| Part of a series on |
| Physical cosmology |
|---|
 |
Early universe |
Components · Structure |
Theexpansion of the universe is the increase indistance betweengravitationally unbound parts of theobservable universe with time.[1] It is anintrinsic expansion, so it does not mean that the universe expands "into" anything or that space exists "outside" it. To any observer in the universe, it appears that all butthe nearest galaxies (which are bound to each other by gravity) move away atspeeds that are proportional to their distance from the observer, on average. While objects cannot movefaster than light, this limitation applies only with respect tolocalreference frames and does not limit the recession rates of cosmologically distant objects.
Cosmic expansion is a key feature ofBig Bang cosmology. It can be modeled mathematically with theFriedmann–Lemaître–Robertson–Walker metric (FLRW), where it corresponds to an increase in the scale of the spatial part of the universe'sspacetimemetric tensor (which governs the size and geometry of spacetime). Within this framework, the separation of objects over time is sometimes interpreted as the expansion of space itself. However, this is not agenerally covariant description but rather only a choice ofcoordinates. Contrary to common misconception, it is equally valid to adopt a description in which space does not expand and objects simply move apart while under the influence of their mutual gravity.[2][3][4] Although cosmic expansion is often framed as a consequence ofgeneral relativity, it is also predicted byNewtonian gravity.[5][6]
According toinflation theory, the universe suddenly expanded during theinflationary epoch (about 10−32 of a second after the Big Bang), and its volume increased by a factor of at least 1078 (an expansion of distance by a factor of at least 1026 in each of the three dimensions). This would be equivalent to expanding an object 1 nanometer across (10−9 m, about half the width of amolecule ofDNA) to one approximately 10.6 light-years across (about1017 m, or 62 trillion miles). Cosmic expansion subsequently decelerated to much slower rates, until around 9.8 billion years after the Big Bang (4 billion years ago) it began to graduallyexpand more quickly, and is still doing so. Physicists have postulated the existence ofdark energy, appearing as acosmological constant in the simplest gravitational models, as a way to explain this late-time acceleration. According to the simplest extrapolation of the currently favored cosmological model, theLambda-CDM model, this acceleration becomes dominant in the future.
In 1912–1914,Vesto Slipher discovered that light from remote galaxies wasredshifted,[7][8] a phenomenonlater interpreted as galaxies receding from the Earth. In 1922,Alexander Friedmann used theEinstein field equations to provide theoretical evidence that the universe is expanding.[9]
Swedish astronomerKnut Lundmark was the first person to find observational evidence for expansion, in 1924. According to Ian Steer of the NASA/IPAC Extragalactic Database of Galaxy Distances, "Lundmark's extragalactic distance estimates were far more accurate than Hubble's, consistent with an expansion rate (Hubble constant) that was within 1% of the best measurements today."[10]
In 1927,Georges Lemaître independently reached a similar conclusion to Friedmann on a theoretical basis, and also presented observational evidence for alinear relationship between distance to galaxies and their recessional velocity.[11]Edwin Hubble observationally confirmed Lundmark's and Lemaître's findings in 1929.[12] Assuming thecosmological principle, these findings would imply that all distant galaxies are moving away from each other.
AstronomerWalter Baade recalculated the size of the known universe in the 1940s, doubling the previous calculation made byHubble in 1929.[13][14][15] He announced this finding to considerable astonishment at the 1952 meeting of theInternational Astronomical Union in Rome. For most of the second half of the 20th century, the value of the Hubble constant was estimated to be between50 and 90 km⋅s−1⋅Mpc−1.
On 13 January 1994, NASA formally announced a completion of its repairs related to the main mirror of theHubble Space Telescope, allowing for sharper images and, consequently, more accurate analyses of its observations.[16] Shortly after the repairs were made,Wendy Freedman's 1994 Key Project analyzed the recession velocity ofM100 from the core of theVirgo Cluster, offering aHubble constant measurement of80±17 km⋅s−1⋅Mpc−1.[17] Later the same year,Adam Riess et al. used an empirical method ofvisual-bandlight-curve shapes to more finely estimate theluminosity ofType Ia supernovae. This further minimized the systematicmeasurement errors of the Hubble constant, to67±7 km⋅s−1⋅Mpc−1. Reiss's measurements on the recession velocity of the nearby Virgo Cluster more closely agree with subsequent and independent analyses ofCepheid variable calibrations ofType Ia supernova, which estimates a Hubble constant of73±7 km⋅s−1⋅Mpc−1.[18] In 2003,David Spergel's analysis of thecosmic microwave background during the first year observations of theWilkinson Microwave Anisotropy Probe satellite (WMAP) further agreed with the estimated expansion rates for local galaxies,72±5 km⋅s−1⋅Mpc−1.[19]
The universe at the largest scales is observed to behomogeneous (the same everywhere) andisotropic (the same in all directions), consistent with thecosmological principle. These constraints demand that any expansion of the universe accord withHubble's law, in which objects recede from each observer with velocities proportional to their positions with respect to that observer. That is, recession velocities scale with (observer-centered) positions according to
where the Hubble rate quantifies the rate of expansion. is a function ofcosmic time.
The expansion of the universe can be understood as resulting from an initial condition in which the contents of the universe are flying apart. The mutual gravitational attraction of the matter and radiation within the universe gradually slows this expansion over time, but their density is too low to prevent continued expansion.[20] In addition, recent observational evidence suggests thatdark energy is now accelerating the expansion.
Mathematically, the expansion of the universe is quantified by thescale factor,, which is proportional to the average separation between objects, such as galaxies. The scale factor is a function of time and is conventionally set to be at the present time. Because the universe is expanding, is smaller in the past and larger in the future. Extrapolating back in time with certain cosmological models will yield a moment when the scale factor was zero; our current understanding of cosmology setsthis time at 13.787 ± 0.020 billion years ago. If the universe continues to expand forever, the scale factor will approach infinity in the future. It is also possible in principle for the universe to stop expanding and begin to contract, which corresponds to the scale factor decreasing in time.
The scale factor is a parameter of theFLRW metric, and its time evolution is governed by theFriedmann equations. The second Friedmann equation,
shows how the contents of the universe influence its expansion rate. Here, is thegravitational constant, is theenergy density within the universe, is thepressure, is thespeed of light, and is the cosmological constant. A positive energy density leads to deceleration of the expansion,, and a positive pressure further decelerates expansion. On the other hand, sufficiently negative pressure with leads to accelerated expansion, and the cosmological constant also accelerates expansion.Nonrelativisticmatter is essentially pressureless, with, while a gas ofultrarelativistic particles (such as aphoton gas) has positive pressure. Negative-pressure fluids, like dark energy, are not experimentally confirmed, but the existence of dark energy is inferred from astronomical observations.
In an expanding universe, it is often useful to study the evolution ofstructure with the expansion of the universe factored out. This motivates the use ofcomoving coordinates, which are defined to grow proportionally with the scale factor. If an object is moving only with theHubble flow of the expanding universe, with no other motion, then it remains stationary in comoving coordinates. The comoving coordinates are the spatial coordinates in theFLRW metric.
The universe is a four-dimensional spacetime, but within a universe that obeys the cosmological principle, there is a natural choice of three-dimensional spatial surface. These are the surfaces on which observers who are stationary in comoving coordinates agree on theage of the universe. In a universe governed byspecial relativity, such surfaces would behyperboloids, because relativistictime dilation means that rapidly receding distant observers' clocks are slowed, so that spatial surfaces must bend "into the future" over long distances. However, withingeneral relativity, the shape of thesecomoving synchronous spatial surfaces is affected by gravity. Current observations are consistent with these spatial surfaces being geometrically flat (so that, for example, the angles of a triangle add up to 180 degrees).
An expanding universe typically has a finite age. Light, and other particles, can have propagated only a finite distance. The comoving distance that such particles can have covered over the age of the universe is known as theparticle horizon, and the region of the universe that lies within our particle horizon is known as theobservable universe.
If the dark energy that is inferred to dominate the universe today is a cosmological constant, then the particle horizon converges to a finite value in the infinite future. This implies that the amount of the universe that we will ever be able to observe is limited. Many systems exist whose light can never reach us, because there is acosmic event horizon induced by the repulsive gravity of the dark energy.
Within the study of the evolution of structure within the universe, a natural scale emerges, known as theHubble horizon.Cosmological perturbations much larger than the Hubble horizon are not dynamical, because gravitational influences do not have time to propagate across them, while perturbations much smaller than the Hubble horizon are straightforwardly governed byNewtonian gravitational dynamics.
For photons, expansion leads to thecosmological redshift. While the cosmological redshift is often explained as the stretching of photon wavelengths due to "expansion of space", it is more naturally viewed as a consequence of theDoppler effect.[3]
An object'speculiar velocity is its velocity with respect to the comoving coordinate grid, i.e., with respect to the average expansion-associated motion of the surrounding material. It is a measure of how a particle's motion deviates from theHubble flow of the expanding universe. The peculiar velocities of nonrelativistic particles decay as the universe expands, in inverse proportion with the cosmicscale factor. This can be understood as a self-sorting effect. A particle that is moving in some direction gradually overtakes the Hubble flow of cosmic expansion in that direction, asymptotically approaching material with the same velocity as its own. More generally, the peculiarmomenta of both relativistic and nonrelativistic particles decay in inverse proportion with the scale factor.
The universe cools as it expands. This follows from the decay of particles' peculiar momenta, as discussed above. It can also be understood asadiabatic cooling. The temperature ofultrarelativistic fluids, often called "radiation" and including thecosmic microwave background, scales inversely with the scale factor (i.e.). The temperature of nonrelativistic matter drops more sharply, scaling as the inverse square of the scale factor (i.e.).
The contents of the universe dilute as it expands. The number of particles within a comoving volume remains fixed (on average), while the volume expands. For nonrelativistic matter, this implies that the energy density drops as, where is thescale factor.
For ultrarelativistic particles ("radiation"), the energy density drops more sharply, as. This is because in addition to the volume dilution of the particle count, the energy of each particle (including therest mass energy) also drops significantly due to the decay of peculiar momenta.
In general, we can consider aperfect fluid with pressure, where is the energy density. The parameter is theequation of state parameter. The energy density of such a fluid drops as
Nonrelativistic matter has while radiation has. For an exotic fluid with negative pressure, like dark energy, the energy density drops more slowly; if it remains constant in time. If, corresponding tophantom energy, the energy density grows as the universe expands.
Inflation is a period of accelerated expansion hypothesized to have occurred at a time of around 10−32 seconds. It would have been driven by theinflaton, afield that has a positive-energyfalse vacuum state. Inflation was originally proposed to explain the absence of exotic relics predicted bygrand unified theories, such asmagnetic monopoles, because the rapid expansion would have diluted such relics. It was subsequently realized that the accelerated expansion would also solve thehorizon problem and theflatness problem. Additionally,quantum fluctuations during inflation would have created initial variations in the density of the universe, which gravity later amplified to yield the observedspectrum of matter density variations.[21]: 157
During inflation, the cosmicscale factor grew exponentially in time. In order to solve the horizon and flatness problems, inflation must have lasted long enough that the scale factor grew by at least a factor of e60 (about 1026).[21]: 162
The history of the universe after inflation but before a time of about 1 second is largely unknown.[22] However, the universe is known to have been dominated by ultrarelativisticStandard Model particles, conventionally calledradiation, by the time ofneutrino decoupling at about 1 second.[23] During radiation domination, cosmic expansion decelerated, with the scale factor growing proportionally with the square root of the time.
Since radiationredshifts as the universe expands, eventually nonrelativisticmatter came to dominate the energy density of the universe. This transition happened at a time of about 50 thousand years after the Big Bang. During the matter-dominated epoch, cosmic expansion also decelerated, with the scale factor growing as the 2/3 power of the time (). Also, gravitational structure formation is most efficient when nonrelativistic matter dominates, and this epoch is responsible for the formation ofgalaxies and thelarge-scale structure of the universe.
Around 3 billion years ago, at a time of about 11 billion years, dark energy is believed to have begun to dominate the energy density of the universe. This transition came about because dark energy does not dilute as the universe expands, instead maintaining a constant energy density. Similarly to inflation, dark energy drives accelerated expansion, such that the scale factor grows exponentially in time.
The most direct way to measure the expansion rate is to independently measure the recession velocities and the distances of distant objects, such as galaxies. The ratio between these quantities gives the Hubble rate, in accordance with Hubble's law. Typically, the distance is measured using astandard candle, which is an object or event for which theintrinsic brightness is known. The object's distance can then be inferred from the observedapparent brightness. Meanwhile, the recession speed is measured through the redshift. Hubble used this approach for his original measurement of the expansion rate, by measuring the brightness ofCepheid variable stars and the redshifts of their host galaxies. More recently, usingType Ia supernovae, the expansion rate was measured to beH0 = 73.24±1.74 (km/s)/Mpc.[24] This means that for every millionparsecs of distance from the observer, recessional velocity of objects at that distance increases by about 73 kilometres per second (160,000 mph).
Supernovae are observable at such great distances that the light travel time therefrom can approach the age of the universe. Consequently, they can be used to measure not only the present-day expansion rate but also the expansion history. In work that was awarded the 2011Nobel Prize in Physics, supernova observations were used to determine that cosmic expansion is accelerating in the present epoch.[25]
By assuming a cosmological model, e.g. theLambda-CDM model, another possibility is to infer the present-day expansion rate from the sizes of the largest fluctuations seen in thecosmic microwave background. A higher expansion rate would imply a smaller characteristic size of CMB fluctuations, and vice versa. ThePlanck collaboration measured the expansion rate this way and determinedH0 =67.4±0.5 (km/s)/Mpc.[26] There is a disagreement between this measurement and the supernova-based measurements, known as theHubble tension.
A third option proposed recently is to use information fromgravitational wave events (especially those involving themerger of neutron stars, likeGW170817), to measure the expansion rate.[27][28] Such measurements do not yet have the precision to resolve the Hubble tension.
In principle, the cosmic expansion history can also be measured by studying redshift drift: how redshifts, distances, fluxes, angular positions, and angular sizes of astronomical objects change over the course of the time that they are being observed. These effects are too small to detect with current equipment. However, changes in redshift or flux could be observed by theSquare Kilometre Array orExtremely Large Telescope in the mid-2030s.[29]: 155
Due to the non-intuitive nature of the subject and what has been described by some as "careless" choices of wording, certain descriptions of the expansion of the universe and the misconceptions to which such descriptions can lead are an ongoing subject of discussion within the fields of education and communication of scientific concepts.[30][31][2] Some of these misconceptions are detailed in the following sections.
It is often erroneously argued that cosmic expansion must be interpreted as the expansion of space itself, such that galaxies are stationary as the space between them stretches. This description suggests the existence of apreferred rest frame, in violation of theprinciple of relativity. On the contrary, the expansion of the universe is naturally interpreted as galaxies moving apart.[2][3][4]
Hubble's law predicts that objects farther than theHubble horizon are recedingfaster than light. This outcome is not in violation ofspecial relativity. Since special relativity treats flat spacetimes, it is only valid over small distances within the context of the curved spacetime of the universe. Cosmic expansion respects special relativity in that nearby objects have relative velocities well below the speed of light. Analyses on cosmological scales require summation or integration over successive small distances.[32]
The relative velocities of cosmologically distant objects are not even well defined.[3] The relative velocity between two objects corresponds to theangle in spacetime between theirworldlines, and there is not a well defined angle between two lines at different points on a curved sheet.[33]
Cosmic expansion is sometimes erroneously described as a force that acts to push objects apart. On the contrary, cosmic expansion does not give rise to any tendency of objects to separate. Rather, it is only a description of how objects in the universe are already separating due to their inertial motion.[34]
Acosmological constant, on the other hand, does give rise to a force that pushes objects apart. This force accelerates cosmic expansion, but expansion can also proceed without it, so the two phenomena should not be conflated.[34]
{{cite web}}: CS1 maint: location (link)
| Background | |
|---|---|
| History of cosmological theories | |
| Past universe | |
| Present universe | |
| Future universe | |
| Components | |
| Structure formation | |
| Experiments | |
