Cold dark matter
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In cosmology and physics, cold dark matter (CDM) is a hypothetical form of matter (a kind of dark matter) whose particles move slowly compared to the speed of light (the cold in CDM) and interact very weakly with electromagnetic radiation (the dark in CDM). It is believed that approximately 80% of matter in the Universe is dark matter, with only a small fraction being the ordinary baryonic matter that composes stars, planets and living organisms. As of 2006[update], most cosmologists favor the cold dark matter theory as a description of how the Universe went from a smooth initial state at early times (as shown by the cosmic microwave background radiation) to the lumpy distribution of galaxies and their clusters we see today — the large-scale structure of the Universe. The theory sees the role that dwarf galaxies played as crucial, as they are thought to be natural building blocks that form larger structures, created by small-scale density fluctuations in the early Universe. The theory was originally published in 1984 by American physicists Joel R. Primack, George Blumenthal, and Sandra Moore Faber with British scientist Martin Rees.
In the cold dark matter theory, structure grows hierarchically, with small objects collapsing under their self-gravity first and merging in a continuous hierarchy to form larger and more massive objects. In the hot dark matter paradigm, popular in the early 1980s, structure does not form hierarchically (bottom-up), but rather forms by fragmentation (top-down), with the largest superclusters forming first in flat pancake-like sheets and subsequently fragmenting into smaller pieces like our galaxy the Milky Way. The predictions of the hot dark matter theory disagree with observations of large-scale structures, whereas the cold dark matter paradigm is in general agreement with the observations.
Dark matter is detected through its gravitational interactions with ordinary matter and radiation. As such, it is very difficult to determine what the constituents of cold dark matter are. The candidates fall roughly into three categories:
- Axions are very light particles with a specific type of self-interaction that makes them a suitable CDM candidate. Axions have the theoretical advantage that their existence solves the Strong CP problem in QCD, but have not been detected.
- MACHOs or Massive Compact Halo Objects are large, condensed objects such as black holes, neutron stars, white dwarfs, very faint stars, or non-luminous objects like planets. The search for these consists of using gravitational lensing to see the effect of these objects on background galaxies. Most experts believe that the constraints from those searches rule out MACHOs as a viable dark matter candidate.
- WIMPs: Dark matter is composed of Weakly Interacting Massive Particles. There is no currently known particle with the required properties, but many extensions of the standard model of particle physics predict such particles. The search for WIMPs involves attempts at direct detection by highly sensitive detectors, as well as attempts at production by particle accelerators. WIMPs are generally regarded as the most promising dark matter candidates. The DAMA/NaI experiment and its successor DAMA/LIBRA have claimed to directly detect dark matter particles passing through the Earth, but many scientists remain skeptical, as no results from similar experiments seem compatible with the DAMA results.
Several discrepancies between the predictions of the particle cold dark matter paradigm and observations of galaxies and their clustering have arisen:
- The cuspy halo problem: the density distributions of DM halos in cold dark matter simulations are much more peaked than what is observed in galaxies by investigating their rotation curves.
- The missing satellites problem: cold dark matter simulations predict much larger numbers of small dwarf galaxies than are observed around galaxies like the Milky Way.
- The disk of satellites problem: dwarf galaxies around the Milky Way and Andromeda galaxies are observed to be orbiting in thin, planar structures whereas the simulations predict that they should be distributed randomly about their parent galaxies.
Some of these problems have proposed solutions but it remains unclear whether they can be solved without abandoning the CDM paradigm.
- Fuzzy cold dark matter
- Meta-cold dark matter
- Dark matter
- Lambda-CDM model
- Modified Newtonian dynamics
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Dwarf galaxies play a crucial role in the CDM scenario for galaxy formation, having been suggested to be the natural building blocks from which larger structures are built up by merging processes. In this scenario dwarf galaxies are formed from small-scale density fluctuations in the primeval Universe.
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