Editing Lambda-CDM model (section)

== Overview ==
The ΛCDM model is based on three postulates on the structure of [[spacetime]]:<ref name="Longair-2009">{{Cite book|date=2008 |title=Galaxy Formation 
|author=Malcolm S. Longair
|url=http://link.springer.com/10.1007/978-3-540-73478-9 |series=Astronomy and Astrophysics Library |language=en |location=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |doi=10.1007/978-3-540-73478-9 |isbn=978-3-540-73477-2}}</ref>{{rp|227}}
# The [[cosmological principle]], that the universe is the same everywhere and in all directions, and that it is expanding,
# A postulate by [[Hermann Weyl]] that the lines of spacetime ([[geodesics]]) intersect at only one point, where time along each line can be synchronized; the behavior resembles an expanding [[perfect fluid]],<ref name="Longair-2009"/>{{rp|175}}
# [[general relativity]] that relates the geometry of spacetime to the distribution of matter and energy.
This combination greatly simplifies the equations of general relativity into a form called the [[Friedmann equations]]. These equations specify the evolution of the [[Scale factor (cosmology)|scale factor]] of the universe in terms of the pressure and density of a perfect fluid. The evolving density is composed of different kinds of energy and matter, each with its own role in affecting the scale factor.<ref>{{Cite book |last=White |first=Simon |title=Physics of the Early Universe: Proceedings of the Thirty Sixth Scottish Universities Summer School in Physics, Edinburgh, July 24 - August 11 1989 |date=1990 |publisher=Taylor & Francis Group |isbn=978-1-040-29413-0 |edition=1 |series=Scottish Graduate Series |location=Milton |chapter=Physical Cosmology}}</ref>{{rp|7}} For example, a model might include [[baryons]], [[photons]], [[neutrinos]], and [[dark matter]].<ref name=PDG-2024>{{Cite journal |last=Navas |first=S. |last2=Amsler |first2=C. |last3=Gutsche |first3=T. |last4=Hanhart |first4=C. |last5=Hernández-Rey |first5=J. J. |last6=Lourenço |first6=C. |last7=Masoni |first7=A. |last8=Mikhasenko |first8=M. |last9=Mitchell |first9=R. E. |last10=Patrignani |first10=C. |last11=Schwanda |first11=C. |last12=Spanier |first12=S. |last13=Venanzoni |first13=G. |last14=Yuan |first14=C. Z. |last15=Agashe |first15=K. |date=2024-08-01 |title=Review of Particle Physics |url=https://link.aps.org/doi/10.1103/PhysRevD.110.030001 |journal=Physical Review D |language=en |volume=110 |issue=3 |doi=10.1103/PhysRevD.110.030001 |issn=2470-0010}}</ref>{{rp|25.1.1}}  These component densities become parameters extracted when the model is constrained to match astrophysical observations. The model aims to describe the observable universe from approximately 0.1&nbsp;s to the present.<ref name=DeruelleUzan/>{{rp|605}}

The most accurate observations which are sensitive to the component densities are consequences of statistical inhomogeneity called "perturbations" in the early universe. Since the Friedmann equations assume homogeneity, additional theory must be added before comparison to experiments. [[Inflation (cosmology)|Inflation]] is a simple model producing perturbations by postulating an extremely rapid expansion early in the universe that separates quantum fluctuations before they can equilibrate. The perturbations are characterized by additional parameters also determined by matching observations.<ref name=PDG-2024/>{{rp|25.1.2}}

Finally, the light which will become astronomical observations must pass through the universe. The latter part of that journey will pass through [[reionization|ionized space]], where the electrons can scatter the light, altering the anisotropies. This effect is characterized by one additional parameter.<ref name=PDG-2024/>{{rp|25.1.3}}

The ΛCDM model includes an expansion of the spatial [[Metric tensor (general relativity)|metric]] that is well documented, both as the [[redshift]] of prominent spectral absorption or emission lines in the light from distant galaxies, and as the time dilation in the light decay of supernova luminosity curves. Both effects are attributed to a [[Doppler shift]] in electromagnetic radiation as it travels across expanding space. Although this expansion increases the distance between objects that are not under shared gravitational influence, it does not increase the size of the objects (e.g. galaxies) in space. Also, since it originates from ordinary general relativity, it, like general relativity, allows for distant galaxies to recede from each other at speeds greater than the speed of light; local expansion is less than the speed of light, but expansion summed across great distances can collectively exceed the speed of light.<ref name=DavisLineweaver>{{Cite journal |last1=Davis |first1=Tamara M. |last2=Lineweaver |first2=Charles H. |date=2004 |title=Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the Universe |url=https://www.cambridge.org/core/product/identifier/S132335800000607X/type/journal_article |journal=Publications of the Astronomical Society of Australia |language=en |volume=21 |issue=1 |pages=97–109 |doi=10.1071/AS03040 |arxiv=astro-ph/0310808 |bibcode=2004PASA...21...97D |issn=1323-3580}}</ref>

The letter Λ ([[lambda]]) represents the [[cosmological constant]], which is associated with a vacuum energy or [[dark energy]] in empty space that is used to explain the contemporary accelerating expansion of space against the attractive effects of gravity. A cosmological constant has negative pressure, <math> p = - \rho c^{2} </math>, which contributes to the [[stress–energy tensor]] that, according to the general theory of relativity, causes accelerating expansion. The fraction of the total energy density of our (flat or almost flat) universe that is dark energy, <math>\Omega_{\Lambda}</math>, is estimated to be 0.669 ± 0.038 based on the 2018 [[Dark Energy Survey]] results using [[Type Ia supernova]]e<ref>{{Cite journal |arxiv = 1811.02374|author=DES Collaboration |title = First Cosmology Results using Type Ia Supernovae from the Dark Energy Survey: Constraints on Cosmological Parameters|journal = The Astrophysical Journal|volume = 872|issue = 2|pages = L30|year = 2018|doi = 10.3847/2041-8213/ab04fa|s2cid = 84833144 |doi-access=free |bibcode=2019ApJ...872L..30A }}</ref> or {{val|0.6847|0.0073}} based on the 2018 release of [[Planck (spacecraft)|''Planck'' satellite]] data, or more than 68.3% (2018 estimate) of the mass–energy density of the universe.<ref>{{Cite journal |arxiv = 1807.06209|author=Planck Collaboration|title = Planck 2018 results. VI. Cosmological parameters|journal = Astronomy & Astrophysics|year = 2020|volume = 641|pages = A6|doi = 10.1051/0004-6361/201833910|bibcode = 2020A&A...641A...6P|s2cid = 119335614}}</ref>

[[Dark matter]] is postulated in order to account for gravitational effects observed in very large-scale structures (the "non-keplerian" [[rotation curve]]s of galaxies;<ref>{{cite journal |author1= Persic, M.|display-authors=etal |title= The universal rotation curve of spiral galaxies — I. The dark matter connection  |journal=[[Monthly Notices of the Royal Astronomical Society]]  |date=1996  |volume=281   |issue=1 |pages=27–47 |doi= 10.1093/mnras/278.1.27 |doi-access=free |bibcode= 1996MNRAS.281...27P |arxiv=astro-ph/9506004}}</ref>                                                                                                                                   the [[gravitational lens]]ing of light by galaxy clusters; and the enhanced clustering of galaxies) that cannot be accounted for by the quantity of observed matter.<ref>{{Cite journal |last1=Bertone |first1=Gianfranco |last2=Hooper |first2=Dan |date=2018-10-15 |title=History of dark matter |url=https://link.aps.org/doi/10.1103/RevModPhys.90.045002 |journal=Reviews of Modern Physics |language=en |volume=90 |issue=4 |page=045002 |doi=10.1103/RevModPhys.90.045002 |issn=0034-6861|arxiv=1605.04909 |bibcode=2018RvMP...90d5002B }}</ref>
The ΛCDM model proposes specifically [[cold dark matter]], hypothesized as:
* Non-baryonic: Consists of matter other than protons and neutrons (and electrons, by convention, although electrons are not baryons)
* Cold: Its velocity is far less than the speed of light at the epoch of radiation–matter equality (thus neutrinos are excluded, being non-baryonic but not cold)
* Dissipationless: Cannot cool by radiating photons
* Collisionless: Dark matter particles interact with each other and other particles only through gravity and possibly the weak force

Dark matter constitutes about 26.5%<ref name="PDG2019">{{cite journal |first1=M. |last1= Tanabashi |display-authors=etal |collaboration=[[Particle Data Group]] |url=http://pdg.lbl.gov/2019/reviews/rpp2019-rev-astrophysical-constants.pdf |title=Astrophysical Constants and Parameters |publisher=[[Particle Data Group]] |year=2019 |access-date=2020-03-08  |journal=Physical Review D |volume=98 |issue=3 |page=030001|doi= 10.1103/PhysRevD.98.030001|doi-access=free |bibcode= 2018PhRvD..98c0001T }}</ref> of the mass–energy density of the universe. The remaining 4.9%<ref name="PDG2019"/> comprises all ordinary matter observed as atoms, chemical elements, gas and plasma, the stuff of which visible planets, stars and galaxies are made. The great majority of ordinary matter in the universe is unseen, since visible stars and gas inside galaxies and clusters account for less than 10% of the ordinary matter contribution to the mass–energy density of the universe.<ref>
{{cite journal
| last1 = Persic
| first1 = Massimo
| last2 = Salucci
| first2 = Paolo
| date = 1992-09-01
| title = The baryon content of the Universe
| url = http://mnras.oxfordjournals.org/content/258/1/14P
| journal = Monthly Notices of the Royal Astronomical Society
| language = en
| volume = 258
| issue = 1
| pages = 14P–18P
| doi = 10.1093/mnras/258.1.14P
| doi-access = free
| issn = 0035-8711
| arxiv = astro-ph/0502178 |bibcode = 1992MNRAS.258P..14P | s2cid = 17945298
}}</ref>

The model includes a single originating event, the "[[Big Bang]]", which was not an explosion but the abrupt appearance of expanding [[spacetime]] containing radiation at temperatures of around 10<sup>15</sup>&nbsp;K. This was immediately (within 10<sup>−29</sup> seconds) followed by an exponential expansion of space by a scale multiplier of 10<sup>27</sup> or more, known as [[cosmic inflation]]. The early universe remained hot (above 10 000 K) for several hundred thousand years, a state that is detectable as a residual [[cosmic microwave background]], or CMB, a very low-energy radiation emanating from all parts of the sky. The "Big Bang" scenario, with cosmic inflation and standard particle physics, is the only cosmological model consistent with the observed continuing expansion of space, the observed distribution of [[Big Bang nucleosynthesis|lighter elements in the universe]] (hydrogen, helium, and lithium), and the spatial texture of minute irregularities ([[Anisotropy|anisotropies]]) in the CMB radiation. Cosmic inflation also addresses the "[[horizon problem]]" in the CMB; indeed, it seems likely that the universe is larger than the observable [[particle horizon]].<ref>{{Cite journal |last=Davis |first=Tamara M. |last2=Lineweaver |first2=Charles H. |date=January 2004 |title=Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the Universe |url=https://www.cambridge.org/core/journals/publications-of-the-astronomical-society-of-australia/article/expanding-confusion-common-misconceptions-of-cosmological-horizons-and-the-superluminal-expansion-of-the-universe/EFEEEFD8D71E59F86DDA82FDF576EFD3 |journal=Publications of the Astronomical Society of Australia |language=en |volume=21 |issue=1 |pages=97–109 |doi=10.1071/AS03040 |issn=1323-3580}}</ref>