Cold dark matter

In cosmology and physics, cold dark matter (CDM) is a hypothetical type of dark matter. Observations indicate that approximately 85% of the matter in the universe is dark matter, 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, while dark indicates that it interacts very weakly with ordinary matter and electromagnetic radiation.

The physical nature of CDM is currently unknown, and there are a wide variety of possibilities. Among them are a new type of weakly interacting massive particle, primordial black holes, and axions.

History

The theory of cold dark matter was originally published in 1982 by three independent groups of cosmologists: James Peebles;[1] 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]
Structure formation

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 and less so now, 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.
Composition

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, very light particles with a specific type of self-interaction that makes them a suitable CDM candidate.[6][7] Axions have the theoretical advantage that their existence solves the strong CP problem in quantum chromodynamics, but axion particles have only been theorized and never detected.

Massive compact halo objects (MACHOs), large, condensed objects such as black holes, neutron stars, white dwarfs, very faint stars, or non-luminous objects like planets. The search for these objects consists of using gravitational lensing to detect the effects of these objects on background galaxies. Most experts believe that the constraints from those searches rule out MACHOs as a viable dark matter candidate.[8][9][10][11][12][13]

Weakly interacting massive particles (WIMPs). 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 of WIMPs by particle accelerators. WIMPs are generally regarded as one of the most promising candidates for the composition of dark matter.[9][11][13] The DAMA/NaI experiment and its successor DAMA/LIBRA have claimed to have directly detected dark matter particles passing through the Earth, but many scientists remain skeptical because no results from similar experiments seem compatible with the DAMA results.

Challenges

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 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.[14]
The missing satellites problem
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.[15]
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.[16]
Galaxy morphology problem
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.[17] That bulgeless fraction was nearly constant for 8 billion years.[18]

Some of these problems have proposed solutions, but it remains unclear whether they can be solved without abandoning the CDM paradigm.[19]
See also

Fuzzy cold dark matter
Hot dark matter
Meta-cold dark matter
Modified Newtonian dynamics
Self-interacting dark matter
Warm dark matter

References

Peebles, P. J. E. (December 1982). "Large-scale background temperature and mass fluctuations due to scale-invariant primeval perturbations". The Astrophysical Journal. 263: L1. Bibcode:1982ApJ...263L...1P. doi:10.1086/183911.
Bond, J. R.; Szalay, A. S.; Turner, M. S. (1982). "Formation of galaxies in a gravitino-dominated universe". Physical Review Letters. 48 (23): 1636–1639. Bibcode:1982PhRvL..48.1636B. doi:10.1103/PhysRevLett.48.1636.
Blumenthal, George R.; Pagels, Heinz; Primack, Joel R. (2 September 1982). "Galaxy formation by dissipationless particles heavier than neutrinos". Nature. 299 (5878): 37–38. Bibcode:1982Natur.299...37B. doi:10.1038/299037a0.
Blumenthal, G. R.; Faber, S. M.; Primack, J. R.; Rees, M. J. (1984). "Formation of galaxies and large-scale structure with cold dark matter". Nature. 311 (517): 517–525. Bibcode:1984Natur.311..517B. doi:10.1038/311517a0.
Battinelli, P.; S. Demers (2005-10-06). "The C star population of DDO 190: 1. Introduction" (PDF). Astronomy and Astrophysics. Astronomy and Astrophysics. 447: 1. Bibcode:2006A&A...447..473B. doi:10.1051/0004-6361:20052829. Archived from the original on 2005-10-06. Retrieved 2012-08-19. "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."
e.g. M. Turner (2010). "Axions 2010 Workshop". U. Florida, Gainesville, USA.
e.g. Pierre Sikivie (2008). "Axion Cosmology". Lect. Notes Phys. 741, 19-50.
Carr, B. J.; et al. (May 2010). "New cosmological constraints on primordial black holes". Physical Review D. 81 (10): 104019.arXiv:0912.5297. Bibcode:2010PhRvD..81j4019C. doi:10.1103/PhysRevD.81.104019.
Peter, A. H. G. (2012). "Dark Matter: A Brief Review". arXiv:1201.3942 [astro-ph.CO].
Bertone, Gianfranco; Hooper, Dan; Silk, Joseph (January 2005). "Particle dark matter: evidence, candidates and constraints". Physics Reports. 405 (5–6): 279–390. arXiv:hep-ph/0404175. Bibcode:2005PhR...405..279B. doi:10.1016/j.physrep.2004.08.031.
Garrett, Katherine; Dūda, Gintaras (2011). "Dark Matter: A Primer". Advances in Astronomy. 2011: 968283.arXiv:1006.2483. Bibcode:2011AdAst2011E...8G. doi:10.1155/2011/968283.. p. 3: "MACHOs can only account for a very small percentage of the nonluminous mass in our galaxy, revealing that most dark matter cannot be strongly concentrated or exist in the form of baryonic astrophysical objects. Although microlensing surveys rule out baryonic objects like brown dwarfs, black holes, and neutron stars in our galactic halo, can other forms of baryonic matter make up the bulk of dark matter? The answer, surprisingly, is no..."
Bertone, Gianfranco (18 November 2010). "The moment of truth for WIMP dark matter". Nature. 468, pp. 389–393
Olive, Keith A. (2003). "TASI Lectures on Dark Matter". Physics. 54: 21. arXiv:astro-ph/0301505. Bibcode:2003astro.ph..1505O.
Gentile, G.; Salucci, P. (2004). "The cored distribution of dark matter in spiral galaxies". Monthly Notices of the Royal Astronomical Society. 351 (3): 903–922. arXiv:astro-ph/0403154. Bibcode:2004MNRAS.351..903G. doi:10.1111/j.1365-2966.2004.07836.x.
Klypin, Anatoly; Kravtsov, Andrey V.; Valenzuela, Octavio; Prada, Francisco (1999). "Where are the missing galactic satellites?". Astrophysical Journal. 522 (1): 82–92. arXiv:astro-ph/9901240. Bibcode:1999ApJ...522...82K. doi:10.1086/307643.
Pawlowski, Marcel; et al. (2014). "Co-orbiting satellite galaxy structures are still in conflict with the distribution of primordial dwarf galaxies". Monthly Notices of the Royal Astronomical Society. 442 (3): 2362–2380.arXiv:1406.1799. Bibcode:2014MNRAS.442.2362P. doi:10.1093/mnras/stu1005.
Kormendy, J.; Drory, N.; Bender, R.; Cornell, M.E. (2010). "Bulgeless giant galaxies challenge our picture of galaxy formation by hierarchical clustering". The Astrophysical Journal. 723 (1): 54–80.arXiv:1009.3015. Bibcode:2010ApJ...723...54K. doi:10.1088/0004-637X/723/1/54.
Sachdeva, S.; Saha, K. (2016). "Survival of pure disk galaxies over the last 8 billion years". The Astrophysical Journal Letters. 820 (1): L4. arXiv:1602.08942. Bibcode:2016ApJ...820L...4S. doi:10.3847/2041-8205/820/1/L4.

Kroupa, P.; Famaey, B.; de Boer, Klaas S.; Dabringhausen, Joerg; Pawlowski, Marcel; Boily, Christian; Jerjen, Helmut; Forbes, Duncan; Hensler, Gerhard (2010). "Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation". Astronomy and Astrophysics. 523: 32–54.arXiv:1006.1647. Bibcode:2010A&A...523A..32K. doi:10.1051/0004-6361/201014892.