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Hypothetical type of dark matter in physics From Wikipedia, the free encyclopedia
In cosmology and physics, cold dark matter (CDM) is a hypothetical type of dark matter. According to the current standard model of cosmology, Lambda-CDM model, approximately 27% of the universe is dark matter and 68% is dark energy, with only a small fraction being the ordinary baryonic matter that composes stars, planets, and living organisms. Cold refers to the fact that the dark matter moves slowly compared to the speed of light, giving it a vanishing equation of state. Dark indicates that it interacts very weakly with ordinary matter and electromagnetic radiation. Proposed candidates for CDM include weakly interacting massive particles, primordial black holes, and axions.
The theory of cold dark matter was originally published in 1982 by James Peebles;[1] while the warm dark matter picture was proposed independently at the same time by J. Richard Bond, Alex Szalay, and Michael Turner;[2] and George Blumenthal, H. Pagels, and Joel Primack.[3] A review article in 1984 by Blumenthal, Sandra Moore Faber, Primack, and Martin Rees developed the details of the theory.[4]
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. Predictions of the cold dark matter paradigm are in general agreement with observations of cosmological large-scale structure.
In the hot dark matter paradigm, popular in the early 1980s but less so in the 1990s, structure does not form hierarchically (bottom-up), but 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.
Since the late 1980s or 1990s, most cosmologists favor the cold dark matter theory (specifically the modern Lambda-CDM model) 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. Dwarf galaxies are crucial to this theory, having been created by small-scale density fluctuations in the early universe;[5] they have now become natural building blocks that form larger structures.
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:
Several discrepancies between the predictions of cold dark matter in the ΛCDM model and observations of galaxies and their clustering have arisen. Some of these problems have proposed solutions, but it remains unclear whether they can be solved without abandoning the ΛCDM model.[15]
The density distributions of dark matter halos in cold dark matter simulations (at least those that do not include the impact of baryonic feedback) are much more peaked than what is observed in galaxies by investigating their rotation curves.[16]
Cold dark matter simulations predict large numbers of small dark matter halos, more numerous than the number of small dwarf galaxies that are observed around galaxies like the Milky Way.[17]
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.[18]
Galaxies in the NGC 3109 association are moving away too rapidly to be consistent with expectations in the ΛCDM model.[19] In this framework, NGC 3109 is too massive and distant from the Local Group for it to have been flung out in a three-body interaction involving the Milky Way or Andromeda Galaxy.[20]
If galaxies grew hierarchically, then massive galaxies required many mergers. Major mergers inevitably create a classical bulge. On the contrary, about 80% of observed galaxies give evidence of no such bulges, and giant pure-disc galaxies are commonplace.[21] The tension can be quantified by comparing the observed distribution of galaxy shapes today with predictions from high-resolution hydrodynamical cosmological simulations in the ΛCDM framework, revealing a highly significant problem that is unlikely to be solved by improving the resolution of the simulations.[22] The high bulgeless fraction was nearly constant for 8 billion years.[23]
If galaxies were embedded within massive halos of cold dark matter, then the bars that often develop in their central regions would be slowed down by dynamical friction with the halo. This is in serious tension with the fact that observed galaxy bars are typically fast.[24]
Comparison of the model with observations may have some problems on sub-galaxy scales, possibly predicting too many dwarf galaxies and too much dark matter in the innermost regions of galaxies. This problem is called the "small scale crisis".[25] These small scales are harder to resolve in computer simulations, so it is not yet clear whether the problem is the simulations, non-standard properties of dark matter, or a more radical error in the model.
Observations from the James Webb Space Telescope have resulted in various galaxies confirmed by spectroscopy at high redshift, such as JADES-GS-z13-0 at cosmological redshift of 13.2[26][27] or JADES-GS-z14-0 at cosmological redshift of 14.32. Such a high rate of large galaxy formation in the early universe appears to contradict the rates of galaxy formation allowed in the existing Lambda CDM model via dark matter halos, as even if galaxy formation were 100% efficient and all mass were allowed to turn into stars in Lambda CDM, it wouldn't be enough to create such large galaxies.[28][29][30] However, this depends upon assuming a stellar initial mass function. If early star formation favored massive stars, this could explain the tension.[31]
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