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In quantum field theory, a false vacuum is a metastable sector of space that appears to be a perturbative vacuum, but is unstable due to instanton effects that may tunnel to a lower energy state. This tunneling can be caused by quantum fluctuations or the creation of high-energy particles. The false vacuum is a local minimum, but not the lowest energy state, even though it may remain stable for some time.
- 1 Stability and instability of the vacuum
- 2 Vacuum metastability event
- 3 Bubble nucleation
- 4 Gravitational effects
- 5 See also
- 6 Notes
- 7 References
- 8 Further reading
- 9 External links
Stability and instability of the vacuum
For decades, scientific models of our universe have included the possibility that it exists as a long-lived, but not completely stable, sector of space, which could potentially at some time be destroyed upon 'toppling' into a more stable vacuum state. If the universe were indeed in such a false vacuum state, a catastrophic bubble of more stable "true vacuum" could theoretically occur at any time or place expanding outward at the speed of light. The Standard Model of particle physics opens the possibility of calculating, from the masses of the Higgs boson and the top quark, whether the universe's present electroweak vacuum state is likely to be stable or merely long-lived. (This was sometimes misreported as the Higgs boson "ending" the universe). A 125–127 GeV Higgs mass seems to be extremely close to the boundary for stability (estimated in 2012 as 123.8–135.0 GeV). However, a definitive answer requires much more precise measurements of the top quark's pole mass, and new physics beyond the Standard Model of Particle Physics could drastically change this picture.
In a 2005 paper published in Nature, as part of their investigation into Global catastrophic risks, MIT physicist Max Tegmark and Oxford philosopher Nick Bostrom calculate the natural risks of the destruction of the Earth at less than 1 per Gigayear from all events, including a transition to a lower vacuum state. They argue that due to observer selection effects, we might underestimate the chances of being destroyed by vacuum decay because any information about this event would reach us only at the instant when we too were destroyed. This is in contrast to events like to risks from impacts, gamma-ray bursts, and supernovae, whose frequencies we have adequate direct measures of.
If measurements of these particles suggests that our universe lies within a false vacuum of this kind, then it would imply—more than likely in many billions of years[Note 1]—that it could cease to exist as we know it, if a true vacuum happened to nucleate.
This is because, if the Standard Model is correct, the particles and forces we observe in our universe exist as they do because of underlying quantum fields. Quantum fields can have states of differing stability, including 'stable', 'unstable', or 'metastable' (meaning, long-lived but capable of being "toppled" in the right circumstances). If a more stable vacuum state were able to arise, then existing particles and forces would no longer arise as they do in the universe's present state. Different particles or forces would arise from (and be shaped by) whatever new quantum states arose. The world we know depends upon these particles and forces, so if this happened, everything around us, from subatomic particles to galaxies, and all fundamental forces, would be reconstituted into new fundamental particles and forces and structures. The universe would lose all of its present structures and become inhabited by new ones (depending upon the exact states involved) based upon the same quantum fields. In a new study posted on the arXiv in March 2015, it was pointed out that the vacuum decay rate could be vastly increased in the vicinity of black holes, which would serve as a nucleation seed. According to the new study, a potentially catastrophic vacuum decay could be triggered any time by primordial black holes, should they exist. It was also discussed that tiny black holes potentially produced at the LHC could trigger such a vacuum decay event, but the results in the existing study were not conclusive.
It would also have implications for other aspects of physics, and would suggest that the Higgs self-coupling λ and its βλ function could be very close to zero at the Planck scale, with "intriguing" implications, including implications for theories of gravity and Higgs-based inflation.:218 A future electron-positron collider would be able to provide the precise measurements of the top quark needed for such calculations.
Vacuum metastability event
A hypothetical vacuum metastability event would be theoretically possible if our universe were part of a metastable (false) vacuum in the first place, an issue that was highly theoretical and far from resolved in 1982. A false vacuum is one that appears stable, and is stable within certain limits and conditions, but is capable of being disrupted and entering a different state which is more stable. If this were the case, a bubble of lower-energy vacuum could come to exist by chance or otherwise in our universe, and catalyze the conversion of our universe to a lower energy state in a volume expanding at nearly the speed of light, destroying all of the observable universe without forewarning. Chaotic Inflation theory suggests that the universe may be in either a false vacuum or a true vacuum state.
A paper by Coleman and de Luccia which attempted to include simple gravitational assumptions into these theories noted that if this was an accurate representation of nature, then the resulting universe "inside the bubble" in such a case would appear to be extremely unstable and would almost immediately collapse:
Such an event would be one possible doomsday event. It was used as a plot device in a science-fiction story in 1988 by Geoffrey A. Landis, in 2000 by Stephen Baxter, and in 2015 by Alastair Reynolds in his novel Poseidon's Wake.
In theory, either high enough energy concentrations or random chance could trigger the tunneling needed to set this event in motion. However an immense number of ultra-high energy particles and events have occurred in the history of our universe, dwarfing by many orders of magnitude any events at human disposal. Hut and Rees note that, because we have observed cosmic ray collisions at much higher energies than those produced in terrestrial particle accelerators, these experiments should not, at least for the foreseeable future, pose a threat to our current vacuum. Particle accelerators have reached energies of only approximately eight tera electron volts (8×1012 eV). Cosmic ray collisions have been observed at and beyond energies of 1018 eV, a million times more powerful – the so-called Greisen–Zatsepin–Kuzmin limit – and other cosmic events may be more powerful yet. Against this, John Leslie has argued that if present trends continue, particle accelerators will exceed the energy given off in naturally occurring cosmic ray collisions by the year 2150. Fears of this kind were raised by critics of both the Relativistic Heavy Ion Collider and the Large Hadron Collider at the time of their respective proposal, and determined to be unfounded by scientific inquiry.
On the other hand, if the many-worlds interpretation of quantum mechanics is correct, the explanation for why there has been no vacuum decay yet despite many high-energy particle collisions changes entirely. In this case, the corresponding collisions did trigger the vacuum decay, and we're not observing it simply because every such event excludes any observers in its causal (light-cone) future - and there are always worlds exactly identical to such a world in everything except the decay event and its future cone. More generally, the same applies to any set of future light-cones - that is, any causally closed patch of spacetime - as long as its content either destroys any observers or is eventually unremarkable. Then observers' transition through the boundary of this patch, such as the (potential) cone of the decay event, will have quantitatively altered probability distributions - but formally and subjectively it will be just the same quantum branching, qualitatively indistinguishable from any other ordinary moment of existence. If this is the case, fine-tuning is an active process, and therefore a vacuum metastability event will never happen.
In the theoretical physics of the false vacuum, the system moves to a lower energy state – either the true vacuum, or another, lower energy vacuum – through a process known as bubble nucleation. In this, instanton effects cause a bubble to appear in which fields have their true vacuum values inside. Therefore, the interior of the bubble has a lower energy. The walls of the bubble (or domain walls) have a surface tension, as energy is expended as the fields roll over the potential barrier to the lower energy vacuum. The most likely size of the bubble is determined in the semi-classical approximation to be such that the bubble has zero total change in the energy: the decrease in energy by the true vacuum in the interior is compensated by the tension of the walls.
Expansion of bubble
Any increase in size of the bubble will decrease its potential energy, as the energy of the wall increases as the area of a sphere but the negative contribution of the interior increases more quickly, as the volume of a sphere . Therefore, after the bubble is nucleated, it quickly begins expanding at very nearly the speed of light. The excess energy contributes to the very large kinetic energy of the walls. If two bubbles are nucleated and they eventually collide, it is thought that particle production would occur where the walls collide.
The tunnelling rate is increased by increasing the energy difference between the two vacua and decreased by increasing the height or width of the barrier.
The addition of gravity to the story leads to a considerably richer variety of phenomena. The key insight is that a false vacuum with positive potential energy density is a de Sitter vacuum, in which the potential energy acts as a cosmological constant and the Universe is undergoing the exponential expansion of de Sitter space. This leads to a number of interesting effects, first studied by Coleman and de Luccia.
Development of theories
Alan Guth, in his original proposal for cosmic inflation, proposed that inflation could end through quantum mechanical bubble nucleation of the sort described above. See History of Chaotic inflation theory. It was soon understood that a homogeneous and isotropic universe could not be preserved through the violent tunneling process. This led Andrei Linde and, independently, Andreas Albrecht and Paul Steinhardt, to propose "new inflation" or "slow roll inflation" in which no tunnelling occurs, and the inflationary scalar field instead rolls down a gentle slope.
- The bubble's effects would be expected to propagate across the universe at the speed of light from wherever it occurred. However space is vast—with even the nearest galaxy being over 2 million lightyears from us, and others being many billions of lightyears distant, so the effect of such an event would be unlikely to arise here for billions of years after first occurring.
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- Peralta, Eyder (2013-02-19). "If Higgs Boson Calculations Are Right, A Catastrophic 'Bubble' Could End Universe". NPR.org. NPR. Retrieved 12 March 2013. Article cites Fermilab's Joseph Lykken: "The bubble forms through an unlikely quantum fluctuation, at a random time and place," Lykken tells us. "So in principle it could happen tomorrow, but then most likely in a very distant galaxy, so we are still safe for billions of years before it gets to us."
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Earth will likely be long gone before any Higgs boson particles set off an apocalyptic assault on the universe
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[T]he Earth will likely be long gone before any Higgs boson particles set off an apocalyptic assault on the universe
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[T]he bad news is that its mass suggests the universe will end in a fast-spreading bubble of doom. The good news? It'll probably be tens of billions of years. The article quotes Fermilab's Joseph Lykken: "[T]he parameters for our universe, including the Higgs [and top quark's masses] suggest that we're just at the edge of stability, in a "metastable" state. Physicists have been contemplating such a possibility for more than 30 years. Back in 1982, physicists Michael Turner and Frank Wilczek wrote in Nature that "without warning, a bubble of true vacuum could nucleate somewhere in the universe and move outwards..."
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- Hans Moravec. Mind Children. p. 188.: "Two builders of a future super (immensely expensive) particle accelerator have a problem. The machine has been completed for months, but so far has failed on each attempt to use it. The problem is not in the design but seemingly just in the designer's bad luck. Lightning caused a power outage just at turn on, or a fuse blew, or a janitor tripped over a cable, or a little earthquake triggered an emergency cutoff; each incident was different, and apparently unrelated to the others. But perhaps the failures are an enormous stroke of luck. New calculations suggest that the machine is powerful enough to trigger a collapse of the vacuum to a lower energy state. A cosmic explosion might radiate out at the speed of light from the accelerator's collision point, eventually destroying the entire universe. What a close call! Or was it? If the universe had been destroyed, there would be no one left to lament the fact. What if the many-worlds idea were correct? In some universes the machine would have worked. For all practical purposes those worlds would have ceased to exist. Only in the remainder would a pair of puzzled physicists be scratching their heads, wondering what had gone wrong this time. Given so many nearly identical universes, the destruction of a few seems of small consequence. An idea strikes them. Why not reinforce the weak points in the machine so that a random failure within it is extremely unlikely, then wire it to a detector of a nuclear attack, like the doomsday machine in Stanley Kubrick's film Dr. Strangelove. Any attack would be met by the destruction of the offending universe. Only those universes in which the attack had not happened, for some reason (the commanding general had a heart attack, the missile launch system failed, the premier had a fit of compassion...), would live to wonder about yet another close call. The machine in Strangelove was ineffective as a deterrent unless the other side was aware of it. Not so the many-worlds version. No attack (that anyone will notice) can occur so long as it operates, no matter how secret its existence."
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