ice. This happened to be the case, and to a surprising extent. Where the researchers hoped for a number of comets sharing similarities, it turned out that all fourteen comets showed the same trend. "There was a single model that fitted each comet best, thereby indicating that they share their origin."

June

Earth-Life Science Institute (ELSI) at Tokyo Institute of Technology. The Tokyo Tech study found that the size and orbit of the satellite systems of large TNOs are best explained if they formed from impacts of molten progenitors. They also found that TNOs which are big enough can retain internal heat and remain molten for a span of only a few million years; especially if their internal heat source is short-lived radioactive isotopes such as Aluminum-26, which has also been implicated in the internal heating of the parent bodies of meteorites. Since these progenitors would need to have a high short-lived radionuclide content in order to be molten, these results suggest that TNO-satellite systems formed before the outward migration of the outer planets, including Neptune, or in the first ~ 700 million years of solar system history.

May

Haumea's ring has never been directly observed. Its existence was inferred in 2017 by an international group of astronomers who took detailed measurements of the light fluctuations as Haumea occulted (passed in front of) a star. In space, an occultation occurs when one object passes in front of another from an observer's perspective.

"The light from the star was observed from Earth as the star was occulted by Haumea. Its brightness decreased as Haumea passed in front of it, enabling the astronomers to obtain information about Haumea's shape," said Othon Cabo Winter, Full Professor at São Paulo State University's Engineering School (FE-UNESP) in Guaratinguetá, Brazil.

"The star's light also faded when the ring passed in front of it, enabling them to obtain information on the ring as well." The researchers who discovered the ring in 2017 suggested that its orbit around Haumea was very close to the 1:3 resonance region, meaning that ring particles make one revolution every three times Haumea rotates.

A new study by Winter, Taís Ribeiro and Gabriel Borderes Motta, who belong to UNESP's Orbital Dynamics and Planetology Group, shows that a degree of eccentricity would be required for this resonance to act on the ring particles. According to Winter, the fact that the ring is narrow and practically circular prevents action by the resonance. However, the group identified a specific type of stable, almost circular, periodic orbit in the same region as Haumea's ring. A periodic orbit is an orbit that repeats over time.

Stretch goal

Swooping within just 2,200 miles (3,500 kilometers), New Horizons flew approximately three times closer to Ultima than it zipped past its primary mission target, Pluto, in July 2015. Not a snowman but a gingerbread man

In addition to being the farthest exploration of an object in history – four billion miles from Earth – the flyby of Ultima Thule was also the first investigation by any space mission of a well-preserved planetesimal, an ancient relic from the era of planet formation.

jan

Edgeworth-Kuiper Belt objects with radii from 1 kilometer to several kilometers have been predicted to exist, but they are too distant, small, and dim for even world-leading telescopes, like the Subaru Telescope, to observe directly. So a research team led by Ko Arimatsu at the National Astronomical Observatory of Japan used a technique known as occultation: monitoring a large number of stars and watching for the shadow of an object passing in front of one of them. The Organized Autotelescopes for Serendipitous Event Survey (OASES) team placed two small 28 cm telescopes on the roof of the Miyako open-air school on Miyako Island, Miyakojima-shi, Okinawa Prefecture, Japan, and monitored approximately 2000 stars for a total of 60 hours.

Analyzing the data, the team found an event consistent with a star appearing to dim as it is occulted by a 1.3 km radius Edgeworth-Kuiper Belt Object. This detection indicates that kilometer sized Edgeworth-Kuiper Belt Objects are more numerous than previously thought. This supports models where planetesimals first grow slowly into kilometer sized objects before runaway growth causes them to merge into planets.

Data transmission from the flyby continues, and will go on until the late summer 2020.

Oumuamua and Berisov

December

In the wake of the discovery of the first two interstellar objects, Tom Hands and Walter Dehnen of the University of Munich, Germany, used computer simulations to study how interstellar objects could be captured by our solar system. "These stowaways form around distant stars before being flung toward us, making a journey of many light-years before encountering Jupiter and being captured into the solar system," explains Hands. "We simulated 400 million such bodies as they approached the sun and Jupiter." The researchers used realistic velocities for these objects based on data from the GAIA mission, and studied how they interact with Jupiter on their journey through the solar system. This work was done on the VESTA cluster of the University of Zürich. "We used an advanced code which runs on graphics processing units rather than traditional computer processors to enable us to simulate such a large number of objects in a short time," explains Hands. "The simulations took two days in total using around 70 graphics cards. [It would have taken] roughly 140 days if we had only used one card and much, much longer if we had used a normal desktop computer processor." The results of the simulations now published in the Monthly Notices of the Royal Astronomical Society (MNRAS) reveal that in a small minority of cases, the trajectories of the objects are altered enough by Jupiter that they become bound to the solar system. "Although the capture probability is small, there could be anywhere from a few hundred to hundreds of thousands of these alien comets orbiting the sun," says the astrophysicist. Captured objects are typically on orbits very similar to those of long-period comets that humanity has observed for centuries, suggesting that they are hiding in plain sight. "If we could identify one, we would have a real possibility to study the composition of material formed in other solar systems in close detail," says Hands.

October

The Hubble Space Telescope has captured the best pictures yet of our newest interstellar visitor. This comet from outside our solar system is zooming by us at a blistering 110,000 mph (177,000 kph). Hubble caught some glam shots over the weekend from a distance of 260 million miles (420 million kilometers). An amateur astronomer from Crimea, Gennady Borisov, discovered the comet in August, two years after the first alien guest, a cigar-shaped rock known as Oumuamua, popped up. "It's a puzzle why these two are so different," David Jewitt of the University of California, Los Angeles, who led the Hubble observation team, said in a statement. On the other hand, it's "very remarkable" that the comet's properties appear to be similar to those of our own solar system's building blocks, said Amaya Moro-Martin of the Space Telescope Science Institute in Baltimore.

identified thanks to their strongly open orbits. "Interstellar Crusher" that scanned tirelessly through online data of newly found comets and asteroids in search of guests from far away. On 8 September 2019 at 04:15 universal time, the program issued a red alert and notified the team of a possible new hyperbolic object arriving from interstellar space. "This code was written specifically for this purpose, and we really hoped to receive this message one day. We didn't know when," said Piotr Guzik of the Jagiellonian University, who led the study. A closer investigation into the object's orbit confirmed its exosolar origin, making it the second-known interstellar interloper. We immediately noticed the familiar coma and tail that were not seen around "Oumuamua," said Michal Drahus of the Jagiellonian University, who co-led the study with Guzik. "This is really cool, because it means that our new visitor is one of these mythical and never-before-seen 'real' interstellar comets," Drahus said. this object appears indistinguishable from the native solar system comets," said Guzik.

Using data gathered by the William Herschel Telescope (WHT), an international team of astronomers found that 2I/Borisov contains cyanide. But as Douglas Adams would famously say, "Don't Panic!"

September

The image shows a very pronounced tail, indicative of outgassing, which is what defines a cometary object. This is the first time an interstellar visitor to our Solar System has clearly shown a tail due to outgassing. The only other interstellar visitor studied in our Solar System was 'Oumuamua which was a very elongated asteroid-like object with no obvious outgassing.

Julia de León adds, "The spectrum of this object is similar to those of solar system comets, and this indicates that their composition must be similar."

July

Matthew Knight, an associate research scientist in the University of Maryland Department of Astronomy, "The motion of 'Oumuamua didn't simply follow gravity along a parabolic orbit as we would expect from an asteroid," Knight said. "But visually, it hasn't ever displayed any of the cometlike characteristics we'd expect. There is no discernable coma—the cloud of ice, dust and gas that surrounds active comets—nor a dust tail or gas jets."

This cross-pollination led to the first comprehensive analysis and the best big-picture summary to date of what we know about the object," Knight explained the object could have been ejected by a gas giant planet orbiting another star. According to theory, Jupiter may have created the Oort cloud—a massive shell of small objects at the outer edge of our solar system—in this way..

May

Our solar system may contain alien comets that were stolen from another star flying past 4.5 billion years ago. Far away in a distant cluster of young stars, a similar close encounter might have also sent the inter-stellar visitor "Oumuamua" flying on its way toward us, and there must be many more of these free-floating objects in the galaxy. These are results of a new study by astrophysicists at the University of Zurich.

A video based on the simulations demonstrates what happens if two young stars in a cluster undergo a close encounter. Each star has a belt of so-called planetesimals, the building blocks of planets, like the Kuiper belt in the outer solar system. When the two stars meet, the Kuiper belt of the smaller star is heavily disrupted by its higher mass sibling. "This causes a bunch of planetesimals to be ejected, flying away to become things like "Oumuamua," explains Tom Hands and adds: "I was surprised by the number of "Oumuamua-like free-floating objects that can be generated in an environment like this on a relatively short time-scale."

Feb

Dr. Zdenek Sekanina of the NASA Jet Propulsion Laboratory, who suggests that 'Oumuamua is the remnant of an interstellar comet that shattered before making its closest pass to the sun (perihelion), leaving behind a cigar-shaped rocky fragment. No outgassing. disintigration as it approached perihelion.

Jan

when 'Oumuamua was first detected, the object was about 0.25 AU from the sun and already on its way out of the solar system.

Europa

November

no one has been able to confirm the presence of water in these plumes by directly measuring the water molecule itself. Now, an international research team led out of NASA's Goddard Space Flight Center in Greenbelt, Maryland, has detected the water vapor for the first time above Europa's surface. The team measured the vapor by peering at Europa through W. M. Keck Observatory in Hawaii, one of the world's biggest telescopes. Confirming that water vapor is present above Europa helps scientists better understand the inner workings of the moon. For example, it helps support an idea, of which scientists are confident, that there's a liquid water ocean, possibly twice as big as Earth's, sloshing beneath this moon's miles-thick ice shell. Another source of water for the plumes, some scientists suspect, could be shallow reservoirs of melted water ice not far below Europa's surface. It's also possible that Jupiter's strong radiation field is stripping water particles from Europa's ice shell, though the recent investigation argued against this mechanism as the source of the observed water. "Essential chemical elements (carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur) and sources of energy, two of three requirements for life, are found all over the solar system. But the third—liquid water—is somewhat hard to find beyond Earth," said Lucas Paganini, a NASA planetary scientist who led the water detection investigation. "While scientists have not yet detected liquid water directly, we've found the next best thing: water in vapor form." In a study published today in the journal Nature Astronomy, Paganini and his team reported that they detected enough water releasing from Europa (5,202 pounds, or 2,360 kilograms, per second) to fill an Olympic-size swimming pool within minutes. Yet, the scientists also found that the water appears infrequently, at least in amounts large enough to detect from Earth, said Paganini: "For me, the interesting thing about this work is not only the first direct detection of water above Europa, but also the lack thereof within the limits of our detection method."

June

Scientists believe that hydrothermal circulation within the ocean, possibly driven by hydrothermal vents might naturally enrich the ocean in sodium chloride, via chemical reactions between the ocean and rock. On Earth, hydrothermal vents are thought to be a source of life, such as bacteria. Plumes emanating from the south pole of Saturn's moon Enceladus, which has a similar ocean, have been found to contain sodium chloride, making both Europa and Enceladus even more enticing targets for exploration. "hydrated" sulfuric acid on the side of Europa that faces backwards along its orbit, the trailing hemisphere. To make sulfuric acid in water ice you need a source of sulfur, and the most likely explanation is that it comes from its sibling volcanic moon, Io. Although there were some hints of salts in the Galileo observations, the newer Hubble data has allowed the scientists to narrow it down to a region on the leading hemisphere called the chaos terrain,

March

École Normale Supérieure, the other Laboratory for Studies of Radiation and Matter in Astrophysics and Atmospheres. JupMagField interacts with Europa's salty ocean, creating a "jet stream" in the water, like the Gulf Stream.

Water

Sorbonne University/University of Paris), has found that one family of comets, the hyperactive comets, contains water similar to terrestrial water. NASA's Stratospheric Observatory for Infrared Astronomy. 46P/Wirtanen approached the Earth in December 2018 it was analysed using the SOFIA airborne observatory, carried aboard a Boeing aircraft. This was the third comet found to exhibit the same D/H ratio as terrestrial water. Like the two previous comets, it belongs to the category of hyperactive comets which, as they approach the Sun, release more water than the surface area of their nucleus should allow. The excess is produced by ice-rich particles present in their atmosphere.

Hyperactive comets, whose water vapour is partially derived from icy grains expelled into their atmosphere thus have a D/H ratio similar to that of terrestrial water, unlike comets whose gas halo is produced only by surface ice. The researchers suggest that the D/H ratios measured in the atmosphere of the latter are not necessarily indicative of the ice present in their nucleus. If this hypothesis is correct, the water in all cometary nuclei may in fact be very similar to terrestrial water, reopening the debate on the origin of Earth's oceans. The farthest object ever explored; New Years Day 2019. "We've never seen anything like this anywhere in the solar system,"

Thanks to the Japanese Hayabusa mission we now have fresh evidence. The spacecraft brought back a precious cargo of grains retrieved from the surface of asteroid 25143 Itokawa in 2010. The researchers behind the new study were able to analyse the water content of two grains. They used a sophisticated piece of kit called an ion microprobe, which bombards a sample with a beam of ions (charged atoms) in order to probe the composition of its surface.

The experiment was not easy – the grains are tiny, less than 40 microns (one millionth of a metre) across, and each grain was made up of several different minerals. The ion microprobe had to be focused on one specific mineral within each grain so that the authors could gather the required data. The species of mineral that they analysed was an iron and magnesium-bearing silicate known as a pyroxene, which is almost entirely free of calcium.

Water is made from hydrogen and oxygen. But those elements occur as different isotopes – meaning they can have a different number of neutrons in their atomic nucleus (neutrons are particles that make up the nucleus together with protons). The researchers looked at the hydrogen isotopic composition of the water and discovered it was very close to that of Earth, suggesting the water on Earth has the same source as that of the Hayabusa grains.

The results raise several interesting questions, the first of which is how so much water came to be in nominally anhydrous minerals? The authors suggest that, during their formation, the grains absorbed hydrogen from the protoplanetary disk, which, at the high temperatures and pressures of the solar nebula, combined with oxygen in the minerals to produce water.

So far, so reasonable. But how is it possible that the water has remained in the minerals? They after all came from an S-type asteroid – one that forms in the inner and hotter part of the solar system. Itokawa has had a complex history of thermal metamorphism and collision, reaching temperatures at least as high as 900°C. But the researchers used computer models to predict how much water would be lost in these processes – and it turned out to be less than 10% of the total.

Enceladus

December

The team was particularly interested in understanding why the stripes are present only on the moon's south pole but were also keen to figure out why the cracks are so evenly spaced. The answer to the first question turns out to be a bit of a coin toss. The researchers revealed that the fissures that make up Enceladus' tiger stripes could have formed on either pole, the south just happened to split open first. Enceladus experiences internal heating due to the eccentricity of its orbit. It is sometimes a little closer to Saturn and sometimes a littler farther, which causes the moon to be slightly deformed—stretched and relaxed—as it responds to the giant planet's gravity. It is this process that keeps the moon from freezing completely solid. Key to the formation of the fissures is the fact that the moon's poles experience the greatest effects of this gravitationally induced deformation, so the ice sheet is thinnest over them. During periods of gradual cooling on Enceladus, some of the moon's subsurface ocean will freeze. Because water expands as it freezes, as the icy crust thickens from below, the pressure in the underlying ocean increases until the ice shell eventually splits open, creating a fissure. Because of their comparatively thin ice, the poles are the most susceptible to cracks. The researchers believe the fissure named after the city of Baghdad was the first to form. (The stripes are named after places referred to in the stories of One Thousand and One Nights, which are also called Arabian Nights.) However, it didn't just freeze back up again. It stayed open, allowing ocean water to spew from its crevasse that, in turn, caused three more parallel cracks to form. "Our model explains the regular spacing of the cracks," Rudolph said. The additional splits formed from the weight of ice and snow building up along the edges of the Baghdad fissure as jets of water from the subsurface ocean froze and fell back down. This weight added a new form of pressure on the ice sheet. "That caused the ice sheet to flex just enough to set off a parallel crack about 35 kilometers away," Rudolph added. That the fissures stay open and erupting is also due to the tidal effects of Saturn's gravity. The moon's deformation acts to keep the wound from healing—repeatedly widening and narrowing the cracks and flushing water in and out of them—preventing the ice from closing up again.

October

New kinds of organic compounds, the ingredients of amino acids, have been detected in the plumes bursting from Saturn's moon Enceladus. The findings are the result of the ongoing deep dive into data from NASA's Cassini mission.

September

Radar observations of Saturn's moons, Mimas, Enceladus and Tethys, show that Enceladus is acting as a "snow-cannon," coating itself and its neighbors with fresh water-ice particles to make them dazzlingly reflective. The extreme radar brightness also points to the presence of "boomerang" structures beneath the surface that boost the moons' efficiency in returning the microwave signals to the spacecraft.

June

The subsurface ocean of Saturn's moon Enceladus probably has higher than previously known concentrations of carbon dioxide and hydrogen and a more Earthlike pH level, possibly providing conditions favorable to life, according to new research from planetary scientists at the University of Washington. This in mind, the team returned to data from the Cassini mission with a computer simulation that accounts for the effects of fractionation, to get a clearer idea of the composition of Enceladus's inner ocean's. They found "significant differences" between Enceladus's plume and ocean chemistry. Previous interpretations, they found, underestimate the presence of hydrogen, methane and carbon dioxide in the ocean.

April

Marc Neveu and Alyssa Rhoden; The researchers designed their model to mimic the behavior of Saturn and its moons over the course of the past 4.5 billion years. The model showed that Enceladus developed a subsurface ocean because of its unique gravitational interactions with the other moons—they forced Enceladus into an oblong orbit. They also found that Saturn's pull on the moon continually distorted it, and that the flexing heated the moon's interior, allowing the water underground to remain liquid. None of the other four moons had conditions similar enough to allow water to remain liquid beneath their surfaces.

K2

At the moment, the study of exoplanets atmospheres is limited, as this kind of measurement requires very high precision, which current instruments were not built to deliver. But molecular signatures from water have been found in the atmospheres of gaseous planets, similar to Jupiter or Neptune. It has never before been seen in smaller planets—until now. K2-18 b was discovered in 2015 and is one of hundreds of "super-Earths"—planets with a mass between Earth and Neptune—found by NASA's Kepler spacecraft. It is a planet with eight times the mass of the Earth that orbits a so called "red dwarf" star, which is much cooler than the sun. The team used archive data from 2016 and 2017 captured by the ESA/NASA Hubble Space Telescope and developed open-source algorithms to analyse the starlight filtered through K2-18b's atmosphere. "Super-Earths are actually the most common type of planet in our galaxy," said Dr. Ingo Waldmann, extrasolar planet explorer at University College London, UK, one of the scientists who reported on the existence of the watery world K2-18 b. Super-Earths are also possible residences of alien life.

Hubble

Kant: nebulae are "island universes".

Born in Missouri, moved to Illinois at 11.

Athlete, broke the state high jump record in secondary school

Spent his vacation time on his grandfather's farm

His scholarship was split by error, he had to take odd jobs during college

Uni Chicago Astromomy Mathematics

"A manly chap between the ages of 19 and 25 a combined good student, athete and leader" - cecil rhodes

began to dress as an english gentleman at Oxford which helped him with the ladies when he became a high school basketball coach, physics and Spanish teacher

120 dollar scholarship to Yerkes plus a dollar a day for room and board

Meeting of the American Astronomical Society, Hubble encouraged to attend. VM Slipher- spectrographic "spiral nebulae" large doppler shifts, larger than indivisual stars. THey were moving rapidly away from the Earth

Hale left Yerkes to build the Mt Wilson Observatory, leaving the remaining faculty to focus on busywork, still largest reflector in the world

Phd thesis on faint nebulae

most nebulae were spiral, some were elliptical. Many occurred in clusters

NGC 2261 changes had occurred within a few years- nearby? "Hubble'a variBLW Nebula"

rushed dissertation but good oral, was allowed to go off to war

promoted to major within a year, but saw little action, much to his chagrin

Mt Wilson, spiral nebulae: gas, star clusters or galaxies?

Harlow Shapley working with cepheid variables

a few years ahead of hubble. Proposed that the distance of the milky way was 300 thousand light years (3x 10x)

1923, Hubble saw Cepheid variable in M31 Andromeda "nebula"

900 thousand light years away from Earth- the andromeda nebula was not a Milky Way object

"island universes"

Why were the vast majority of galaxies red shifted?

Moving away at a very rapid rate

Einstein's cosmological constant

Georges Lamaitre: proposed big bang theory

1929: linear relationship between the distance of the galaxy and its redshift

the farther away a galaxy is, the faster it is moving

Edwin did not initially accept an expanding universe

spirals ellipticals, lenticulars and irregulars.

Shapley

reborn geocentrism: the universe was the galaxy, and the Solar System was at the middle of it.

While at Princeton, studied binary stars with Henry Norris Russell, who would go on to co discover the Hertzsprung Russell diagram

1914: Moumt Wilson Observatory, Shapley w russell concludes that cepheids are not eclipsing binaries but pulsating

1915 to 1921: studies of globular clusters and cephied variables within them led him to conclude that the Earth was 30,000 light years from the centre of the galaxy.

Very few people initially accepted his arguments.

1920: George Elory Hale arranges Symposium on the Scale of the Universe (Great Debate) at the National Academy of Sciences in Washington, between Shapley and Herber D Curtis.

Curtis: attacked Shapley's period-luminosity calculations, believed spiral nebulae were galaxies.

Shapley: sheer size of the galaxy was enough.

1921: Shapley becomes director of the Harvard Observatory

Used Harvard's southern hemisphere obseratories to study the Magellanic clouds

Lemaitre

Georges Lemaitre; catholic priest, professor of astrophysics

1927: explained Hubble's law with the expanding universe.

homogeneous and isotropic universe, expanding sphere

solutions to einstein's equations; einstein had blocked Alexander Friedmann from discovering this first because he felt the paper conflicted with his ideal of a static universe

"primeval atom hypothesis"; "initial quantum"

Arthur Eddington: the idea of a creation is philosophically repugnant. Reportedly because as a quaker he considered God's creation mysterious and wonderful.

Einstein believed Lemaitre invented the Big Bang to reconcile science with his faith.

Kragh, 1999

Kant, 1755: Our universe (the Milky Way) was just one of many, and will eventually die, but creation would be renewed elsewhere.

Auguste Comte, 1835: stars, "we can never by any means investigate their chemical composition or mineralogical structure". The astronomical spectroscope would make its first discovery in 1859.

8 Feb 1917: Einstein applies his general relativity to the whole universe, ushering in modern cosmology.

Newton believed the universe was infinite, because a finite universe would eventually collapse under the influence of gravitation

Slipher initially believed the galaxies approached on the south side of the galaxy and receded on the north side, and that he had discovered a flow of galaxies toward a specific point.

Lamaitre concluded that the non-static model was supported by the redshifting nebulae.

Cepheid variables are named for delta Cephei.

1929: Hubble punblishes results for receding nebulae. Most of the redshifts were taken from Slipher. 100 inch at mt wilson

Shapley could see no evidence of a linear law, though it was generally accepted

Huble concluded that the law was 526 km/s/mpc, later revised to 558.

not a motion through space, but an expansion of space.

Hubble never quite endorsed the idea of an expanding universe, and Russell claimed that "the notion that the galaxies were initially close together is philosophically rather unsatisfactory."

1930: Lamaitre's work became known.

Accepting the finite age of the universe was not necessary to accept the expanding universe. Lamaitre's primeval atom was not accepted during the 1930s.

Friedmann was the first to propose that the equations led to a finite time for the universe, and thus to a creation. He also proposed, linking it to the Hindu idea of a cyclic universe, that the universe could be born, reach a certain size, and then contract back to a point, before being born again.

Einstein considered the idea of a physically expanding universe, "abominable". (1927, to Lemaitre)

1930: Eddington goes to bat for lamaitre. With his support, the notion of an expanding universe gained credence

Fritz Zwicky was the first to suggest a form of tired light; that photons would lose energy to interstellar matter as they travelled.

"I think that everyone who believes in a supreme being supporting every being and every acting, believes also that god is essentially hidden, and may be glad to see how the present physics provides a veil hiding the creation. Lemaitre, 1931, later crossed out. He considered the expansion of the universe a scientific fact that required no revelation from an oracle, and felt an oracle would deny others the pleasure if discovering it.

"relatively small" radius. "resulted from the disintigration of the primeval atom". The radius of space began at zero.

Lemaitre imagined a time of stagnation, during which the cosmological constant exactly balanced gravitational force, and estimated the maximum age of the universe as 100 billion years, a Hubble time comfortably longer than the age of the Earth.

Because it could not be physical, he denied the possibility of a primal singularity.

Both he and Einstein were perfectly happy to imagine Einstein's equations breaking down before a singularity could form.

Much like Copernicus's model of the universe, fellow scientists were happy to accept Lemaitre's model as a mathematical curiosity, but not as an explanation of the origin of the universe

Lemaitre 1961: BB "old fashioned cosmology" unchanged since 1931. Fred Hoyle "Aristotle" veneration.

Lemaitre: seeing science in the Bible is like seeing religious dogma in the binomial theorem. God is beyond mathematical reasoning and empirical testing.

By 1921 the age of the Earth had already been established at 4 billion years.

1931 Jan Oort HC 290, or 3.4 billion years

In 1936, Hubble concluded that "further revision is expected to be of minor importance".

For 20 years, astromomers didn't really care about the time discrepancy- either the geologists were wrong, or the past value for the HC was lower.

Belenkiy 2013

Alexander Friedman Born in Russia, died in 1925

The cosmological constant λ was introduced in 1917 by Einstein in the hope of finding a stable, static cosmological solution of the field equations

Unlike Einstein, who favoured a static universe, Friedmann showed that general relativity could function in a non-static, evolving universe

1922. Einstein called Friedman's work "suspicious" and claimed he'd made a mathematical error. He had not, but his letter didn't reach einstein until late 1923, whereupon he said that the calculations had no phyical meaning

Friedman determined that GR cannot determine the shape of the cosmos. Only observation can.

He contemplated that an expanding universe may have originated in a singularity.

In 1931 Einstein recognized Friedmann’s achievement and suggested that his old nemesis, the cosmological constant, be expunged from GR.

Discover

1923: Hubble identifies a Cepheid variable in Andromeda.

“You will be interested to hear that I have found a Cepheid variable in the Andromeda Nebula,” the letter began.

Shapley needed to read no farther to understand the significance of Hubble’s words. “Here is the letter that destroyed my universe,” Shapley morosely told Cecilia Payne-Gaposchkin,

Hubble was cautious to publish his result.

Van Maanen never figured out where he went wrong and refused to admit his mistake. Hubble finally reexamined his colleague’s photographic plates and declared that “the large rotations previously found arose from obscure systematic errors and did not indicate motion, either real or apparent, in the nebulae themselves.”

Even with the prospect of a thousand dollar reward for the best paper from the AAAS, Hubble still wouldn't publish.

Jan 1 1925

Longair 2004

Einstein stated that the introduction of the cosmological constant was “the biggest blunder of my life”

Using Hubble’s estimate of H0 = 500 kms−1 Mpc−1, the age of the Universe must be less than 2*10^9 years old, a figure in conflict with the age of the Earth derived from studies of the ratios of abundances of long-lived radioactive species, which gave ages significantly greater than this value

Steady-state cosmology was invented by Hermann Bondi, Thomas Gold and Fred Hoyle in 1948

Because of the expansion of the Universe, matter has to be continuously created in order to replace the dispersing matter, the rate of creation amounting to only one particle m−3 every 300,000 years.

nucleosynthesis in stars and these considerations led to his remarkable prediction of the carbon resonance

Lemaitre 1961: BB "old fashioned cosmology" unchanged since 1931. Fred Hoyle "Aristotle" veneration.

there was a much greater comoving number density of extragalactic radio sources at large distances, and hence at earlier cosmic epochs, than nearby. As Ryle expressed it in his Halley Lecture in 1955, “there seems no way in which the observations can be explained in terms of a Steady-State theory”

1965, the microwave background radiation was discovered, more or less by accident, by Arno Penzias and Robert Wilson (1965). During the commissioning of a 20-foot horn antenna designed for telecommunications, they found an excess noise temperature of about 3:5 � 1 K wherever they pointed their telescope at the sky.

van biezen

HST: 12.8 gyr, 13.4 gyr, 13.8 gyr

10 million degrees to overcome strong force

75 percent hydrogen, 25 percent helium (ralph Alpher and Robert Herman 1940s)

primordial nucleosynthesis

atmosphere opaque to 1 to 10 mm

1989 COBE: almost no variation in the frequency and radiation

2.74 kelvin

extremely uniform throught the universe- the universe must have been very small and uniformly distributed.

wavelength of fusion era vs wavelength now. Expanded with the size of the universe, ergo size of universe has grown

3.7 millionx

higher energy in the direction of Leo

Lower in Aquarius

difference= 0.124 percent, same in both directions

371 km/sec blue shift red shift sun's direction

photons can become protons at 4.4 trillion k; as the universe expanded the possible particles became smaller

There is no centre of the universe, just as there is no centre of the surface of the Earth

10^50 expansion in size during the electroweak era

uniformity of the CMB- not possible after 13.8 billion years

Even the slightest initial difference in wavelength would be a massive difference today

particles and antiparticles expanded too quickly to anhillate each other 10-25 seconds

Areas of matter density created gravitation which attracted light and redshfited it. Areas of less matter density left light alone.

quantum fluctuations, the structure of the universe today was set by the fluctuations at the start.

space time

Acoustic soundwaves created by light pressure travelling at half the speed of light

density fluctuations from random quantum fluctuations from when the universe was subatomic in size

density made gravity, gravity pulled more matter

more matter, more photons, more pressure, acoustic waves

at cmb wall, plasma hits 3000 k, low enough for atoms to form

after decoupling, the wall's radius expanded with the universe

Its initial radius was the distance sound could travel in 370000 years at that time- about 500 thousand light years

today the rings of overdensity in the CMB are seen in the structure of the observable universe- rings of galaxies

2005 sloane digital sky survey found a distribution marker that ties the current structure of the universe to the CMB occilations.

Because we know how big the initial acoustic waves were, we can map them onto the current size of the universe to measure the rate of expansion, confirming dark energy.

CMB variations mapped to one part in 10,000

At the moment of recombination

Because the structure of the CMB was created by a number of discrete occillations, by finding a single, complete occillation we can use as a yard stick. It must be as big as the speed of sound at that time multiplied by the duration of a single occillation.

Basic trig confirms the predicted size of these occillations, which also suggests the universe is Euclidian

Speed is measured through redshift; distance using standard candles (cepheids, type 1as)

Acceleration first detected in the 1990s.

SHOES measures galactic recession to about two billion light years, thus current expansion rate New standard candles (type 1a supernovae can be seen much farther than cepheids)

In the 1990s supernovae were used to determine the acceleration of the universe

Adam Reiss improve the calibration of type 1as hubble space telescope using newer cepheids

73.5 +- 1.7 km/s/mpc

Planck result: 66.9 +- 0.6

one in 7 thousand chance that this could be due to error

withstood rechecking

systematic sources of uncertainty in either method?

unknown physics in the CMB calculation?

very fast moving particle in sufficient numbers could skew the result (sterile neutrino)?

Dark matter particles behave differently? React more strongly?

Dark energy isn't constant? No cosmological constant?

Riess

Friedmann equations are only applicable if the universe is homogeneous and isotropic

HST went from factor of 2 to ten percent precision

standard candles: type 1a supernovae (Riess thesis paper)

variations in luminosity in type 1a supernovae- correct for these

dust dims standard candles but scatter more light in the blue than red

after accounting for variations, type 1as were good to 6 percent distance

1998: distant supernvae dimmer than expected. "Grey dust"? Or perhaps were supernovae dimmer in the past?

10 years: extending supernovae into the distance to see effects more obviously. Dark energy looks like a cosmological constant

various checking tests using different data sets have pretty much stuck us with this result.

reiss: 10 to 1 percent

reduce systematic errors, near infrared to reduce dust contamination

simplify the calculations

parallax to cepheids, look for cepheids in galaxies with type 1as, type 1as in the distant universe (gigaparsec), measure redshift

parallax is good up to 3 kpc (1/100 a pixel on HST), you can spread out the image over several pixels by moving the HST whilst imaging- 40 mas order of magnitude

Gaia was about to be launched, which would render their work relatively null

measuring the absolute size of a star using an eclipsing binary and gaining its angular diameter using interferometry

water masers around a supermassive black hole

checked 23 variables

myrids, tip of the red giant branch

lensed quasars

So little time

November

More than a hundred scientists met this summer at the Kavli Institute for Theoretical Physics at the University of California (U.S.) to try to clarify what is happening with the discordant data on the expansion rate of the universe, an issue that affects the very origin, evolution and fate of our cosmos. Their conclusions have been published in Nature Astronomy journal. "The problem lies in the Hubble constant (H0), a parameter which value—it is actually not a constant because it changes with time—indicates how fast the Universe is currently expanding," points out cosmologist Licia Verde, an ICREA researcher at the Institute of Cosmos Sciences of the University of Barcelona (ICC-UB) and the main author of the article. "There are different ways of measuring this quantity," she explains, "but they can be divided into two major classes: those relying on the late Universe (the closest to us in space and time) and those based on the early Universe, and they do not give exactly the same result."

the H0 value is approximately 73.9 kilometres per second per megaparsec. However, measurements based on the early Universe provide an average H0 value of 67.4 km/s/Mpc. These other records, obtained with data from the European Space Agency's Planck Satellite and other instruments, are obtained indirectly on the basis of the success of the standard cosmological model (Lambda-CDM model), which proposes a Universe made up of 5 % atoms or ordinary matter, 27 % dark matter (made up of particles, as yet detected, that provide additional gravitational attraction so that galaxies can form and clusters of galaxies are held together) and 68 % dark energy, which is responsible for accelerating the expansion of the Universe. In particular, these measurements of the primordial Universe focus on the farthest light that can be observed: the cosmic microwave background, produced when the Universe was only 380,000 years old, in the so-called recombination era (where protons recombined with electrons to form atoms)," says Licia Verde. The researcher highlights a relevant fact: "There are very different and independent ways (with totally different instruments and scientific tools) to measure the H0 on the basis of the early Universe, and the same goes for the late Universe. What is interesting is that all the measurements of one type are in mutual agreement with one another, at an exquisite precision of 1 or 2 %, as are those of the other type, with the same great precision; but when we compare the measurements of one class with those of the other, the discrepancy arises." "It looks like a small difference, only 7%, but it is significant considering that we are talking about precisions of 1 or 2% in the value of the Hubble constant," as emphasised by Licia Verde, who jokes: "It is like trying to thread a 'cosmic needle' where its hole is the H0 value measured today and the thread is brought by the model from the furthest Universe we can observe: the cosmic microwave background." In addition, she points out some of the consequences of the discrepancy: "The lower the H0 is, the older the Universe is. Its current age is calculated at about 13.8 billion years considering that the Hubble constant is 67 or 68 km/s/Mpc; but if its value were 74 km/s/Mpc, our universe would be younger: it would be approximately 12.8 billion years old." The authors point out in their study that this anomaly does not seem to depend on the instrument or method used for measuring, or on human equipment or sources. "If there are no errors in the data or measurements, could it be a problem with the model?" the researcher asks. "After all, the H0 values of the primordial Universe class are based on the standard cosmological model, which is very well established, very successful, but which we can try to change a little to solve the discrepancy," says the expert. "However, we cannot tamper with the characteristics of the model that work very well". If the data continue to confirm the problem, theoretical physicists seem to agree that the most promising route for solving it is to modify the model just before the light observed of the cosmic microwave background was formed, i.e. just before recombination (in which there was already 63 % dark matter, 15 % photons, 10 % neutrinos and 12 % atoms). One of the ideas proposed is that, shortly after the Big Bang, an intense episode of dark energy could have occurred that expanded the Universe faster than previously calculated. "Although it is still highly speculative, with this fine-tuned model, the H0 value obtained with measurements based on the primordial Universe could coincide with local measurements," notes Licia Verde, who concludes: "It won't be easy, but in this way we could thread the cosmic needle without breaking what works well in the model."

The observed acceleration of the Hubble expansion rate has been attributed to a mysterious "dark energy" which supposedly makes up about 70% of the universe. Professor Subir Sarkar from the Rudolf Peierls Centre for Theoretical Physics, Oxford along with collaborators at the Institut d'Astrophysique, Paris and the Niels Bohr Institute, Copenhagen have used observations of 740 Type Ia supernovae to show that this acceleration is a relatively local effect—it is directed along the direction we seem to be moving with respect to the cosmic microwave background (which exhibits a similar dipole anisotropy). While the physical reason for this acceleration is unknown, it cannot be ascribed to dark energy which would have caused equal acceleration in all directions. Professor Sarkar explains: "The cosmological standard model rests on the assumption that the Universe is isotropic around all observers. This cosmological principle is an extension of the Copernican principle—namely that we are not privileged observers. It affords a vast simplification in the mathematical construction of the cosmological model using Einstein's theory of general relativity. However when observational data are interpreted within this framework we are led to the astonishing conclusion that about 70% of the universe is constituted of Einstein's Cosmological Constant or more generally "dark energy." This has been interpreted as due to quantum zero-point fluctuations of the vacuum but the associated energy scale is set by H0, the present rate of expansion of the universe. This is however a factor of 1044 below the energy scale of the standard model of particle physics—the well-established quantum field theory that precisely describes all subatomic phenomena. Its zero-point fluctuations have therefore a huge energy density which would have prevented the universe from reaching its present age and size if they indeed influence the expansion rate via gravity. To this cosmological constant problem must be added the "why now?" problem, namely why has dark energy come to dominate the universe only recently? It was negligible at earlier times, in particular at an age of ~400,000 years when the primordial plasma cooled sufficiently to form atoms and the cosmic microwave background (CMB) radiation was released (hence the CMB is not directly sensitive to dark energy)." It is against this background that he, along with Jacques Colin and Roya Mohayaee (Institut d'Astrophysique, Paris) and Mohamed Rameez (Niels Bohr Institute, Copenhagen), set out to examine whether dark energy really exists. The primary evidence—rewarded with the 2011 Nobel prize in physics—concerns the "discovery of the accelerated expansion of the universe through observations of distant supernovae" in 1998 by two teams of astronomers. This was based on observations of about 60 Type Ia supernovae, but meanwhile, the sample had grown, and in 2014, the data was made available for 740 objects scattered over the sky (Joint Lightcurve Analysis catalog). "First, we worked out the supernova redshifts and apparent magnitudes as measured (in the heliocentric system), undoing the corrections that had been made in the JLA catalog for local 'peculiar' (non-Hubble) velocities. This had been done to determine their values in the CMB frame in which the universe should look isotropic—however, previous work by our team had shown that such corrections are suspect because peculiar velocities do not fall off with increasing distance, hence there is no convergence to the CMB frame even as far out as a billion light years," says Professor Sarkar. "When we then employed the standard maximum likelihood estimator statistic to extract parameter values, we made an astonishing finding. The supernova data indicate, with a statistical significance of 3.9σ, a dipole anisotropy in the inferred acceleration (see figure) in the same direction as we are moving locally, which is indicated by a similar, well-known, dipole in the CMB. By contrast, any isotropic (monopole) acceleration that can be ascribed to dark energy is 50 times smaller and consistent with being zero at 1.4σ. By the Bayesian information criterion, the best fit to the data has, in fact, no isotropic component. We showed that allowing for evolution with redshift of the parameters used to fit the supernova light curves does not change the conclusion—thus refuting previous criticism of our method. "Our analysis is data-driven but supports the theoretical proposal due to Christos Tsagas (University of Thessaloniki) that acceleration may be inferred when we are not Copernican observers, as is usually assumed, but are embedded in a local bulk flow shared by nearby galaxies, as is, indeed, observed. This is unexpected in the standard cosmological model, and the reason for such a flow remains unexplained. But independently of that, it appears that the acceleration is an artifact of our local flow, so dark energy cannot be invoked as its cause. "There are, indeed, other probes of our expansion history, e.g. the imprint of baryon acoustic oscillations (BAO) in the distribution of galaxies, the ages of the oldest stars, the rate of growth of structure, etc., but such data is still too sparse, and presently equally well consistent with a non-accelerating universe. The precisely measured temperature fluctuations in the CMB are not directly sensitive to dark energy, although its presence is usually inferred from the sum rule that while the CMB measures the spatial curvature of the universe to be close to zero, its matter content does not add up to the critical density to make it so. This is, however, true only under the assumptions of exact homogeneity and isotropy—which are now in question." Professor Sarkar concludes: "But progress will soon be made. The Large Synoptic Survey Telescope will measure many more supernovae and confirm or rule out a dipole in the deceleration parameter. The Dark Energy Spectroscopic Instrument and Euclid satellite will measure BAO and lensing precisely. The European Extremely Large Telescope will measure the 'redshift drift' of distant sources over a period of time, and thus make a direct measurement of the expansion history of the universe."

October

The study comes on the heels of a hot debate over just how fast the universe is ballooning; measurements thus far are in disagreement. The team's new measurement of the Hubble Constant, or the expansion rate of the universe, involved a different method. They used NASA's Hubble Space Telescope (HST) in combination with W. M. Keck Observatory's Adaptive Optics (AO) system to observe three gravitationally-lensed systems. This is the first time ground-based AO technology has been used to obtain the Hubble Constant. "When I first started working on this problem more than 20 years ago, the available instrumentation limited the amount of useful data that you could get out of the observations," says co-author Chris Fassnacht, Professor of Physics at UC Davis. "In this project, we are using Keck Observatory's AO for the first time in the full analysis. I have felt for many years that AO observations could contribute a lot to this effort." The team's results are published in the latest online issue of the Monthly Notices of the Royal Astronomical Society. To rule out any bias, the team conducted a blind analysis; during the processing, they kept the final answer hidden from even themselves until they were convinced that they had addressed as many possible sources of error as they could think of. This prevented them from making any adjustments to get to the "correct" value, avoiding confirmation bias. "When we thought that we had taken care of all possible problems with the analysis, we unblind the answer with the rule that we have to publish whatever value that we find, even if it's crazy. It's always a tense and exciting moment," says lead author Geoff Chen, a graduate student at the UC Davis Physics Department. The unblinding revealed a value that is consistent with Hubble Constant measurements taken from observations of "local" objects close to Earth, such as nearby Type Ia supernovae or gravitationally-lensed systems; Chen's team used the latter objects in their blind analysis. The team's results add to growing evidence that there is a problem with the standard model of cosmology, which shows the universe was expanding very fast early in its history, then the expansion slowed down due to the gravitational pull of dark matter, and now the expansion is speeding up again due to dark energy, a mysterious force.

April

Hubble to observe 70 pulsating stars called Cepheid variables in the Large Magellanic Cloud.

reduced the uncertainty in their Hubble constant value to 1.9% from an earlier estimate of 2.2%.

At odds with measurements were made by Planck, which maps the cosmic microwave background, a relic afterglow from 380,000 years after the big bang.

"This is not just two experiments disagreeing," Riess explained. "We are measuring something fundamentally different. One is a measurement of how fast the universe is expanding today, as we see it. The other is a prediction based on the physics of the early universe and on measurements of how fast it ought to be expanding. If these values don't agree, there becomes a very strong likelihood that we're missing something in the cosmological model that connects the two eras."

The Hubble astronomers then combined their result with another set of observations, made by the Araucaria Project, a collaboration between astronomers from institutions in Chile, the U.S., and Europe. This group made distance measurements to the Large Magellanic Cloud by observing the dimming of light as one star passes in front of its partner in eclipsing binary-star systems.

The new estimate of the Hubble constant is 74 kilometers per second per megaparsec.

The number indicates that the universe is expanding at a 9% faster rate than the prediction of 67 kilometers (41.6 miles) per second per megaparsec, which comes from Planck's observations of the early universe, coupled with our present understanding of the universe.

One explanation for the mismatch involves an unexpected appearance of dark energy in the young universe, which is thought to now comprise 70% of the universe's contents. Proposed by astronomers at Johns Hopkins, the theory is dubbed "early dark energy," and suggests that the universe evolved like a three-act play.

Astronomers have already hypothesized that dark energy existed during the first seconds after the big bang and pushed matter throughout space, starting the initial expansion. Dark energy may also be the reason for the universe's accelerated expansion today. The new theory suggests that there was a third dark-energy episode not long after the big bang, which expanded the universe faster than astronomers had predicted. The existence of this "early dark energy" could account for the tension between the two Hubble constant values, Riess said.

Another idea is that the universe contains a new subatomic particle that travels close to the speed of light. Such speedy particles are collectively called "dark radiation" and include previously known particles like neutrinos, which are created in nuclear reactions and radioactive decays.

Yet another attractive possibility is that dark matter (an invisible form of matter not made up of protons, neutrons, and electrons) interacts more strongly with normal matter or radiation than previously assumed.