Editing Lambda-CDM model (section)

=== Missing baryon problem ===
{{main|Missing baryon problem}}
Massimo Persic and Paolo Salucci<ref>{{Cite journal|last1=Persic|first1=M.|last2=Salucci|first2=P.|date=1992-09-01|title=The baryon content of the Universe|journal=Monthly Notices of the Royal Astronomical Society|volume=258|issue=1|pages=14P–18P|doi=10.1093/mnras/258.1.14P|arxiv=astro-ph/0502178|bibcode=1992MNRAS.258P..14P |issn=0035-8711|doi-access=free}}</ref> first estimated the baryonic density today present in ellipticals, spirals, groups and clusters of galaxies.
They performed an integration of the baryonic mass-to-light ratio over luminosity (in the following <math display="inline">  M_{\rm b}/L </math>), weighted with the luminosity function <math display="inline">\phi(L)</math> over the previously mentioned classes of astrophysical objects:

<math display="block">\rho_{\rm b} = \sum \int L\phi(L) \frac{M_{\rm b}}{L} \, dL.</math>

The result was:

<math display="block"> \Omega_{\rm b}=\Omega_*+\Omega_\text{gas}=2.2\times10^{-3}+1.5\times10^{-3}\;h^{-1.3}\simeq0.003 ,</math>

where <math> h\simeq 0.72 </math>.

Note that this value is much lower than the prediction of standard cosmic nucleosynthesis <math> \Omega_{\rm b}\simeq0.0486 </math>, so that stars and gas in galaxies and in galaxy groups and clusters account for less than 10% of the primordially synthesized baryons. This issue is known as the problem of the "missing baryons".

The missing baryon problem is claimed to be resolved. Using observations of the kinematic [[Sunyaev–Zeldovich effect|Sunyaev–Zel'dovich effect]] spanning more than 90% of the lifetime of the Universe, in 2021 astrophysicists found that approximately 50% of all baryonic matter is outside [[dark matter halo]]es, filling the space between galaxies.<ref>{{Cite journal|last1=Chaves-Montero|first1=Jonás|last2=Hernández-Monteagudo|first2=Carlos|last3=Angulo|first3=Raúl E|last4=Emberson|first4=J D|date=2021-03-25|title=Measuring the evolution of intergalactic gas from z = 0 to 5 using the kinematic Sunyaev–Zel'dovich effect|url=https://academic.oup.com/mnras/article/503/2/1798/6184230|journal=Monthly Notices of the Royal Astronomical Society|language=en|volume=503|issue=2|pages=1798–1814|doi=10.1093/mnras/staa3782|doi-access=free |arxiv=1911.10690 |issn=0035-8711}}</ref> Together with the amount of baryons inside galaxies and surrounding them, the total amount of baryons in the late time Universe is compatible with early Universe measurements.