Editing Cold dark matter

{{short description|Hypothetical type of dark matter in physics}}

In [[Physical cosmology|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 [[star]]s, [[planet]]s, 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 (cosmology)|equation of state]]. ''Dark'' indicates that it interacts very weakly with ordinary matter and [[electromagnetic radiation]].  Proposed candidates for CDM include [[weakly interacting massive particle]]s, [[primordial black hole]]s, and [[axion]]s.

== History ==

The theory of cold dark matter was originally published in 1982 by [[Jim Peebles|James Peebles]];<ref>{{cite journal|last1=Peebles|first1=P. J. E.|title=Large-scale background temperature and mass fluctuations due to scale-invariant primeval perturbations|journal=The Astrophysical Journal|date=December 1982|volume=263|pages=L1|doi=10.1086/183911|bibcode = 1982ApJ...263L...1P |doi-access=free}}  </ref> while the warm dark matter picture was proposed independently at the same time by [[J. Richard Bond]], [[Alex Szalay]], and [[Michael Turner (cosmologist)|Michael Turner]];<ref>{{cite journal|doi=10.1103/PhysRevLett.48.1636|bibcode=1982PhRvL..48.1636B|title=Formation of galaxies in a gravitino-dominated universe | volume=48|issue=23|journal=Physical Review Letters|pages=1636–1639|year=1982|last1=Bond|first1=J. R.|last2=Szalay|first2=A. S.|last3=Turner|first3=M. S.}}</ref> and [[George Blumenthal (astrophysicist)|George Blumenthal]], H. Pagels, and [[Joel Primack]].<ref>{{cite journal|last1=Blumenthal|first1=George R.|last2=Pagels|first2=Heinz|last3=Primack|first3=Joel R.|title=Galaxy formation by dissipationless particles heavier than neutrinos|journal=Nature|date=2 September 1982|volume=299|issue=5878|pages=37–38|doi=10.1038/299037a0|bibcode = 1982Natur.299...37B |s2cid=4351645}}</ref>
A review article in 1984 by Blumenthal, [[Sandra Moore Faber]], Primack, and [[Martin Rees]] developed the details of the theory.<ref>{{cite journal
| last1        = Blumenthal
| first1       = G. R.
| last2        =  Faber
| first2       = S. M.
| last3        = Primack
| first3       = J. R.
| last4        =  Rees
| first4       = M. J.
| date         = 1984
| title        = Formation of galaxies and large-scale structure with cold dark matter
| journal      = Nature
| volume       = 311
| issue        = 517
| pages        = 517–525
| doi          = 10.1038/311517a0
| bibcode      = 1984Natur.311..517B
| osti = 1447148
| s2cid = 4324282
}}</ref>

==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 [[Observable universe|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 [[supercluster]]s 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 [[galaxy cluster|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;<ref>{{cite journal
 |url          = http://www.aanda.org/
 |title        = The C star population of DDO 190: 1. Introduction
 |first        = P.
 |last         = Battinelli
 |author2      = S. Demers
 |date         = 2005-10-06
 |publisher    = Astronomy & Astrophysics
 |page         = 473
 |doi          = 10.1051/0004-6361:20052829
 |archive-url  = https://web.archive.org/web/20120815011424/http://www.aanda.org/
 |archive-date = 2012-08-15
 |access-date  = 2012-08-19
 |quote        = 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.
 |bibcode      = 2006A&A...447..473B
 |volume       = 447
 |journal      = Astronomy and Astrophysics
 |issue        = 2
 |doi-access   = free
 |url-status   = bot: unknown
|url-access= subscription
 }}</ref> 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:

* [[Axion]]s, very light particles with a specific type of self-interaction that makes them a suitable CDM candidate.<ref name="Axion, first CDM candidate">
{{cite news
 | author1 = Turner, M.
 | display-authors=etal
 | year = 2010
 | title = Axions 2010 Workshop
 | publisher = U. Florida
 | place = Gainesville, USA
}}{{full citation|date=August 2021}}
</ref><ref name="Axion Cosmology">
{{cite news
 | author1 = Sikivie, Pierre
 | display-authors=etal
 | year= 2008
 | title = Axion Cosmology
 | work = Lect. Notes Phys.
 | volume = 741
 | pages = 19–50
}}{{full citation|date=August 2021}}
</ref> Since the late 2010s, axions have become one of the most promising candidates for dark matter.<ref name="Chadha-Day et al">{{cite journal |title=Axion dark matter: What is it and why now?|author1=Francesca Chadha-Day|author2=John Ellis|author3=David J. E. Marsh|journal=Science Advances|volume=8|issue=8|doi=10.1126/sciadv.abj3618|pmid=35196098|date=23 February 2022|pages=eabj3618 |pmc=8865781 |arxiv=2105.01406 |bibcode=2022SciA....8J3618C }}</ref> 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 (particle physics)|WISP]] ([[WISP (particle physics)|weakly interacting "slender" or "slim" particle]]), which are the low-mass counterparts of WIMPs. 

* [[Massive compact halo object]]s (MACHOs), large, condensed objects such as [[black hole]]s, [[neutron star]]s, [[white dwarf]]s, very faint [[star]]s, or non-luminous objects like [[planet]]s. 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.<ref name=Carr>
{{cite journal
 | last1=Carr | first1=B.J.
 | display-authors=etal
 | date=May 2010
 | title=New cosmological constraints on primordial black holes
 | journal=Physical Review D
 | volume=81 | issue=10 | page=104019
 | doi=10.1103/PhysRevD.81.104019 |arxiv=0912.5297
 |bibcode = 2010PhRvD..81j4019C |s2cid=118946242
}}
</ref><ref name=Peter>
{{cite arXiv
 |last=Peter |first=A.H.G.
 |year=2012
 |title=Dark matter: A brief review
 |eprint=1201.3942
 |class=astro-ph.CO
}}
</ref><ref name="bertone hooper silk">
{{cite journal
 |last1=Bertone | first1=Gianfranco
 | last2=Hooper | first2=Dan
 | last3=Silk   | first3=Joseph | author3-link=Joseph Silk
 | date=January 2005
 | title=Particle dark matter: evidence, candidates and constraints
 | doi=10.1016/j.physrep.2004.08.031
 | arxiv = hep-ph/0404175
 | journal=Physics Reports
 | volume=405 |issue=5–6 | pages=279–390
 | bibcode=2005PhR...405..279B | s2cid=118979310
}}
</ref><ref name=Garrett>
{{cite journal
 | last1 = Garrett | first1 = Katherine
 | last2 = Dūda    | first2 = Gintaras
 | year = 2011
 | title = Dark Matter: A Primer
 | journal = Advances in Astronomy
 | volume = 2011 | page = 968283
 | doi = 10.1155/2011/968283 |arxiv = 1006.2483
 |bibcode = 2011AdAst2011E...8G | s2cid = 119180701
 |quote=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''{{'}}&nbsp;...
| doi-access = free
 }}
</ref><ref name=Bertone>
{{cite journal
 |last=Bertone |first=Gianfranco
 |date=18 November 2010 
 |title=The moment of truth for WIMP dark matter
 |journal=Nature
 |volume=468 |issue=7322
 |pages=389–393
|doi=10.1038/nature09509
 |pmid=21085174
 |arxiv=1011.3532
 |bibcode=2010Natur.468..389B
 |s2cid=4415912
 |url=https://www.zora.uzh.ch/id/eprint/41577/1/1011.3532v1.pdf
 }}
</ref><ref name=Olive>
{{cite journal
 | last1 = Olive | first1 = Keith A.
 | year = 2003
 | title = TASI lectures on dark matter
 | journal = Physics
 | volume = 54 | page = 21
 | bibcode = 2003astro.ph..1505O | arxiv = astro-ph/0301505
}}
</ref>

* [[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 accelerator]]s. Historically, WIMPs were regarded as one of the most promising candidates for the composition of dark matter,<ref name=Peter/><ref name=Garrett/><ref name=Olive/> but since the late 2010s, WIMPs have been supplanted by axions with the non-detection of WIMPs in experiments.<ref name="Chadha-Day et al"/> 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 ==
{{See also|Lambda-CDM model#Challenges}}
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.<ref>{{Cite journal |arxiv=1006.1647 |title=Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation |year=2010 |last1=Kroupa |first1=P. |last2=Famaey |first2=B. |last3=de Boer |first3=Klaas S. |last4=Dabringhausen |first4=Joerg |last5=Pawlowski |first5=Marcel |last6=Boily |first6=Christian |last7=Jerjen |first7=Helmut |last8=Forbes |first8=Duncan |last9=Hensler |first9=Gerhard |journal=Astronomy and Astrophysics |volume=523 |pages=32–54 |doi=10.1051/0004-6361/201014892 |bibcode=2010A&A...523A..32K|s2cid=11711780 }}</ref>

===Cuspy halo problem===
{{main|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.<ref>{{Cite journal |title=The cored distribution of dark matter in spiral galaxies|year=2004 |last1=Gentile |first1=G. |last2=Salucci |first2=P. |journal=Monthly Notices of the Royal Astronomical Society |volume=351 |issue=3 |pages=903–922 |doi=10.1111/j.1365-2966.2004.07836.x |doi-access=free |arxiv=astro-ph/0403154 |bibcode = 2004MNRAS.351..903G|s2cid=14308775 }}</ref>

===Dwarf galaxy problem===
{{main|Dwarf galaxy problem}} 
Cold dark matter simulations predict large numbers of small clumps in dark matter halos, consequently many [[dwarf galaxy|dwarf galaxies]] clustered around [[spiral galaxy|spiral]] and [[elliptical galaxy|elliptical galaxies]] – more numerous than the number of small dwarf galaxies that are observed around large galaxies like the [[Milky Way]].<ref name=Klypin>{{cite journal |last1=Klypin |first1=Anatoly |last2=Kravtsov |first2=Andrey V. |last3=Valenzuela |first3=Octavio |last4=Prada |first4=Francisco |year=1999 |title=Where are the missing galactic satellites? |journal=Astrophysical Journal |volume=522 |issue=1 |pages=82–92 |doi=10.1086/307643 |bibcode=1999ApJ...522...82K |arxiv=astro-ph/9901240|s2cid=12983798 }}</ref>

===Satellite disk problem===
Dwarf galaxies around the [[Milky Way]] and [[Andromeda Galaxy|Andromeda]] galaxies are observed to be orbiting in thin, planar structures whereas the simulations predict that they should be distributed randomly, in a roughly [[galactic halo|spherical halos]] about their parent galaxies, similar to the orbits observed for [[globular cluster]]s.<ref name=Pawlowski>{{cite journal |first1=Marcel |last1=Pawlowski |display-authors=etal |year=2014 |title=Co-orbiting satellite galaxy structures are still in conflict with the distribution of primordial dwarf galaxies |journal=Monthly Notices of the Royal Astronomical Society |volume=442 |issue=3 |pages=2362–2380 |arxiv=1406.1799 |doi=10.1093/mnras/stu1005 |doi-access=free |bibcode=2014MNRAS.442.2362P }}</ref>

===High-velocity galaxy problem===
Galaxies in the [[NGC 3109]] association are moving away too rapidly to be consistent with expectations in the ΛCDM model.<ref>{{Cite journal |last1=Banik |first1=Indranil |last2=Zhao |first2=H |date=2018-01-21 |title=A plane of high velocity galaxies across the Local Group |journal=Monthly Notices of the Royal Astronomical Society |volume=473 |issue=3 |pages=4033–4054 |arxiv=1701.06559 |doi=10.1093/mnras/stx2596 |bibcode=2018MNRAS.473.4033B |issn=0035-8711 |doi-access=free}}</ref> 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]].<ref>{{Cite journal |last1=Banik |first1=Indranil |last2=Haslbauer |first2=Moritz |last3=Pawlowski |first3=Marcel S. |last4=Famaey |first4=Benoit |last5=Kroupa |first5=Pavel |date=2021-06-21 |title=On the absence of backsplash analogues to NGC 3109 in the ΛCDM framework |journal=Monthly Notices of the Royal Astronomical Society |volume=503 |issue=4 |pages=6170–6186 |doi=10.1093/mnras/stab751 |arxiv=2105.04575 |bibcode=2021MNRAS.503.6170B |issn=0035-8711 |doi-access=free}}</ref>

===Galaxy morphology problem===
If galaxies grew hierarchically, then massive galaxies required many mergers. [[Galaxy merger|Major mergers]] inevitably create a classical [[Bulge (astronomy)|bulge]]. On the contrary, about 80% of observed galaxies give evidence of no such bulges, and giant pure-disc galaxies are commonplace.<ref name="kormendy2010">{{cite journal |last1=Kormendy |first1=J. |author1-link=John Kormendy |last2=Drory |first2=N. |last3=Bender |first3=R. |last4=Cornell |first4=M.E. |title=Bulgeless giant galaxies challenge our picture of galaxy formation by hierarchical clustering |year=2010 |journal=[[The Astrophysical Journal]] |volume=723 |issue=1 |pages=54–80 |doi=10.1088/0004-637X/723/1/54 |arxiv=1009.3015 |bibcode=2010ApJ...723...54K|s2cid=119303368 }}</ref> 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.<ref name="Haslbauer2022">{{cite journal |last1=Haslbauer |first1=M |last2=Banik |first2=I |last3=Kroupa |first3=P |last4=Wittenburg |first4=N |last5=Javanmardi |first5=B |title=The High Fraction of Thin Disk Galaxies Continues to Challenge ΛCDM Cosmology |date=2022-02-01 |journal=[[The Astrophysical Journal]] |volume=925 |issue=2 |pages=183 |doi=10.3847/1538-4357/ac46ac |issn=1538-4357 |arxiv=2202.01221 |bibcode=2022ApJ...925..183H |doi-access=free}}</ref> The high bulgeless fraction was nearly constant for 8&nbsp;billion years.<ref name="sachdeva2016">{{cite journal |last1=Sachdeva |first1=S. |last2=Saha |first2=K. |title=Survival of pure disk galaxies over the last 8&nbsp;billion years |year=2016 |journal=The Astrophysical Journal Letters |volume=820 |issue=1 |pages=L4 |doi=10.3847/2041-8205/820/1/L4 |arxiv=1602.08942 |bibcode=2016ApJ...820L...4S |s2cid=14644377 |doi-access=free }}</ref>

===Fast galaxy bar problem===
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.<ref name="Roshan2021">{{Cite journal |last1=Mahmood |first1=R |last2=Ghafourian |first2=N |last3=Kashfi |first3=T |last4=Banik |first4=I |last5=Haslbauer |first5=M |last6=Cuomo |first6=V |last7=Famaey |first7=B |last8=Kroupa |first8=P |date=2021-11-01 |title=Fast galaxy bars continue to challenge standard cosmology |journal=Monthly Notices of the Royal Astronomical Society |volume=508 |issue=1 |pages=926–939 |doi=10.1093/mnras/stab2553 |doi-access=free |arxiv=2106.10304 |bibcode=2021MNRAS.508..926R |hdl=10023/24680 |issn=0035-8711}}</ref>

===Small-scale crisis===
Comparison of the model with observations may have some problems on sub-galaxy scales, possibly predicting [[Dwarf galaxy problem|too many dwarf galaxies]] and too much dark matter in the innermost regions of galaxies. This problem is called the "small scale crisis".<ref>{{Cite journal
 | title =Synopsis: Tackling the Small-Scale Crisis
 |journal = Physical Review D|volume = 95|issue = 12|pages = 121302| last =Rini
 | first =Matteo
 |doi = 10.1103/PhysRevD.95.121302|year = 2017|arxiv = 1703.10559|bibcode = 2017PhRvD..95l1302N|s2cid = 54675159}}</ref>  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.

===High redshift galaxies===
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<ref name="NASA-milestone">{{cite web|title = NASA's Webb Reaches New Milestone in Quest for Distant Galaxies|url = https://blogs.nasa.gov/webb/2022/12/09/nasas-webb-reaches-new-milestone-in-quest-for-distant-galaxies/ |first = Thaddeus|last = Cesari|date = 9 December 2022|accessdate = 9 December 2022}}</ref><ref name="Curtis-Lake2022">{{cite journal |display-authors = etal|first1 = Emma|last1 = Curtis-Lake|title = Spectroscopic confirmation of four metal-poor galaxies at z=10.3–13.2 | journal=Nature Astronomy |date = 27 February 2023 | volume=7 | issue=5 | page=622 | doi=10.1038/s41550-023-01918-w |arxiv=2212.04568| bibcode=2023NatAs...7..622C }}</ref> 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.<ref name="SciAm2022">{{cite web |last=O'Callaghan |first=Jonathan |date=6 December 2022 |title=Astronomers Grapple with JWST's Discovery of Early Galaxies |url=https://www.scientificamerican.com/article/astronomers-grapple-with-jwsts-discovery-of-early-galaxies1/ |access-date=10 December 2022 |website=[[Scientific American]]}}</ref><ref name="BehrooziEtAl">{{cite journal |author1=Behroozi, Peter |author2=Conroy, Charlie |author3=Wechsler, Risa H. |author4=Hearin, Andrew |author5=Williams, Christina C. |author6=Moster, Benjamin P. |author7=Yung, L. Y. Aaron |author8=Somerville, Rachel S. |author9=Gottlöber, Stefan |author10=Yepes, Gustavo |author11=Endsley, Ryan |date=December 2020 |title=The Universe at z > 10: predictions for JWST from the UNIVERSEMACHINE DR1 |journal=Monthly Notices of the Royal Astronomical Society |volume=499 |issue=4 |pages=5702–5718 |arxiv=2007.04988 |bibcode=2020MNRAS.499.5702B |doi=10.1093/mnras/staa3164|doi-access=free }}</ref><ref name="SpringelHernquist">{{cite journal |author1=Volker Springel |author2=Lars Hernquist |date=February 2003 |title=The history of star formation in a Λ cold dark matter universe |journal=Monthly Notices of the Royal Astronomical Society |volume=339 |issue=2 |pages=312–334 |arxiv=astro-ph/0206395 |bibcode=2003MNRAS.339..312S |doi=10.1046/j.1365-8711.2003.06207.x |doi-access=free |s2cid=8715136}}</ref> However, this depends upon assuming a stellar [[initial mass function]]. If early star formation favored massive stars, this could explain the tension.<ref name="Boylan-Kolchin">{{cite journal |title=Stress testing ΛCDM with high-redshift galaxy candidates|first=Michael|last=Boylan-Kolchin |journal=Nature Astronomy |year=2023 |volume=7 |issue=6 |pages=731–735 |doi=10.1038/s41550-023-01937-7 |pmid=37351007 |pmc=10281863 |arxiv=2208.01611 |bibcode=2023NatAs...7..731B |s2cid=251252960}}</ref>

==See also==
* [[Fuzzy cold dark matter]]
* [[Hot dark matter]]
* [[Meta-cold dark matter]]
* [[Modified Newtonian dynamics]]
* [[Self-interacting dark matter]]
* [[Warm dark matter]]
*[[2CDM model of dark matter]]

==References==
{{Reflist|2}}

==Further reading==
*{{Cite book
  | last = Bertone
  | first = Gianfranco
  | title = Particle Dark Matter: Observations, Models and Searches
  | publisher = [[Cambridge University Press]]
  | year = 2010
  | pages = 762
  | isbn = 978-0-521-76368-4| title-link = Particle Dark Matter
 }}

{{Dark matter}}

{{DEFAULTSORT:Cold Dark Matter}}
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[[Category:Dark matter]]
[[Category:Physical cosmological concepts]]