Editing General relativity (section)

=== Cosmology ===
{{Main|Physical cosmology}}
[[File:Lensshoe hubble.jpg|thumb|This blue horseshoe is a distant galaxy that has been magnified and warped into a nearly complete ring by the strong gravitational pull of the massive foreground luminous red galaxy.]]
The current models of cosmology are based on [[Einstein's field equations]], which include the cosmological constant <math>\Lambda</math> since it has important influence on the large-scale dynamics of the cosmos,
:<math> R_{\mu\nu} - {\textstyle 1 \over 2}R\,g_{\mu\nu} + \Lambda\ g_{\mu\nu} = \frac{8\pi G}{c^{4}}\, T_{\mu\nu} </math>
where ''<math>g_{\mu\nu}</math>'' is the spacetime metric.<ref>{{Harvnb|Einstein|1917}}; cf. {{Harvnb|Pais|1982|pp=285–288}}</ref> [[Isotropic]] and homogeneous solutions of these enhanced equations, the [[Friedmann–Lemaître–Robertson–Walker metric|Friedmann–Lemaître–Robertson–Walker solutions]],<ref>{{Harvnb|Carroll|2001|loc=ch. 2}}</ref> allow physicists to model a universe that has evolved over the past 14&nbsp;[[1000000000 (number)|billion]]&nbsp;years from a hot, early Big Bang phase.<ref>{{Harvnb|Bergström|Goobar|2003|loc=ch. 9–11}}; use of these models is justified by the fact that, at large scales of around hundred million [[light-year]]s and more, our own universe indeed appears to be isotropic and homogeneous, cf. {{Harvnb|Peebles|Schramm|Turner|Kron|1991}}</ref> Once a small number of parameters (for example the universe's mean matter density) have been fixed by astronomical observation,<ref>E.g. with [[WMAP]] data, see {{Harvnb|Spergel|Verde|Peiris|Komatsu|2003}}</ref> further observational data can be used to put the models to the test.<ref>These tests involve the separate observations detailed further on, see, e.g., fig. 2 in {{Harvnb|Bridle|Lahav|Ostriker|Steinhardt|2003}}</ref> Predictions, all successful, include the initial abundance of chemical elements formed in a period of [[Big Bang nucleosynthesis|primordial nucleosynthesis]],<ref>{{Harvnb|Peebles|1966}}; for a recent account of predictions, see {{Harvnb|Coc, Vangioni‐Flam et al.|2004}}; an accessible account can be found in {{Harvnb|Weiss|2006}}; compare with the observations in {{Harvnb|Olive|Skillman|2004}}, {{Harvnb|Bania|Rood|Balser|2002}}, {{Harvnb|O'Meara|Tytler|Kirkman|Suzuki|2001}}, and {{Harvnb|Charbonnel|Primas|2005}}</ref> the large-scale structure of the universe,<ref>{{Harvnb|Lahav|Suto|2004}}, {{Harvnb|Bertschinger|1998}}, {{Harvnb|Springel|White|Jenkins|Frenk|2005}}</ref> and the existence and properties of a "[[thermal radiation|thermal]] echo" from the early cosmos, the [[cosmic background radiation]].<ref>{{Harvnb|Alpher|Herman|1948}}, for a pedagogical introduction, see {{Harvnb|Bergström|Goobar|2003|loc=ch. 11}}; for the initial detection, see {{Harvnb|Penzias|Wilson|1965}} and, for precision measurements by satellite observatories, {{Harvnb|Mather|Cheng|Cottingham|Eplee|1994}} ([[Cosmic Background Explorer|COBE]]) and {{Harvnb|Bennett|Halpern|Hinshaw|Jarosik|2003}} (WMAP). Future measurements could also reveal evidence about gravitational waves in the early universe; this additional information is contained in the background radiation's [[polarized light|polarization]], cf. {{Harvnb|Kamionkowski|Kosowsky|Stebbins|1997}} and {{Harvnb|Seljak|Zaldarriaga|1997}}</ref>

Astronomical observations of the cosmological expansion rate allow the total amount of matter in the universe to be estimated, although the nature of that matter remains mysterious in part. About 90% of all matter appears to be dark matter, which has mass (or, equivalently, gravitational influence), but does not interact electromagnetically and, hence, cannot be observed directly.<ref>Evidence for this comes from the determination of cosmological parameters and additional observations involving the dynamics of galaxies and galaxy clusters cf. {{Harvnb|Peebles|1993|loc=ch. 18}}, evidence from gravitational lensing, cf. {{Harvnb|Peacock|1999|loc=sec. 4.6}}, and simulations of large-scale structure formation, see {{Harvnb|Springel|White|Jenkins|Frenk|2005}}</ref> There is no generally accepted description of this new kind of matter, within the framework of known [[particle physics]]<ref>{{Harvnb|Peacock|1999|loc=ch. 12}}, {{Harvnb|Peskin|2007}}; in particular, observations indicate that all but a negligible portion of that matter is not in the form of the usual [[elementary particle]]s ("non-[[baryon]]ic matter"), cf. {{Harvnb|Peacock|1999|loc=ch. 12}}</ref> or otherwise.<ref>Namely, some physicists have questioned whether or not the evidence for dark matter is, in fact, evidence for deviations from the Einsteinian (and the Newtonian) description of gravity cf. the overview in {{Harvnb|Mannheim|2006|loc=sec. 9}}</ref> Observational evidence from redshift surveys of distant supernovae and measurements of the cosmic background radiation also show that the evolution of our universe is significantly influenced by a cosmological constant resulting in an acceleration of cosmic expansion or, equivalently, by a form of energy with an unusual [[equation of state]], known as [[dark energy]], the nature of which remains unclear.<ref>{{Harvnb|Carroll|2001}}; an accessible overview is given in {{Harvnb|Caldwell|2004}}. Here, too, scientists have argued that the evidence indicates not a new form of energy, but the need for modifications in our cosmological models, cf. {{Harvnb|Mannheim|2006|loc=sec. 10}}; aforementioned modifications need not be modifications of general relativity, they could, for example, be modifications in the way we treat the inhomogeneities in the universe, cf. {{Harvnb|Buchert|2008}}</ref>

An [[cosmic inflation|inflationary phase]],<ref>A good introduction is {{Harvnb|Linde|2005}}; for a more recent review, see {{Harvnb|Linde|2006}}</ref> an additional phase of strongly accelerated expansion at cosmic times of around 10<sup>−33</sup> seconds, was hypothesized in 1980 to account for several puzzling observations that were unexplained by classical cosmological models, such as the nearly perfect homogeneity of the cosmic background radiation.<ref>More precisely, these are the [[flatness problem]], the [[horizon problem]], and the [[monopole problem]]; a pedagogical introduction can be found in {{Harvnb|Narlikar|1993|loc=sec. 6.4}}, see also {{Harvnb|Börner|1993|loc=sec. 9.1}}</ref> Recent measurements of the cosmic background radiation have resulted in the first evidence for this scenario.<ref>{{Harvnb|Spergel|Bean|Doré|Nolta|2007|loc=sec. 5,6}}</ref> However, there is a bewildering variety of possible inflationary scenarios, which cannot be restricted by current observations.<ref>More concretely, the [[potential]] function that is crucial to determining the dynamics of the [[inflaton]] is simply postulated, but not derived from an underlying physical theory</ref> An even larger question is the physics of the earliest universe, prior to the inflationary phase and close to where the classical models predict the big bang [[Gravitational singularity|singularity]]. An authoritative answer would require a complete theory of quantum gravity, which has not yet been developed<ref>{{Harvnb|Brandenberger|2008|loc=sec. 2}}</ref> (cf. the section on [[#Quantum gravity|quantum gravity]], below).