m →Extended models: fix error introduced in previous edit, where table was moved to the beginning of the section
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There has been debate on whether early massive galaxies and supermassive black holes are in conflict with LCDM<ref>{{Cite journal |last1=Steinhardt |first1=Charles. L. |last2=Capak |first2=Peter |last3=Masters |first3=Dan |last4=Speagle |first4=Josh S. |date=2016-06-10 |title=The Impossibly Early Galaxy Problem |journal=The Astrophysical Journal |volume=824 |issue=1 |pages=21 |doi=10.3847/0004-637X/824/1/21 |arxiv=1506.01377 |bibcode=2016ApJ...824...21S |doi-access=free |issn=0004-637X}}</ref>. To make such a comparison, one must model the complex physics of galaxy formation, as well as the underlying LCDM cosmology.<ref>{{Cite journal |last1=Behroozi |first1=Peter |last2=Silk |first2=Joseph |date=2018-07-11 |title=The most massive galaxies and black holes allowed by ΛCDM |url=https://academic.oup.com/mnras/article/477/4/5382/4975781 |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=477 |issue=4 |pages=5382–5387 |doi=10.1093/mnras/sty945 |doi-access=free |issn=0035-8711}}</ref> Tests using galaxies are therefore less direct, as they require assumptions about how galaxies form.
Using some of the first data from the [[James WebWebb Space Telescope]], a team of astronomers selected candidate massive galaxies in the early universe.<ref>{{Cite journal |last1=Labbé |first1=Ivo |last2=van Dokkum |first2=Pieter |last3=Nelson |first3=Erica |last4=Bezanson |first4=Rachel |last5=Suess |first5=Katherine A. |last6=Leja |first6=Joel |last7=Brammer |first7=Gabriel |last8=Whitaker |first8=Katherine |last9=Mathews |first9=Elijah |last10=Stefanon |first10=Mauro |last11=Wang |first11=Bingjie |date=April 2023 |title=A population of red candidate massive galaxies ~600 Myr after the Big Bang |url=https://www.nature.com/articles/s41586-023-05786-2 |journal=Nature |language=en |volume=616 |issue=7956 |pages=266–269 |doi=10.1038/s41586-023-05786-2 |pmid=36812940 |arxiv=2207.12446 |bibcode=2023Natur.616..266L |issn=1476-4687}}</ref> The existence of such massive galaxies in the early universe would challenge standard cosmology. <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> Follow up spectroscopy revealed that most of these objects have [[Active galactic nucleus|Active Galactic Nuclei]], which boosts the galaxies brightness and caused the masses to be overestimated. <ref>{{Cite web |date=2025-07-01 |title=JWST's early galaxies didn't break the Universe. They revealed it. |url=https://bigthink.com/starts-with-a-bang/jwst-break-universe-revealed/ |access-date=2025-07-24 |website=Big Think |language=en-US}}</ref><ref>{{Cite journal |last1=Kocevski |first1=Dale D. |last2=Finkelstein |first2=Steven L. |last3=Barro |first3=Guillermo |last4=Taylor |first4=Anthony J. |last5=Calabrò |first5=Antonello |last6=Laloux |first6=Brivael |last7=Buchner |first7=Johannes |last8=Trump |first8=Jonathan R. |last9=Leung |first9=Gene C. K. |last10=Yang |first10=Guang |last11=Dickinson |first11=Mark |last12=Pérez-González |first12=Pablo G. |last13=Pacucci |first13=Fabio |last14=Inayoshi |first14=Kohei |last15=Somerville |first15=Rachel S. |date=June 2025 |title=The Rise of Faint, Red Active Galactic Nuclei at z > 4: A Sample of Little Red Dots in the JWST Extragalactic Legacy Fields |journal=The Astrophysical Journal |language=en |volume=986 |issue=2 |pages=126 |doi=10.3847/1538-4357/adbc7d |arxiv=2404.03576 |bibcode=2025ApJ...986..126K |doi-access=free |issn=0004-637X}}</ref> The high redshift galaxies which have been spectroscopically confirmed, such as [[JADES-GS-z13-0]], are much less massive and are consistent with the predictions from LCDM simulations run before JWST<ref>{{Cite journal |last1=McCaffrey |first1=Joe |last2=Hardin |first2=Samantha |last3=Wise |first3=John H. |last4=Regan |first4=John A. |date=2023-09-27 |title=No Tension: JWST Galaxies at \(z > 10\) Consistent with Cosmological Simulations |url=http://localhost:58547/article/88302-no-tension-jwst-galaxies-at-z-10-consistent-with-cosmological-simulations,%20https://astro.theoj.org/article/88302-no-tension-jwst-galaxies-at-z-10-consistent-with-cosmological-simulations |journal=The Open Journal of Astrophysics |language=en |volume=6 |page=47 |doi=10.21105/astro.2304.13755 |arxiv=2304.13755 |bibcode=2023OJAp....6E..47M }}</ref>. As a population, the confirmed high redshift galaxies are brighter than expected from simulations, but not to the extent that they violate cosmological limits.<ref>{{Cite journal |last1=Xiao |first1=Mengyuan |last2=Oesch |first2=Pascal A. |last3=Elbaz |first3=David |last4=Bing |first4=Longji |last5=Nelson |first5=Erica J. |last6=Weibel |first6=Andrea |last7=Illingworth |first7=Garth D. |last8=van Dokkum |first8=Pieter |last9=Naidu |first9=Rohan P. |last10=Daddi |first10=Emanuele |last11=Bouwens |first11=Rychard J. |last12=Matthee |first12=Jorryt |last13=Wuyts |first13=Stijn |last14=Chisholm |first14=John |last15=Brammer |first15=Gabriel |date=November 2024 |title=Accelerated formation of ultra-massive galaxies in the first billion years |url=https://ui.adsabs.harvard.edu/abs/2024Natur.635..311X/abstract |journal=Nature |language=en |volume=635 |issue=8038 |pages=311–315 |doi=10.1038/s41586-024-08094-5 |pmid=39537883 |arxiv=2309.02492 |bibcode=2024Natur.635..311X |issn=0028-0836}}</ref><ref>{{Citation |last1=Yung |first1=L. Y. Aaron |title=$Λ$CDM is still not broken: empirical constraints on the star formation efficiency at $z \sim 12-30$ |date=2025 |url=https://arxiv.org/abs/2504.18618 |access-date=2025-07-24 |arxiv=2504.18618 |last2=Somerville |first2=Rachel S. |last3=Iyer |first3=Kartheik G.}}</ref> Theorists are studying many possible explanations, including modifying cosmology, more efficient star formation and different stellar populations.<ref>{{Cite journal |last1=Sun |first1=Guochao |last2=Faucher-Giguère |first2=Claude-André |last3=Hayward |first3=Christopher C. |last4=Shen |first4=Xuejian |last5=Wetzel |first5=Andrew |last6=Cochrane |first6=Rachel K. |date=2023-10-01 |title=Bursty Star Formation Naturally Explains the Abundance of Bright Galaxies at Cosmic Dawn |journal=The Astrophysical Journal Letters |volume=955 |issue=2 |pages=L35 |doi=10.3847/2041-8213/acf85a |arxiv=2307.15305 |bibcode=2023ApJ...955L..35S |doi-access=free |issn=2041-8205}}</ref><ref>{{Cite journal |last1=Dekel |first1=Avishai |last2=Sarkar |first2=Kartick C |last3=Birnboim |first3=Yuval |last4=Mandelker |first4=Nir |last5=Li |first5=Zhaozhou |date=2023-06-08 |title=Efficient formation of massive galaxies at cosmic dawn by feedback-free starbursts |url=https://academic.oup.com/mnras/article/523/3/3201/7179993 |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=523 |issue=3 |pages=3201–3218 |doi=10.1093/mnras/stad1557 |doi-access=free |issn=0035-8711}}</ref>
=== Missing baryon problem ===
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Extended models allow one or more of the "fixed" parameters above to vary, in addition to the basic six; so these models join smoothly to the basic six-parameter model in the limit that the additional parameter(s) approach the default values. For example, possible extensions of the simplest ΛCDM model allow for spatial curvature (<math>\Omega_\text{tot}</math> may be different from 1); or [[quintessence (physics)|quintessence]] rather than a [[cosmological constant]] where the [[Equation of state (cosmology)|equation of state]] of dark energy is allowed to differ from −1. Cosmic inflation predicts tensor fluctuations ([[gravitational wave]]s). Their amplitude is parameterized by the tensor-to-scalar ratio (denoted <math>r</math>), which is determined by the unknown energy scale of inflation. Other modifications allow [[hot dark matter]] in the form of [[neutrino]]s more massive than the minimal value, or a running spectral index; the latter is generally not favoured by simple cosmic inflation models.
Allowing additional variable parameter(s) will generally ''increase'' the uncertainties in the standard six parameters quoted above, and may also shift the central values slightly. The table belowabove shows results for each of the possible "6+1" scenarios with one additional variable parameter; this indicates that, as of 2015, there is no convincing evidence that any additional parameter is different from its default value.
Some researchers have suggested that there is a running spectral index, but no statistically significant study has revealed one. Theoretical expectations suggest that the tensor-to-scalar ratio <math>r</math> should be between 0 and 0.3, and the latest results are within those limits.
