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Physical cosmology is a branch ofcosmology concerned with the study of cosmological models. Acosmological model, or simplycosmology, provides a description of the largest-scale structures and dynamics of theuniverse and allows study of fundamental questions about itsorigin, structure,evolution, andultimate fate.[1] Cosmology as ascience originated with theCopernican principle, which implies thatcelestial bodies obey identicalphysical laws to those on Earth, andNewtonian mechanics, which first allowed those physical laws to be understood.
Physical cosmology, as it is now understood, began in 1915 with the development ofAlbert Einstein'sgeneral theory of relativity, followed by major observational discoveries in the 1920s: first,Edwin Hubble discovered that the universe contains a huge number of externalgalaxies beyond theMilky Way; then, work byVesto Slipher and others showed that the universe isexpanding. These advances made it possible to speculate about the origin of the universe, and allowed the establishment of theBig Bang theory, byGeorges Lemaître, as the leading cosmological model. A few researchers still advocate a handful ofalternative cosmologies;[2] however, most cosmologists agree that the Big Bang theory best explains the observations.
Dramatic advances in observational cosmology since the 1990s, including thecosmic microwave background, distantsupernovae and galaxyredshift surveys, have led to the development of astandard model of cosmology. This model requires the universe to contain large amounts ofdark matter anddark energy whose nature is currently not well understood, but the model gives detailed predictions that are in excellent agreement with many diverse observations.[3]
Cosmology draws heavily on the work of many disparate areas of research intheoretical andapplied physics. Areas relevant to cosmology includeparticle physics experiments andtheory, theoretical and observationalastrophysics, general relativity,quantum mechanics, andplasma physics.
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Modern cosmology developed along tandem tracks of theory and observation. In 1916, Albert Einstein published his theory ofgeneral relativity, which provided a unified description of gravity as a geometric property of space and time.[4] At the time, Einstein believed in astatic universe, but found that his original formulation of the theory did not permit it.[5] This is because masses distributed throughout the universe gravitationally attract, and move toward each other over 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. Einstein published his first paper on relativistic cosmology in 1917, in which he added thiscosmological constant to his field equations in order to force them to model a static universe.[7] The Einstein model describes a static universe; space is finite and unbounded (analogous to the surface of a sphere, which has a finite area but no edges). However, this so-called Einstein model is unstable to small perturbations—it will eventually start toexpand or contract.[5] It was later realized that Einstein's model was just one of a larger set of possibilities, all of which were consistent with general relativity and thecosmological principle. The cosmological solutions of general relativity were found byAlexander Friedmann in the early 1920s.[8] His equations describe theFriedmann–Lemaître–Robertson–Walker universe, which may expand or contract, and whose geometry may be open, flat, or closed.
In the 1910s,Vesto Slipher (and laterCarl Wilhelm Wirtz) interpreted thered shift ofspiral nebulae as aDoppler shift that indicated they were receding from Earth.[12][13] However, it is difficult to determine the distance to astronomical objects. One way is to compare the physical size of an object to itsangular size, but a physical size must be assumed in order to do this. Another method is to measure thebrightness of an object and assume an intrinsicluminosity, from which the distance may be determined using theinverse-square law. Due to the difficulty of using these methods, they did not realize that the nebulae were actually galaxies outside our ownMilky Way, nor did they speculate about the cosmological implications. In 1927, theBelgianRoman CatholicpriestGeorges 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 "primevalatom"[14]—which was later called the Big Bang. In 1929,Edwin Hubble provided an observational basis for Lemaître's theory. Hubble showed that the spiral nebulae were galaxies by determining their distances using measurements of the brightness ofCepheid variable stars. He discovered a relationship between the redshift of a galaxy and its distance. He interpreted this as evidence that the galaxies are receding from Earth in every direction at speeds proportional to their distance from Earth.[15] This fact is now known asHubble's law, though the numerical factor Hubble found relating recessional velocity and distance was off by a factor of ten, due to not knowing about the types of Cepheid variables.
Given the cosmological principle, Hubble's law suggested that the universe was expanding. Two primary explanations were proposed for the expansion. One was Lemaître's Big Bang theory, advocated and developed byGeorge Gamow. The other explanation wasFred Hoyle'ssteady state model in which new matter is created as the galaxies move away from each other. In this model, the universe is roughly the same at any point in time.[16][17]
For a number of years, support for these theories was evenly divided. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. The discovery of the cosmic microwave background in 1965 lent strong support to the Big Bang model,[17] and since the precise measurements of the cosmic microwave background by theCosmic Background Explorer in the early 1990s, few cosmologists have seriously proposed other theories of the origin and evolution of the cosmos. One consequence of this is that in standard general relativity, the universe began with asingularity, as demonstrated byRoger Penrose andStephen Hawking in the 1960s.[18]
An alternative view to extend the Big Bang model, suggesting the universe had no beginning or singularity and the age of the universe is infinite, has been presented.[19][20][21]
In September 2023, astrophysicists questioned the overall current view of theuniverse, in the form of theStandard Model of Cosmology, based on the latestJames Webb Space Telescope studies.[22]
The lightestchemical elements, primarilyhydrogen andhelium, were created during the Big Bang through the process ofnucleosynthesis.[23] In a sequence ofstellar nucleosynthesis reactions, smaller atomic nuclei are then combined into larger atomic nuclei, ultimately forming stableiron group elements such asiron andnickel, which have the highest nuclearbinding energies.[24] The net process results in alater energy release, meaning subsequent to the Big Bang.[25] Such reactions of nuclear particles can lead tosudden energy releases fromcataclysmic variable stars such asnovae. Gravitational collapse of matter intoblack holes also powers the most energetic processes, generally seen in the nuclear regions of galaxies, formingquasars andactive galaxies.
Cosmologists cannot explain all cosmic phenomena exactly, such as those related to theaccelerating expansion of the universe, using conventionalforms of energy. Instead, cosmologists propose a new form of energy calleddark energy that permeates all space.[26] One hypothesis is that dark energy is just thevacuum energy, a component of empty space that is associated with thevirtual particles that exist due to theuncertainty principle.[27]
There is no clear way to define the total energy in the universe using the most widely accepted theory of gravity, general relativity. Therefore, it remains controversial whether the total energy is conserved in an expanding universe. For instance, eachphoton that travels through intergalactic space loses energy due to theredshift effect. This energy is not transferred to any other system, so seems to be permanently lost. On the other hand, some cosmologists insist that energy is conserved in some sense; this follows the law ofconservation of energy.[28]
Different forms of energy may dominate the cosmos—relativistic particles which are referred to asradiation, or non-relativistic particles referred to as matter. Relativistic particles are particles whoserest mass is zero or negligible compared to theirkinetic energy, and so move at the speed of light or very close to it; non-relativistic particles have much higher rest mass than their energy and so move much slower than the speed of light.
As the universe expands, both matter and radiation become diluted. However, theenergy densities of radiation and matter dilute at different rates. As a particular volume expands, mass-energy density is changed only by the increase in volume, but the energy density of radiation is changed both by the increase in volume and by the increase in thewavelength of the photons that make it up. Thus the energy of radiation becomes a smaller part of the universe's total energy than that of matter as it expands. The very early universe is said to have been 'radiation dominated' and radiation controlled the deceleration of expansion. Later, as the average energy per photon becomes roughly 10eV and lower, matter dictates the rate of deceleration and the universe is said to be 'matter dominated'. The intermediate case is not treated wellanalytically. As the expansion of the universe continues, matter dilutes even further and the cosmological constant becomes dominant, leading to an acceleration in the universe's expansion.
The history of the universe is a central issue in cosmology. The history of the universe is divided into different periods called epochs, according to the dominant forces and processes in each period. The standard cosmological model is known as theLambda-CDM model.
Within thestandard cosmological model, theequations of motion governing the universe as a whole are derived from general relativity with a small, positive cosmological constant.[29] The solution is an expanding universe; due to this expansion, the radiation and matter in the universe cool and become diluted. At first, the expansion is slowed down bygravitation attracting the radiation and matter in the universe. However, as these become diluted, the cosmological constant becomes more dominant and the expansion of the universe starts to accelerate rather than decelerate. In our universe this happened billions of years ago.[30]
During the earliest moments of the universe, the average energy density was very high, making knowledge ofparticle physics critical to understanding this environment. Hence,scattering processes anddecay of unstableelementary particles are important for cosmological models of this period.
As a rule of thumb, a scattering or a decay process is cosmologically important in a certain epoch if the time scale describing that process is smaller than, or comparable to, the time scale of the expansion of the universe.[clarification needed] The time scale that describes the expansion of the universe is with being theHubble parameter, which varies with time. The expansion timescale is roughly equal to the age of the universe at each point in time.
Observations suggest that the universe began around 13.8 billion years ago.[31] Since then, the evolution of the universe has passed through three phases. The very early universe, which is still poorly understood, was the split second in which the universe was so hot thatparticles had energies higher than those currently accessible inparticle accelerators on Earth. 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.
Following this, in the early universe, the evolution of the universe proceeded according to knownhigh energy physics. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, thecosmic microwave background was emitted. Finally, the epoch of structure formation began, when matter started to aggregate into the firststars andquasars, and ultimately galaxies,clusters of galaxies andsuperclusters formed. The future of the universe is not yet firmly known, but according to theΛCDM model it will continue expanding forever.
Below, some of the most active areas of inquiry in cosmology are described, in roughly chronological order. This does not include all of the Big Bang cosmology, which is presented inTimeline of the Big Bang.
The early, hot universe appears to be well explained by the Big Bang from roughly 10−33 seconds onwards, but there are severalproblems. One is that there is no compelling reason, using current particle physics, for the universe to beflat, homogeneous, andisotropic(see thecosmological principle). Moreover,grand unified theories of particle physics suggest that there should bemagnetic monopoles in the universe, which have not been found. These problems are resolved by a brief period ofcosmic inflation, which drives the universe toflatness, smooths outanisotropies and inhomogeneities to the observed level, and exponentially dilutes the monopoles.[32] The physical model behind cosmic inflation is extremely simple, but it has not yet been confirmed by particle physics, and there are difficult problems reconciling inflation andquantum field theory.[vague] Some cosmologists think thatstring theory andbrane cosmology will provide an alternative to inflation.[33]
Another major problem in cosmology is what caused the universe to contain far more matter thanantimatter. Cosmologists can observationally deduce that the universe is not split into regions of matter and antimatter. If it were, there would beX-rays andgamma rays produced as a result ofannihilation, but this is not observed. Therefore, some process in the early universe must have created a small excess of matter over antimatter, and this (currently not understood) process is calledbaryogenesis. Three required conditions for baryogenesis were derived byAndrei Sakharov in 1967, and requires a violation of the particle physicssymmetry, calledCP-symmetry, between matter and antimatter.[34] However, particle accelerators measure too small a violation of CP-symmetry to account for the baryon asymmetry. Cosmologists and particle physicists look for additional violations of the CP-symmetry in the early universe that might account for thebaryon asymmetry.[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 andexperiment, rather than through observations of the universe.[speculation?]
Big Bang nucleosynthesis is the theory of the formation of the elements in the early universe. It finished when the universe was about three minutes old and itstemperature dropped below that at whichnuclear fusion could occur. Big Bang nucleosynthesis had a brief period during which it could operate, so only the very lightest elements were produced. Starting from hydrogenions (protons), it principally produceddeuterium,helium-4, andlithium. Other elements were produced in only trace abundances. The basic theory of nucleosynthesis was developed in 1948 by George Gamow,Ralph Asher Alpher, andRobert Herman.[36] It was used for many years as a probe of physics at the time of the Big Bang, as the theory of Big Bang nucleosynthesis connects the abundances of primordial light elements with the features of the early universe.[23] Specifically, it can be used to test theequivalence principle,[37] to probedark matter, and testneutrino physics.[38] Some cosmologists have proposed that Big Bang nucleosynthesis suggests there is a fourth "sterile" species of neutrino.[39]
TheΛCDM (Lambda cold dark matter) orLambda-CDM model is aparametrization of the Big Bang cosmological model in which the universe contains a cosmological constant, denoted byLambda (GreekΛ), associated with dark energy, andcold dark matter (abbreviatedCDM). It is frequently referred to as thestandard model of Big Bang cosmology.[40][41]
The cosmic microwave background is radiation left over fromdecoupling after the epoch ofrecombination when neutral atoms first formed. At this point, radiation produced in the Big Bang stoppedThomson scattering from charged ions. The radiation, first observed in 1965 byArno Penzias andRobert Woodrow Wilson, has a perfect thermalblack-body spectrum. It has a temperature of 2.7kelvins today and is isotropic to one part in 105.Cosmological perturbation theory, which describes the evolution of slight inhomogeneities in the early universe, has allowed cosmologists to precisely calculate the angularpower spectrum of the radiation, and it has been measured by the recent satellite experiments (COBE andWMAP)[42] and many ground and balloon-based experiments (such asDegree Angular Scale Interferometer,Cosmic Background Imager, andBoomerang).[43] One of the goals of these efforts is to measure the basic parameters of the Lambda-CDM model with increasing accuracy, as well as to test the predictions of the Big Bang model and look for new physics. The results of measurements made by WMAP, for example, have placed limits on the neutrino masses.[44]
Newer experiments, such asQUIET and theAtacama Cosmology Telescope, are trying to measure thepolarization of the cosmic microwave background.[45] These measurements are expected to provide further confirmation of the theory as well as information about cosmic inflation, and the so-called secondary anisotropies,[46] such as theSunyaev-Zel'dovich effect andSachs-Wolfe effect, which are caused by interaction betweengalaxies andclusters with the cosmic microwave background.[47][48]
On 17 March 2014, astronomers of theBICEP2 Collaboration announced the apparent detection ofB-mode polarization of the CMB, considered to be evidence ofprimordial gravitational waves that are predicted by the theory ofinflation to occur during the earliest phase of the Big Bang.[9][10][11][49] However, later that year thePlanck collaboration provided a more accurate measurement ofcosmic dust, concluding that the B-mode signal from dust is the same strength as that reported from BICEP2.[50][51] On 30 January 2015, a joint analysis of BICEP2 andPlanck data was published and theEuropean Space Agency announced that the signal can be entirely attributed to interstellar dust in the Milky Way.[52]
Understanding the formation and evolution of the largest and earliest structures (i.e., quasars, galaxies,clusters andsuperclusters) is one of the largest efforts in cosmology. Cosmologists study a model ofhierarchical structure formation in which structures form from the bottom up, with smaller objects forming first, while the largest objects, such as superclusters, are still assembling.[53] One way to study structure in the universe is to survey the visible galaxies, in order to construct a three-dimensional picture of the galaxies in the universe and measure the matterpower spectrum. This is the approach of theSloan Digital Sky Survey and the2dF Galaxy Redshift Survey.[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 intofilaments, superclusters andvoids. Most simulations contain only non-baryoniccold dark matter, which should suffice to understand the universe on the largest scales, as there is much more dark matter in the universe than visible, baryonic matter. More advanced simulations are starting to include baryons and study the formation of individual galaxies. Cosmologists study these simulations to see if they agree with the galaxy surveys, and to understand any discrepancy.[56]
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Other, complementary observations to measure the distribution of matter in the distant universe and to probereionization include:
These will help cosmologists settle the question of when and how structure formed in the universe.
Evidence fromBig Bang nucleosynthesis, thecosmic microwave background, structure formation, andgalaxy rotation curves suggests that about 23% of the mass of the universe consists of non-baryonic dark matter, whereas only 4% consists of visible,baryonic matter. The gravitational effects of dark matter are well understood, as it behaves like a cold,non-radiative fluid that formshaloes around galaxies. Dark matter has never been detected in the laboratory, and the particle physics nature of dark matter remains completely unknown. Without observational constraints, there are a number of candidates, such as a stablesupersymmetric particle, aweakly interacting massive particle, a gravitationally-interacting massive particle, anaxion, and amassive compact halo object. Alternatives to the dark matter hypothesis include a modification of gravity at small accelerations (MOND) or an effect from brane cosmology.TeVeS is a version of MOND that can explain gravitational lensing.[60]
If the universe isflat, there must be an additional component making up 73% (in addition to the 23% dark matter and 4% baryons) of the energy density of the universe. This is called dark energy. 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. There is strong observational evidence for dark energy, as the total energy density of the universe is known through constraints on the flatness of the universe, but the amount of clustering matter is tightly measured, and is much less than this. The case for dark energy was strengthened in 1999, when measurements demonstrated that the expansion of the universe has begun to gradually accelerate.[61]
Apart from its density and its clustering properties, nothing is known about dark energy.Quantum field theory predicts a cosmological constant (CC) much like dark energy, but 120orders of magnitude larger than that observed.[62]Steven Weinberg and a number of string theorists(seestring landscape) have invoked the 'weakanthropic principle': i.e. the reason that physicists observe a universe with such a small cosmological constant is that no physicists (or any life) could exist in a universe with a larger cosmological constant. Many cosmologists find this an unsatisfying explanation: perhaps because while the weak anthropic principle is self-evident (given that living observers exist, there must be at least one universe with a cosmological constant (CC) which allows for life to exist) it does not attempt to explain the context of that universe.[63] For example, the weak anthropic principle alone does not distinguish between:
Other possible explanations for dark energy includequintessence[64] or a modification of gravity on the largest scales.[65] The effect on cosmology of the dark energy that these models describe is given by the dark energy'sequation of state, which varies depending upon the theory. The nature of dark energy is one of the most challenging problems in cosmology.
A better understanding of dark energy is likely to solve the problem of theultimate fate of the universe. In the current cosmological epoch, the accelerated expansion due to dark energy is preventing structures larger thansuperclusters from forming. It is not known whether the acceleration will continue indefinitely, perhaps even increasing until abig rip, or whether it will eventually reverse, lead to aBig Freeze, or follow some other scenario.[66]
Gravitational waves are ripples in thecurvature ofspacetime that propagate aswaves at the speed of light, generated in certain gravitational interactions that propagate outward from their source.Gravitational-wave astronomy is an emerging branch ofobservational astronomy which aims to use gravitational waves to collect observational data about sources of detectable gravitational waves such asbinary star systems composed ofwhite dwarfs,neutron stars, andblack holes; and events such assupernovae, and the formation of theearly universe shortly after the Big Bang.[67]
In 2016, theLIGO Scientific Collaboration andVirgo Collaboration teams announced that they had made thefirst observation of gravitational waves, originating from apair ofmerging black holes using the Advanced LIGO detectors.[68][69][70] On 15 June 2016, asecond detection of gravitational waves from coalescing black holes was announced.[71] Besides LIGO, many othergravitational-wave observatories (detectors) are under construction.[72]
Cosmologists also study:
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(subtitle) Could cosmic inflation be a sign that our universe is embedded in a far vaster realm?
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