View source for Friedmann equations

{{Short description|Equations in physical cosmology}}
{{Physical cosmology |expansion}}

The '''Friedmann equations''', also known as the '''Friedmann–Lemaître''' ('''FL''') '''equations''', are a set of [[equation]]s in [[physical cosmology]] that govern [[Expansion of the universe|cosmic expansion]] in [[Homogeneity (physics)|homogeneous]] and [[Isotropy|isotropic]] models of the universe within the context of [[general relativity]]. They were first derived by [[Alexander Friedmann]] in 1922 from [[Einstein field equations|Einstein's field equations]] of [[gravitation]] for the [[Friedmann–Lemaître–Robertson–Walker metric]] and a [[perfect fluid]] with a given [[Density|mass density]] {{mvar|[[Rho (letter)|ρ]]}} and [[pressure]] {{mvar|p}}.<ref name="af1922">{{cite journal |first=A |last=Friedman |author-link=Alexander Alexandrovich Friedman |title=Über die Krümmung des Raumes|url=https://web.phys.ntnu.no/~mika/friedmann1.pdf|journal=[[Z. Phys.]] |volume=10 |date=29 May 1922 |issue=1 |pages=377–386|location=[[Saint Petersburg|Petrograd]]|publisher=[[Springer-Verlag GmbH & Co. KG]] |doi=10.1007/BF01332580 |bibcode = 1922ZPhy...10..377F|s2cid=125190902 |language=de|via=Michael Kachelrieß: [[Norges Teknisk-Naturvitenskapelige Universitet]]}} (English translation: {{cite journal |first=A |last=Friedman |translator1=F.R. Ellis|translator2=H. van Elst|title=On the Curvature of Space |journal=[[General Relativity and Gravitation]] |volume=31 |issue=12 |year=1999 |pages= 1991–2000 |publisher=[[Springer-Verlag GmbH & Co. KG]] |bibcode=1999GReGr..31.1991F |doi=10.1023/A:1026751225741|s2cid=122950995 }}). The original Russian manuscript of this paper is preserved in the [http://ilorentz.org/history/Friedmann_archive Ehrenfest archive].</ref> The equations for negative spatial curvature were given by Friedmann in 1924.<ref name="af1924">{{cite journal |first=A |last=Friedmann |author-link=Alexander Alexandrovich Friedman |title=Über die Möglichkeit einer Welt mit konstanter negativer Krümmung des Raumes |journal=Z. Phys. |volume=21 |year=1924 |issue=1 |pages=326–332 |doi=10.1007/BF01328280 |bibcode=1924ZPhy...21..326F|s2cid=120551579 |language=de}}  (English translation: {{cite journal |first=A |last=Friedmann |title=On the Possibility of a World with Constant Negative Curvature of Space |journal=General Relativity and Gravitation |volume=31 |issue=12 |year=1999 |pages=2001–2008 |bibcode=1999GReGr..31.2001F |doi=10.1023/A:1026755309811|s2cid=123512351 }})</ref>
The physical models built on the Friedmann equations are called FRW or FLRW models and form the ''Standard Model'' of modern [[Physical cosmology|cosmology]], although such a description is also associated with the further developed [[Lambda-CDM model]]. The FLRW model was developed independently by the named authors in the 1920s and 1930s.

== Assumptions ==
{{main|Friedmann–Lemaître–Robertson–Walker metric}}

The Friedmann equations build on three assumptions:<ref name=PDG-2024/>{{rp|22.1.3}} 
# the [[Friedmann–Lemaître–Robertson–Walker metric]],
# [[Einstein field equations|Einstein's equations]] for [[general relativity]], and
# a [[perfect fluid]] source.
The metric in turn starts with the simplifying assumption that the universe is spatially homogeneous and [[Isotropic manifold|isotropic]], that is, the [[cosmological principle]]; empirically, this is justified on scales larger than the order of 100 [[Parsec|Mpc]].

The metric can be written as:<ref name=Peacock-1998>{{Cite book |last=Peacock |first=J. A. |url=https://www.cambridge.org/core/product/identifier/9780511804533/type/book |title=Cosmological Physics |date=1998-12-28 |publisher=Cambridge University Press |isbn=978-0-521-41072-4 |edition=1 |doi=10.1017/cbo9780511804533}}</ref>{{rp|65}}
<math display="block">c^2d\tau^2 = c^2dt^2 - R^2(t) \left(dr^2 + S^2_k(r) d\psi^2\right)</math>
where 
<math display="block">S_{-1}(r) = \sinh(r), S_0 = 1, S_1 = \sin(r).</math>
These three possibilities correspond to parameter {{mvar|k}} of '''(0)''' flat space, '''(+1)''' a sphere of constant positive curvature or '''(−1)''' a hyperbolic space with constant negative curvature.
Here the radial position has been decomposed into a time-dependent scale factor, <math>R(t)</math>, and a comoving coordinate, <math>r</math>. 
Inserting this metric into Einstein's field equations relate the evolution of this scale factor to the pressure and energy of the matter in the universe. With the [[stress–energy tensor]] for a perfect fluid,  results in the equations are described below.<ref name=Peacock-1998/>{{rp|p=73}}

== Equations ==
{{General relativity sidebar |equations}}

There are two independent Friedmann equations for modelling a homogeneous, isotropic universe.
The first is:<ref name="PDG-2024">{{Cite journal |last=Navas, S. |year=2024 |title=Review of Particle Physics |journal=[[Physical Review D]]  |volume=110 |issue=3 |pages=1–708 |doi=10.1103/PhysRevD.110.030001 |collaboration=[[Particle Data Group]]|hdl=20.500.11850/695340 |hdl-access=free }} 22.1.3 The Friedmann equations of motion</ref> 
<math display="block"> H^2\equiv {\left(\frac{\dot{R}}{R}\right)}^2  = \frac{8 \pi G \rho}{3} - \frac{k}{R^2} + \frac{\Lambda}{3},</math> and second is:
<math display="block">\frac{\ddot{R}}{R} = \frac{\Lambda}{3} -\frac{4 \pi G}{3}\left(\rho+3p\right).  </math>
The term ''Friedmann equation'' sometimes is used only for the first equation.<ref name=PDG-2024/>
In these equations,
{{mvar|H}} is the Hubble parameter, {{mvar|R(t)}} is the [[scale factor (universe)|cosmological scale factor]], <math>G</math> is the [[Newtonian constant of gravitation]],  {{math|Λ}} is the [[cosmological constant]] with dimension length<sup>&minus;2</sup>, {{mvar|ρ}}&nbsp;is the energy density and {{mvar|p}} is the isotropic pressure. {{mvar|k}} is constant throughout a particular solution, but may vary from one solution to another. The units set the [[speed of light|speed of light in vacuum]] to one.

In previous equations, {{mvar|R}}, {{mvar|ρ}}, and {{mvar|p}} are functions of time. If the cosmological constant, {{math|Λ}}, is ignored, the term <math>-k/R^2</math> in the first Friedmann equation can be interpreted as a Newtonian total energy, so the evolution of the universe pits gravitational potential energy, <math>8\pi G\rho/3</math> against kinetic energy, <math>\dot{R}/R</math>. The winner depends upon the {{mvar|k}} value in the total energy: if k is +1, gravity eventually causes the universe to contract. These conclusions will be altered if the {{math|Λ}} is not zero.<ref name=PDG-2024/>

Using the first equation, the second equation can be re-expressed as:<ref name=PDG-2024/>
<math display="block">\dot{\rho} = -3 H \left(\rho + \frac{p}{c^2}\right),</math>
which eliminates {{math|Λ}}. Alternatively the conservation of [[mass–energy]]:
<math display="block"> T^{\alpha\beta}{}_{;\beta}= 0</math>
leads to the same result.<ref name=PDG-2024/>

=== Spatial curvature ===

The first Friedmann equation contains the discrete parameter {{math|1=''k''}}, the value of which determines the [[shape of the universe]]: 
*{{math|+1}} is a [[3-sphere]],<ref name=RDI2008>{{Cite book |last=D'Inverno |first=Ray |title=Introducing Einstein's relativity |date=2008 |publisher=Clarendon Press |isbn=978-0-19-859686-8 |edition=Repr |location=Oxford}}</ref> the universe is "closed": starting off on some paths through the universe return to the starting point - analogous to a sphere: finite but unbounded.<ref name=Peacock3:3.1>{{Cite book|author=John. A. Peacock|author-link=John A. Peacock|chapter=3. THE ISOTROPIC UNIVERSE 3.1 The Robertson-Walker Metric |chapter-url=https://ned.ipac.caltech.edu/level5/Peacock/Peacock3_1.html|title=Cosmological Physics |date=1999|publisher=[[Cambridge University Press]]: [[NASA]]/[[Infrared Processing and Analysis Center|IPAC]] Extragalactic Database - [[California Institute of Technology]] }} {{web archive|url=https://web.archive.org/web/20250908125511/https://ned.ipac.caltech.edu/level5/Peacock/Peacock3_1.html|date=8 September 2025}}</ref>
* {{math|0}} is [[Euclidean space|flat Euclidean space]]<ref name=RDI2008/> and infinite.<ref name=Peacock3:3.1/>
* {{math|−1}} is a 3-[[hyperboloid]]<ref name=RDI2008/> the universe is "open": infinite and no paths return.<ref name=Peacock3:3.1/>

In the Friedmann model the choice between these different shapes is controlled by density, with the critical density, <math>\rho_c</math>, for <math>k=0</math> given by:
<math display="block">\rho_c = \frac{3H^2}{8\pi G}</math>
where <math>H</math> is the Hubble parameter and <math>G</math> is the gravitational constant. Higher density will give a closed universe and lower density will give an open one.<ref name=Peacock-1998/>{{rp|73}}

== Dimensionless scale factor ==
A dimensionless scale factor can be defined:
<math display="block">a(t) \equiv \frac{R(t)}{R_0}</math>
using the present day value
<math display="block">R_0 = R(\text{now}).</math>
The Friedmann equations can be written in terms of this dimensionless scale factor:
<math display="block">H^2(t) = \left( \frac{\dot{a}}{a}\right)^2 = \frac{8\pi G}{3}\left[ \rho(t) + \frac{\rho_c - \rho_0}{a^2(t)}\right]</math>
where <math>\dot{a} = da/dt</math>, <math>\rho_c = 3H^2_0/8\pi G</math>, and <math>\rho_0 = \rho(t=\text{now})</math>.<ref>{{Cite book |last=Dodelson |first=Scott |title=Modern cosmology |date=2003 |publisher=Academic Press |isbn=978-0-12-219141-1 |location=San Diego, Calif}}</ref>{{rp|3}}

== Critical density<span class="anchor" id="Critical density"></span> ==
That value of the mass-energy density, <math>\rho</math> that gives <math>k=0</math> when <math>\Lambda=0</math> is called the '''critical density''': 
<math display="block">\rho_c \equiv \frac{3H^2}{8\pi G}. </math>
If the universe has higher density, <math>\rho \ge \rho_c</math>, then it is called "spatially closed": in this simple approximation the universe would eventually contract. On the other hand, if has lower density, <math>\rho \le \rho_c</math>, then it is called "spatially open" and expands forever. Therefore the geometry of the universe is directly connected to its density.<ref name=Peacock-1998/>{{rp|p=73}}

== Density parameter ==<!--[[Density parameter]] links here-->

The '''density parameter'''<!--boldface per WP:R#PLA--> {{mvar|Ω}} is defined as the ratio of the actual (or observed) density {{mvar|ρ}} to the critical density {{math|''ρ''<sub>c</sub>}} of the Friedmann universe:<ref name=Peacock-1998/>{{rp|p=74}}
<math display="block">\Omega := \frac{\rho}{\rho_{c}} = \frac{8 \pi G\rho}{3 H^2}.</math>
Both the density <math>\rho(t)</math> and the Hubble parameter <math>H(t)</math> depend upon time and thus the density parameter varies with time.<ref name=Peacock-1998/>{{rp|p=74}}

The critical density is equivalent to approximately five atoms (of [[monatomic]] [[hydrogen]]) per cubic metre, whereas the average density of [[Baryons#Baryonic matter|ordinary matter]] in the Universe is believed to be 0.2–0.25 atoms per cubic metre.<ref>{{Cite book |last=Rees |first=Martin |title=Just six numbers: the deep forces that shape the universe |date=2001 |publisher=Basic Books |isbn=978-0-465-03673-8 |edition=Repr. |series=Astronomy/science |location=New York, NY}}</ref><ref>{{cite web | publisher=[[NASA]] | title=Universe 101 | url=http://map.gsfc.nasa.gov/universe/uni_matter.html | access-date=September 9, 2015 | quote=The actual density of atoms is equivalent to roughly 1 proton per 4 cubic meters.}}</ref>
[[File:UniverseComposition.svg|thumb|right|375px|Estimated relative distribution for components of the energy density of the universe. [[Dark energy]] dominates the total energy (74%) while [[dark matter]] (22%) constitutes most of the mass.  Of the remaining baryonic matter (4%), only one tenth is compact. In February 2015, the European-led research team behind the [[Planck (spacecraft)|Planck cosmology probe]] released new data refining these values to 4.9% ordinary matter, 25.9% dark matter and 69.1% dark energy.]]
A much greater density comes from the unidentified [[dark matter]], although both ordinary and dark matter contribute in favour of contraction of the universe. However, the largest part comes from so-called [[dark energy]], which accounts for the cosmological constant term. Although the total density is equal to the critical density (exactly, up to measurement error), dark energy does not lead to contraction of the universe but rather may accelerate its expansion.

An expression for the critical density is found by assuming {{mvar|Λ}} to be zero (as it is for all basic Friedmann universes) and setting the normalised spatial curvature, {{mvar|k}}, equal to zero. When the substitutions are applied to the first of the Friedmann equations given the new <math>H_0</math> value we find:<ref>{{Cite journal |last1=Scolnic |first1=Daniel |author1-link=Daniel M. Scolnic|last2=Riess |first2=Adam G. |last3=Murakami |first3=Yukei S. |last4=Peterson |first4=Erik R. |last5=Brout |first5=Dillon |last6=Acevedo |first6=Maria |last7=Carreres |first7=Bastien |last8=Jones |first8=David O. |last9=Said |first9=Khaled |last10=Howlett |first10=Cullan |last11=Anand |first11=Gagandeep S. |date=2025-01-15 |title=The Hubble Tension in Our Own Backyard: DESI and the Nearness of the Coma Cluster |journal=The Astrophysical Journal Letters |volume=979 |issue=1 |pages=L9 |doi=10.3847/2041-8213/ada0bd |doi-access=free |bibcode=2025ApJ...979L...9S |issn=2041-8205|arxiv=2409.14546 }}</ref>
<math display="block">\begin{align}
\rho = \frac{3 H_0^2}{8 \pi G} 
&\approx 1.10 \times 10^{-26} \mathrm{kg \, m^{-3}}
\\&\approx 1.88 \times 10^{-26} h^2\, {\rm kg}\, {\rm m}^{-3}
\\&\approx 2.78 \times 10^{11} h^2 M_\odot\,{\rm Mpc}^{-3}
\end{align}</math>
where:
* <math display="inline" id="https://iopscience.iop.org/article/10.3847/2041-8213/ada0bd">H_0 = 76.5 \pm 2.2 \, \mathrm{km \, s^{-1} \, Mpc^{-1}} 
\approx 2.48 \times 10^{-18} \mathrm{s^{-1}}</math>
* <math display="inline" id="https://iopscience.iop.org/article/10.3847/2041-8213/ada0bd">h = \frac{H_0}{100 \, \mathrm{(km/s)/Mpc}}</math>
* <math>\rho_c = 8.5 \times 10^{-27} \mathrm{kg / m^3} </math>

Given the value of dark energy to be <math>\Omega_\Lambda = 0.647</math>
This term originally was used as a means to determine the [[shape of the universe|spatial geometry]] of the universe, where {{math|''ρ''<sub>c</sub>}} is the critical density for which the spatial geometry is flat (or Euclidean). Assuming a zero vacuum energy density, if {{mvar|Ω}} is larger than unity, the space sections of the universe are closed; the universe will eventually stop expanding, then collapse. If {{mvar|Ω}} is less than unity, they are open; and the universe expands forever. However, one can also subsume the spatial curvature and vacuum energy terms into a more general expression for {{mvar|Ω}} in which case this density parameter equals exactly unity. Then it is a matter of measuring the different components, usually designated by subscripts. According to the [[Lambda-CDM model|ΛCDM model]], there are important components of {{mvar|Ω}} due to [[baryon]]s, [[cold dark matter]] and [[dark energy]]. The spatial geometry of the [[universe]] has been measured by the [[Wilkinson Microwave Anisotropy Probe|WMAP]] spacecraft to be nearly flat. This means that the universe can be well approximated by a model where the spatial curvature parameter {{mvar|k}} is zero; however, this does not necessarily imply that the universe is infinite: it might merely be that the universe is much larger than the part we see.

The first Friedmann equation is often seen in terms of the present values of the density parameters, that is<ref>{{cite journal | last=Nemiroff | first=Robert J. | author-link=Robert J. Nemiroff | author2=Patla, Bijunath |arxiv = astro-ph/0703739| doi = 10.1119/1.2830536 | volume=76 | title=Adventures in Friedmann cosmology: A detailed expansion of the cosmological Friedmann equations | journal=American Journal of Physics | year=2008 | issue=3 | pages=265–276 | bibcode = 2008AmJPh..76..265N| s2cid=51782808 }}</ref>
<math display="block">\frac{H^2}{H_0^2} = \Omega_{0,\mathrm R} a^{-4} + \Omega_{0,\mathrm M} a^{-3} + \Omega_{0,k} a^{-2} + \Omega_{0,\Lambda}.</math>
Here {{math|Ω<sub>0,R</sub>}} is the radiation density today (when {{math|1=''a'' = 1}}), {{math|Ω<sub>0,M</sub>}} is the matter ([[dark matter|dark]] plus [[baryon]]ic) density today, {{math|1=Ω<sub>0,''k''</sub> = 1 − Ω<sub>0</sub>}} is the "spatial curvature density" today, and {{math|Ω<sub>0,Λ</sub>}} is the cosmological constant or vacuum density today.

=== Other forms ===
{{Unreferenced section|date=September 2024}}
The Hubble parameter can change over time if other parts of the equation are time dependent (in particular the mass density, the vacuum energy, or the spatial curvature). Evaluating the Hubble parameter at the present time yields Hubble's constant which is the proportionality constant of [[Hubble's law]]. Applied to a fluid with a given [[equation of state (cosmology)|equation of state]], the Friedmann equations yield the time evolution and geometry of the universe as a function of the fluid density.

== FLRW models ==

Relativisitic cosmology models based on the FLRW metric and obeying the Friedmann equations are called '''FRW models'''.<ref name=Peacock-1998/>{{rp|p=73}}
Direct observation of stars has shown their velocities to be dominated by radial recession, validating these assumptions for cosmological models.<ref name=Peacock-1998/>{{rp|p=65}}
These models are the basis of the standard model<ref name="Goobar">{{Cite book |last1=Bergström |first1=Lars |url=https://books.google.com/books?id=CQYu_sutWAoC&pg=PA61 |title=Cosmology and particle astrophysics |last2=Goobar |first2=Ariel |date=2008 |publisher=Praxis Publ |isbn=978-3-540-32924-4 |edition=2. ed., reprinted |series=Springer Praxis books in astronomy and planetary science |location=Chichester, UK |page=61}}</ref> of [[Big Bang]] cosmological including the current [[Lambda-CDM model|ΛCDM]] model.<ref name=PDG-2024/>{{rp|loc=25.1.3}}

To apply the metric to cosmology and predict its time evolution via the scale factor <math>a(t)</math> requires Einstein's field equations together with a way of calculating the density, <math>\rho (t),</math> such as a [[equation of state (cosmology)|cosmological equation of state]].
This process allows an approximate analytic solution [[Einstein field equations|Einstein's field equations]] <math>G_{\mu\nu} + \Lambda g_{\mu\nu} = \kappa T_{\mu\nu}</math> giving the Friedmann equations when the [[energy–momentum tensor]] is similarly assumed to be isotropic and homogeneous. The resulting equations are:<ref>{{Cite journal |last1=Rosu |first1=H. C. |last2=Ojeda-May |first2=P. |date=June 2006 |title=Supersymmetry of FRW Barotropic Cosmologies |journal=International Journal of Theoretical Physics |language=en |volume=45 |issue=6 |pages=1152–1157 |arxiv=gr-qc/0510004 |bibcode=2006IJTP...45.1152R |doi=10.1007/s10773-006-9123-2 |issn=0020-7748 |s2cid=119496918}}</ref>
<math display="block">\begin{align}
{\left(\frac{\dot a}{a}\right)}^2 + \frac{kc^2}{a^2} - \frac{\Lambda c^2}{3} &= \frac{\kappa c^4}{3}\rho \\[4pt]
2\frac{\ddot a}{a} + {\left(\frac{\dot a}{a}\right)}^2 + \frac{kc^2}{a^2} - \Lambda c^2 &= -\kappa c^2 p .
\end{align}</math>

Because the FLRW model assumes homogeneity, some popular accounts mistakenly assert that the Big Bang model cannot account for the observed lumpiness of the universe. In a strictly FLRW model, there are no clusters of galaxies or stars, since these are objects much denser than a typical part of the universe. Nonetheless, the FLRW model is used as a first approximation for the evolution of the real, lumpy universe because it is simple to calculate, and models that calculate the lumpiness in the universe are added onto the FLRW models as extensions. Most cosmologists agree that the [[observable universe]] is well approximated by an ''almost FLRW model'', i.e., a model that follows the FLRW metric apart from [[primordial fluctuations|primordial density fluctuations]]. {{As of|2003}}, the theoretical implications of the various extensions to the FLRW model appear to be well understood, and the goal is to make these consistent with observations from [[Cosmic Background Explorer|COBE]] and [[WMAP]].

=== Interpretation ===
The pair of equations given above is equivalent to the following pair of equations
<math display="block">\begin{align}
\dot{\rho} &= - 3 \frac{\dot a}{a} \left(\rho+\frac{p}{c^2}\right) \\[1ex]
\frac{\ddot{a}}{a} &= - \frac{\kappa c^4}{6} \left(\rho + \frac{3p}{c^2}\right) + \frac{\Lambda c^2}{3}
\end{align}</math>
with <math>k</math>, the spatial curvature index, serving as a [[constant of integration]] for the first equation.

The first equation can be derived also from thermodynamical considerations and is equivalent to the [[first law of thermodynamics]], assuming the expansion of the universe is an [[adiabatic process]] (which is implicitly assumed in the derivation of the Friedmann–Lemaître–Robertson–Walker metric).

The second equation states that both the energy density and the pressure cause the expansion rate of the universe <math>{\dot a}</math> to decrease, i.e., both cause a deceleration in the expansion of the universe. This is a consequence of [[gravitation]], with pressure playing a similar role to that of energy (or mass) density, according to the principles of [[general relativity]]. The [[cosmological constant]], on the other hand, [[Dark energy|causes an acceleration in the expansion]] of the universe.

=== Cosmological constant ===
The [[cosmological constant]] term can be omitted if we make the following replacements
<math display="block">\begin{align}
\rho &\to \rho - \frac{\Lambda}{\kappa c^2}, &
p &\to p + \frac{\Lambda}{\kappa}.
\end{align}</math>

Therefore, the [[cosmological constant]] can be interpreted as arising from a form of energy that has negative pressure, equal in magnitude to its (positive) mass-energy density:
<math display="block">p = - \rho c^2 \,,</math>
which is an equation of state of vacuum with [[dark energy]].

An attempt to generalize this to
<math display="block">p =  w \rho c^2</math>
would not have [[General covariance|general invariance]] without further modification.

In fact, in order to get a term that causes an acceleration of the universe expansion, it is enough to have a [[scalar field theory|scalar field]] that satisfies
<math display="block">p < - \frac {\rho c^2} {3} .</math>
Such a field is sometimes called [[Quintessence (physics)|quintessence]].

===Dust models===
Setting the pressure of the perfect fluid in the Friedmann equations to zero (<math>p=0</math>) gives a cosmological ''dust model''.<ref name=Longair-2023/>{{rp|231}}

=== Newtonian analog ===
In 1934 McCrea and Milne<ref>{{cite journal |last1=McCrea |first1=W. H. |last2=Milne |first2=E. A. |year=1934 |title=Newtonian universes and the curvature of space |journal=Quarterly Journal of Mathematics |volume=5 |pages=73–80 |doi=10.1093/qmath/os-5.1.73 |bibcode=1934QJMat...5...73M }}</ref> showed that the Friedmann equations in the case of a pressureless fluid can be derived with non-relativistic Newtonian dynamics.<ref name=Longair-2023>{{Cite book |last=Longair |first=Malcolm S. |url=https://link.springer.com/10.1007/978-3-662-65891-8 |title=Galaxy Formation |date=2023 |publisher=Springer Berlin Heidelberg |isbn=978-3-662-65890-1 |series=Astronomy and Astrophysics Library |location=Berlin, Heidelberg |language=en |doi=10.1007/978-3-662-65891-8}}</ref>{{rp|231}}
<math display="block">\begin{align}
 - a^3 \dot{\rho} = 3 a^2 \dot{a} \rho + \frac{3 a^2 p \dot{a}}{c^2} \, \\[1ex]
 \frac{\dot{a}^2}{2} - \frac{\kappa c^4 a^3 \rho}{6a} = - \frac{k c^2}{2} \,.
\end{align}</math>

The first equation says that the decrease in the mass contained in a fixed cube (whose side is momentarily ''a'') is the amount that leaves through the sides due to the expansion of the universe plus the mass equivalent of the work done by pressure against the material being expelled. This is the conservation of mass–energy ([[first law of thermodynamics]]) contained within a part of the universe.

The second equation says that the kinetic energy (seen from the origin) of a particle of unit mass moving with the expansion plus its (negative) [[gravitational energy|gravitational potential energy]] (relative to the mass contained in the sphere of matter closer to the origin) is equal to a constant related to the curvature of the universe. In other words, the energy (relative to the origin) of a co-moving particle in free-fall is conserved. General relativity merely adds a connection between the spatial curvature of the universe and the energy of such a particle: positive total energy implies negative curvature and negative total energy implies positive curvature.

== Useful solutions ==

The Friedmann equations can be solved exactly in presence of a [[perfect fluid]] with equation of state
<math display="block">p = w \rho c^2,</math>
where {{mvar|p}} is the [[pressure]], {{mvar|ρ}} is the mass density of the fluid in the comoving frame and {{mvar|w}} is some constant.

In spatially flat case ({{math|1=''k'' = 0}}), the solution for the scale factor is
<math display="block"> a(t) = a_0 \, t^{\frac{2}{3(w+1)}} </math>
where {{math|''a''<sub>0</sub>}} is some integration constant to be fixed by the choice of initial conditions. This family of solutions labelled by {{mvar|w}} is extremely important for cosmology. For example, {{math|1=''w'' = 0}} describes a [[matter-dominated era|matter-dominated]] universe, where the pressure is negligible with respect to the mass density. From the generic solution one easily sees that in a matter-dominated universe the scale factor goes as
<math display="block">a(t) \propto t^{2/3} \qquad \text{matter-dominated}</math>

Another important example is the case of a [[radiation-dominated era|radiation-dominated]] universe, namely when {{math|1=''w'' = {{sfrac|1|3}}}}. This leads to
<math display="block">a(t) \propto t^{1/2} \qquad \text{radiation-dominated}</math>

Note that this solution is not valid for domination of the cosmological constant, which corresponds to an {{math|1=''w'' = −1}}. In this case the energy density is constant and the scale factor grows exponentially.

Solutions for other values of {{mvar|k}} can be found at {{cite web | last=Tersic | first=Balsa | title=Lecture Notes on Astrophysics | url=https://www.academia.edu/5025956|access-date=24 February 2022}}

===Mixtures===
If the matter is a mixture of two or more non-interacting fluids each with such an equation of state, then
<math display="block">\dot{\rho}_{f} = -3 H \left( \rho_{f} + \frac{p_{f}}{c^2} \right) </math>
holds separately for each such fluid {{mvar|f}}. In each case,
<math display="block">\dot{\rho}_{f} = -3 H \left( \rho_{f} + w_{f} \rho_{f} \right) \,</math>
from which we get
<math display="block">{\rho}_{f} \propto a^{-3 \left(1 + w_{f}\right)} \,.</math>

For example, one can form a linear combination of such terms
<math display="block">\rho = A a^{-3} + B a^{-4} + C a^0 \,</math>
where {{mvar|A}} is the density of "dust" (ordinary matter, {{math|1=''w'' = 0}}) when {{math|1=''a'' = 1}}; {{mvar|B}} is the density of radiation ({{math|1=''w'' = {{sfrac|1|3}}}}) when {{math|1=''a'' = 1}}; and {{mvar|C}} is the density of "dark energy" ({{math|1=''w'' = −1}}). One then substitutes this into
<math display="block">\left(\frac{\dot{a}}{a}\right)^2 = \frac{8 \pi G}{3} \rho - \frac{kc^2}{a^2} </math>
and solves for {{mvar|a}} as a function of time.

== History ==
[[File:Aleksandr Fridman.png|thumb|236px|[[Alexander Friedmann]]]]
Friedmann published two cosmology papers in the 1922-1923 time frame. He adopted the same homogeneity and isotropy assumptions used by [[Albert Einstein]] and by [[Willem de Sitter]] in their papers, both published in 1917. Both of the earlier works also assumed the universe was static, eternally unchanging. Einstein postulated an additional term to his equations of general relativity to ensure this stability. In his paper, de Sitter showed that spacetime had curvature even in the absence of matter: the new equations of general relativity implied that a vacuum had properties that altered spacetime.<ref name=Tropp-1993>{{Cite book |last1=Tropp |first1=Eduard A. |url=https://www.cambridge.org/core/product/identifier/9780511608131/type/book |title=Alexander A Friedmann: The Man who Made the Universe Expand |last2=Frenkel |first2=Viktor Ya. |last3=Chernin |first3=Artur D. |date=1993-06-03 |publisher=Cambridge University Press |isbn=978-0-521-38470-4 |edition=1 |translator-last=Dron |translator-first=Alexander |doi=10.1017/cbo9780511608131 |translator-last2=Burov |translator-first2=Michael}}</ref>{{rp|152}}

The idea of static universe was a fundamental assumption of philosophy and science. However, Friedmann abandoned the idea in his first paper "On the curvature of space". Starting with Einstein's 10 equations of relativity, Friedmann applies the symmetry of an isotropic universe and a simple model for mass-energy density to derive a relationship between that density and the curvature of spacetime. He demonstrates that in addition to one solution is static, many time dependent solutions also exist.<ref name=Tropp-1993/>{{rp|p=157}}

Friedmann's second paper, "On the possibility of a world with constant negative curvature," published in 1924 explored more complex geometrical ideas. This paper establish the idea that that the finiteness of spacetime was not a property that could be established based on the equations of general relativity alone: both finite and infinite geometries could be used to give solutions. Friedmann used two concepts of a three dimensional sphere as analogy: a trip at constant latitude could return to the starting point or the sphere might have an infinite number of sheets and the trip never repeats.<ref name=Tropp-1993/>{{rp|p=167}}

Friedmann's paper were largely ignored except – initially – by Einstein who actively dismissed them. However once [[Edwin Hubble]] published astronomical [[Hubble's law|evidence that the universe was expanding]], Einstein became convinced. Unfortunately for Friedmann, [[Georges Lemaître]] discovered some aspects of the same solutions and wrote persuasively about the concept of a universe born from a "primordial atom". Thus historians give these two scientists equal billing for the discovery.<ref>{{Cite journal |last=Belenkiy |first=Ari |date=2012-10-01 |title=Alexander Friedmann and the origins of modern cosmology |url=https://pubs.aip.org/physicstoday/article/65/10/38/413748/Alexander-Friedmann-and-the-origins-of-modern |journal=Physics Today |language=en |volume=65 |issue=10 |pages=38–43 |doi=10.1063/PT.3.1750 |bibcode=2012PhT....65j..38B |issn=0031-9228|url-access=subscription }}</ref>

== In popular culture ==

Several students at [[Tsinghua University]] ([[Chinese Communist Party|CCP]] [[Leader of the Chinese Communist Party|leader]] [[Xi Jinping]]'s [[alma mater]]) participating in the [[2022 COVID-19 protests in China]] carried placards with Friedmann equations scrawled on them, interpreted by some as a play on the words "Free man". Others have interpreted the use of the equations as a call to “open up” China and stop its Zero Covid policy, as the Friedmann equations relate to the expansion, or “opening” of the universe.<ref>{{Cite news |last=Murphy |first=Matt |date=November 28, 2022 |title=China's protests: Blank paper becomes the symbol of rare demonstrations |url=https://www.bbc.com/news/world-asia-china-63778871 |work=[[BBC News]]}}</ref>

== See also ==
* [[Mathematics of general relativity]]
* [[Solutions of the Einstein field equations]]

== Sources ==
{{reflist}}

== Further reading ==
* {{cite book |first=Dierck-Ekkehard |last=Liebscher |chapter=Expansion |title=Cosmology |location=Berlin |publisher=Springer |year=2005 |isbn=3-540-23261-3 |pages=53–77 |chapter-url=https://books.google.com/books?id=VK_rbBR61eUC&pg=PA53 }}

{{Relativity}}
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[[Category:General relativity]]