Physical cosmology
Physical cosmology, as it is now understood, began in 1915 with the development of Albert Einstein's general theory of relativity, followed by major observational discoveries in the 1920s: first, Edwin Hubble discovered that the universe contains a huge number of external galaxies beyond the Milky Way; then, work by Vesto Slipher and others showed that the universe is expanding.These advances made it possible to speculate about the origin of the universe, and allowed the establishment of the Big Bang theory, by Georges Lemaître, as the leading cosmological model.A few researchers still advocate a handful of alternative cosmologies;[2] however, most cosmologists agree that the Big Bang theory best explains the observations.In 1916, Albert Einstein published his theory of general relativity, which provided a unified description of gravity as a geometric property of space and time.[6] However, he realized that his equations permitted the introduction of a constant term which could counteract the attractive force of gravity on the cosmic scale.In 1927, the Belgian Roman Catholic priest Georges Lemaître independently derived the Friedmann–Lemaître–Robertson–Walker equations and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval atom"[14]—which was later called the Big Bang.Hubble showed that the spiral nebulae were galaxies by determining their distances using measurements of the brightness of Cepheid variable stars.One consequence of this is that in standard general relativity, the universe began with a singularity, as demonstrated by Roger Penrose and Stephen Hawking in the 1960s.[19][20][21] In September 2023, astrophysicists questioned the overall current view of the universe, in the form of the Standard Model of Cosmology, based on the latest James Webb Space Telescope studies.Later, as the average energy per photon becomes roughly 10 eV and lower, matter dictates the rate of deceleration and the universe is said to be 'matter dominated'.[30] During the earliest moments of the universe, the average energy density was very high, making knowledge of particle physics critical to understanding this environment.Therefore, while the basic features of this epoch have been worked out in the Big Bang theory, the details are largely based on educated guesses.These problems are resolved by a brief period of cosmic inflation, which drives the universe to flatness, smooths out anisotropies and inhomogeneities to the observed level, and exponentially dilutes the monopoles.[35] Both the problems of baryogenesis and cosmic inflation are very closely related to particle physics, and their resolution might come from high energy theory and experiment, rather than through observations of the universe.[speculation?]Cosmological perturbation theory, which describes the evolution of slight inhomogeneities in the early universe, has allowed cosmologists to precisely calculate the angular power spectrum of the radiation, and it has been measured by the recent satellite experiments (COBE and WMAP)[42] and many ground and balloon-based experiments (such as Degree Angular Scale Interferometer, Cosmic Background Imager, and Boomerang).[47][48] On 17 March 2014, astronomers of the BICEP2 Collaboration announced the apparent detection of B-mode polarization of the CMB, considered to be evidence of primordial gravitational waves that are predicted by the theory of inflation to occur during the earliest phase of the Big Bang.[50][51] On 30 January 2015, a joint analysis of BICEP2 and Planck data was published and the European Space Agency announced that the signal can be entirely attributed to interstellar dust in the Milky Way.[54][55] Another tool for understanding structure formation is simulations, which cosmologists use to study the gravitational aggregation of matter in the universe, as it clusters into filaments, superclusters and voids.The gravitational effects of dark matter are well understood, as it behaves like a cold, non-radiative fluid that forms haloes around galaxies.Alternatives to the dark matter hypothesis include a modification of gravity at small accelerations (MOND) or an effect from brane cosmology.In order not to interfere with Big Bang nucleosynthesis and the cosmic microwave background, it must not cluster in haloes like baryons and dark matter.Quantum field theory predicts a cosmological constant (CC) much like dark energy, but 120 orders of magnitude larger than that observed.[63] For example, the weak anthropic principle alone does not distinguish between: Other possible explanations for dark energy include quintessence[64] or a modification of gravity on the largest scales.In the current cosmological epoch, the accelerated expansion due to dark energy is preventing structures larger than superclusters from forming.Gravitational-wave astronomy is an emerging branch of observational astronomy which aims to use gravitational waves to collect observational data about sources of detectable gravitational waves such as binary star systems composed of white dwarfs, neutron stars, and black holes; and events such as supernovae, and the formation of the early universe shortly after the Big Bang.
CosmologyBig BangUniverseAge of the universeChronology of the universeInflationNucleosynthesisGravitational wave (GWB)Microwave (CMB)Neutrino (CNB)Hubble's lawRedshiftExpansion of the universeFLRW metricFriedmann equationsInhomogeneous cosmologyFuture of an expanding universeUltimate fate of the universeLambda-CDM modelDark energyDark matterShape of the universeGalaxy filamentGalaxy formationLarge quasar groupReionizationStructure formationExperimentsBlack Hole Initiative (BHI)BOOMERanGCosmic Background Explorer (COBE)Dark Energy SurveyPlanck space observatorySloan Digital Sky Survey (SDSS)2dF Galaxy Redshift Survey ("2dF")Wilkinson Microwave AnisotropyProbe (WMAP)AaronsonAlfvénAlpherCopernicusde SitterEhlersEinsteinFriedmannGalileoHawkingHubbleHuygensKeplerLemaîtreMatherNewtonPenrosePenziasSchmidtSuntzeffSunyaevTolmanWilsonZeldovichList of cosmologistsDiscovery of cosmic microwavebackground radiationHistory of the Big Bang theoryTimeline of cosmological theoriesoriginevolutionultimate fatescienceCopernican principlecelestial bodiesphysical lawsNewtonian mechanicsAlbert Einsteingeneral theory of relativityEdwin HubblegalaxiesMilky WayVesto SlipherexpandingGeorges Lemaîtrealternative cosmologiescosmic microwave backgroundsupernovaeredshift surveysstandard model of cosmologytheoreticalapplied physicsparticle physicstheoryastrophysicsquantum mechanicsplasma physicsNature timelineAccelerated expansionSingle-celled lifePhotosynthesisMulticellularlifeVertebratesEarliest UniverseEarliest galaxyquasarblack holeOmega CentauriAndromeda GalaxyNGC 188 star clusterAlpha CentauriSolar SystemEarliest known lifeEarliest oxygenAtmospheric oxygenSexual reproductionEarliest fungiEarliest animalsCambrian explosionEarliest mammalsEarliest apeshumansbillion years agoTimeline of cosmologygeneral relativitystatic universecosmological constantexpandcosmological principleAlexander FriedmannFriedmann–Lemaître–Robertson–Walker