Editing Cold dark matter (section)

== 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>