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; 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:
Axions, very light particles with a specific type of self-interaction that makes them a suitable CDM candidate. In recent years, axions have become one of the most promising candidates for dark matter. 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. Axions are an example of a more general category of particle called a WISP (weakly interacting "slender" or "slim" particle), which are the low-mass counterparts of WIMPs.
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. Historically, WIMPs were regarded as one of the most promising candidates for the composition of dark matter, but in recent years WIMPs have since been supplanted by axions with the non-detection of WIMPs in experiments. 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.
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.
Cuspy halo problemEdit
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.
Dwarf galaxy problemEdit
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.
Satellite disk problemEdit
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.
High-velocity galaxy problemEdit
Galaxies in the NGC 3109 association are moving away too rapidly to be consistent with expectations in the ΛCDM model. 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.
Galaxy morphology problemEdit
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. 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. The high bulgeless fraction was nearly constant for 8 billion years.
Fast galaxy bar problemEdit
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.
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". 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.
^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. S2CID 4351645.
^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. OSTI 1447148. S2CID 4324282.
^Battinelli, P.; S. Demers (2005-10-06). "The C star population of DDO 190: 1. Introduction" (PDF). Astronomy and Astrophysics. Astronomy & Astrophysics. 447 (2): 473. 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.
^ abFrancesca Chadha-Day; John Ellis; David J. E. Marsh (23 February 2022). "Axion dark matter: What is it and why now?". Science Advances. 8 (8): eabj3618. arXiv:2105.01406. Bibcode:2022SciA....8J3618C. doi:10.1126/sciadv.abj3618. PMC8865781. PMID35196098.
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. S2CID 118946242.
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. S2CID 118979310.
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. S2CID 119180701. 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. Vol. 468. pp. 389–393.
^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. S2CID 11711780.
^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. S2CID 14308775.
^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. S2CID 12983798.
^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.
^Banik, Indranil; Zhao, H (2018-01-21). "A plane of high velocity galaxies across the Local Group". Monthly Notices of the Royal Astronomical Society. 473 (3): 4033–4054. arXiv:1701.06559. Bibcode:2018MNRAS.473.4033B. doi:10.1093/mnras/stx2596. ISSN 0035-8711.
^Banik, Indranil; Haslbauer, Moritz; Pawlowski, Marcel S.; Famaey, Benoit; Kroupa, Pavel (2021-06-21). "On the absence of backsplash analogues to NGC 3109 in the ΛCDM framework". Monthly Notices of the Royal Astronomical Society. 503 (4): 6170–6186. arXiv:2105.04575. Bibcode:2021MNRAS.503.6170B. doi:10.1093/mnras/stab751. ISSN 0035-8711.
^Haslbauer, M; Banik, I; Kroupa, P; Wittenburg, N; Javanmardi, B (2022-02-01). "The High Fraction of Thin Disk Galaxies Continues to Challenge ΛCDM Cosmology". The Astrophysical Journal. 925 (2): 183. arXiv:2202.01221. Bibcode:2022ApJ...925..183H. doi:10.3847/1538-4357/ac46ac. ISSN 1538-4357.
^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. S2CID 14644377.
^Mahmood, R; Ghafourian, N; Kashfi, T; Banik, I; Haslbauer, M; Cuomo, V; Famaey, B; Kroupa, P (2021-11-01). "Fast galaxy bars continue to challenge standard cosmology". Monthly Notices of the Royal Astronomical Society. 508 (1): 926–939. arXiv:2106.10304. Bibcode:2021MNRAS.508..926R. doi:10.1093/mnras/stab2553. hdl:10023/24680. ISSN 0035-8711.
^Rini, Matteo (2017). "Synopsis: Tackling the Small-Scale Crisis". Physical Review D. 95 (12): 121302. arXiv:1703.10559. Bibcode:2017PhRvD..95l1302N. doi:10.1103/PhysRevD.95.121302. S2CID 54675159.