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Observations show that theexpansion of the universe isaccelerating, such that thevelocity at which a distantgalaxy recedes from the observer is continuously increasing with time.[1][2][3] The accelerated expansion of theuniverse was discovered in 1998 by two independent projects, theSupernova Cosmology Project and theHigh-Z Supernova Search Team, which used distanttype Ia supernovae to measure the acceleration.[4][5][6] The idea was that as type Ia supernovae have almost the same intrinsic brightness (astandard candle), and since objects that are further away appear dimmer, the observed brightness of these supernovae can be used to measure the distance to them. The distance can then be compared to the supernovae's cosmologicalredshift, which measures how much the universe has expanded since the supernova occurred; theHubble law established that the further away an object is, the faster it is receding. The unexpected result was that objects in the universe are moving away from one another at an accelerating rate. Cosmologists at the time expected that recession velocity would always be decelerating, due to the gravitational attraction of the matter in the universe. Three members of these two groups have subsequently been awardedNobel Prizes for their discovery.[7] Confirmatory evidence has been found inbaryon acoustic oscillations, and in analyses of theclustering of galaxies.
The accelerated expansion of the universe is thought to have begun since the universe entered itsdark-energy-dominated era roughly 5 billion years ago.[8][notes 1]Within the framework ofgeneral relativity, an accelerated expansion can be accounted for by a positive value of thecosmological constantΛ, equivalent to the presence of a positivevacuum energy, dubbed "dark energy". While there are alternative possible explanations, the description assuming dark energy (positiveΛ) is used in the standard model ofcosmology, which also includescold dark matter (CDM) and is known as theLambda-CDM model.
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In the decades since the detection ofcosmic microwave background (CMB) in 1965,[9] theBig Bang model has become the most accepted model explaining the evolution of our universe. TheFriedmann equation defines how theenergy in the universe drives its expansion.
whereκ represents thecurvature of the universe,a(t) is thescale factor,ρ is the total energy density of the universe, andH is theHubble parameter.[10]
Thecritical density is defined asand thedensity parameter
The Hubble parameter can then be rewritten aswhere the four currently hypothesized contributors to the energy density of the universe arecurvature,matter,radiation anddark energy.[11] Each of the components decreases with the expansion of the universe (increasing scale factor), except perhaps the dark energy term. It is the values of these cosmological parameters which physicists use to determine the acceleration of the universe.
Theacceleration equation describes the evolution of the scale factor with timewhere thepressureP is defined by the cosmological model chosen(see also:Explanatory models).
Physicists at one time were so assured of the deceleration of the universe's expansion that they introduced a so-calleddeceleration parameterq0.[12] Recent observations indicate this deceleration parameter is negative.
According to the theory ofcosmic inflation, the very early universe underwent a period of very rapid, quasi-exponential expansion. While the time-scale for this period of expansion was far shorter than that of the existing expansion, this was a period of accelerated expansion with some similarities to the current epoch.
The definition of "accelerating expansion" is that the second time derivative of the cosmic scale factor,, is positive, which is equivalent to thedeceleration parameter,, being negative. However, note this doesnot imply that theHubble parameter is increasing with time. Since the Hubble parameter is defined as, it follows from the definitions that the derivative of the Hubble parameter is given byso the Hubble parameter is decreasing with time unless. Observations prefer, which implies that is positive but is negative. Essentially, this implies that the cosmic recession velocity of any one particular galaxy is increasing with time, but its velocity/distance ratio is still decreasing; thus different galaxies expanding across a sphere of fixed radius cross the sphere more slowly at later times.
It is seen from above that the case of "zero acceleration/deceleration" corresponds to is a linear function of,,, and.
The rate of expansion of the universe can be analyzed using themagnitude-redshift relationship of astronomical objects usingstandard candles, or their distance-redshift relationship usingstandard rulers. Also a factor is the growth oflarge-scale structure, finding that the observed values of the cosmological parameters are best described by models which include an accelerating expansion.
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In 1998, the first evidence for acceleration came from the observation ofType Ia supernovae, which are explodingwhite dwarf stars that have exceeded theirstability limit. Because they all have similar masses, their intrinsicluminosity can be standardized. Repeated imaging of selected areas of the sky is used to discover the supernovae, then follow-up observations give their peak brightness, which is converted into a quantity known as luminosity distance (seedistance measures in cosmology for details).[13]Spectral lines of their light can be used to determine theirredshift.
For supernovae at redshift less than around 0.1, or light travel time less than 10 percent of the age of the universe, this gives a nearly linear distance–redshift relation due toHubble's law. At larger distances, since the expansion rate of the universe has changed over time, the distance-redshift relation deviates from linearity, and this deviation depends on how the expansion rate has changed over time. The full calculation requires computer integration of the Friedmann equation, but a simple derivation can be given as follows: the redshiftz directly gives thecosmic scale factor at the time the supernova exploded.
So a supernova with a measured redshiftz = 0.5 implies the universe was1/1 + 0.5 = 2/3 of its present size when the supernova exploded. In the case of accelerated expansion, is positive; therefore, was smaller in the past than today. Thus, an accelerating universe took a longer time to expand from 2/3 to 1 times its present size, compared to a non-accelerating universe with constant and the same present-day value of the Hubble constant. This results in a larger light-travel time, larger distance and fainter supernovae, which corresponds to the actual observations.Adam Riesset al. found that "the distances of the high-redshift SNe Ia were, on average, 10% to 15% further than expected in a low mass densityΩM = 0.2 universe without a cosmological constant".[14] This means that the measured high-redshift distances were too large, compared to nearby ones, for a decelerating universe.[15]
Several researchers have questioned the majority opinion on the acceleration or the assumption of the "cosmological principle" (that the universe is homogeneous and isotropic).[16] For example, a 2019 paper analyzed theJoint Light-curve Analysis catalog of Type Ia supernovas, containing ten times as many supernova as were used in the 1998 analyses, and concluded that there was little evidence for a "monopole", that is, for an isotropic acceleration in all directions[17][18](see also:§ Alternative theories, below).
In the early universe beforerecombination anddecoupling took place,photons and matter existed in aprimordial plasma. Points of higher density in the photon-baryon plasma would contract, being compressed by gravity until the pressure became too large and they expanded again.[12] This contraction and expansion created vibrations in the plasma analogous tosound waves. Sincedark matter only interactsgravitationally, it stayed at the centre of the sound wave, the origin of the original overdensity. When decoupling occurred, approximately 380,000 years after the Big Bang,[19] photons separated from matter and were able tostream freely through the universe, creating thecosmic microwave background as we know it. This left shells ofbaryonic matter at a fixed radius from the overdensities of dark matter, a distance known as the sound horizon. As time passed and the universe expanded, it was at these inhomogeneities of matter density where galaxies started to form. So by looking at the distances at which galaxies at different redshifts tend to cluster, it is possible to determine a standardangular diameter distance and use that to compare to the distances predicted by different cosmological models.
Peaks have been found in the correlation function (the probability that two galaxies will be a certain distance apart) at100h−1Mpc,[11] (whereh is thedimensionless Hubble constant) indicating that this is the size of the sound horizon today, and by comparing this to the sound horizon at the time of decoupling (using the CMB), we can confirm the accelerated expansion of the universe.[20]
Measuring the mass functions ofgalaxy clusters, which describe thenumber density of the clusters above a threshold mass, also provides evidence for dark energy[further explanation needed].[21] By comparing these mass functions at high and low redshifts to those predicted by different cosmological models, values forw andΩm are obtained which confirm a low matter density and a non-zero amount of dark energy.[15]
Given a cosmological model with certain values of the cosmological density parameters, it is possible to integrate theFriedmann equations and derive the age of the universe.
By comparing this to actual measured values of the cosmological parameters, we can confirm the validity of a model which is accelerating now, and had a slower expansion in the past.[15]
Recent discoveries ofgravitational waves throughLIGO andVIRGO[22][23][24] not only confirmed Einstein's predictions but also opened a new window into the universe. These gravitational waves can work as sort ofstandard sirens to measure the expansion rate of the universe. Abbot et al. 2017 measured the Hubble constant value to be approximately 70 kilometres per second per megaparsec.[22] The amplitudes of the strain 'h' is dependent on the masses of the objects causing waves, distances from observation point and gravitational waves detection frequencies. The associated distance measures are dependent on the cosmological parameters like the Hubble Constant for nearby objects[22] and will be dependent on other cosmological parameters like the dark energy density, matter density, etc. for distant sources.[25][24]
The most important property of dark energy is that it has negative pressure (repulsive action) which is distributed relatively homogeneously in space.
wherec is the speed of light andρ is the energy density. Different theories of dark energy suggest different values ofw, withw < −1/3 for cosmic acceleration (this leads to a positive value ofä in theacceleration equation above).
The simplest explanation for dark energy is that it is a cosmological constant orvacuum energy; in this casew = −1. This leads to theLambda-CDM model, which has generally been known as the Standard Model of Cosmology from 2003 through the present, since it is the simplest model in good agreement with a variety of recent observations. Riesset al. found that their results from supernova observations favoured expanding models with positive cosmological constant (Ωλ > 0) and an accelerated expansion (q0 < 0).[14]
These observations allow the possibility of a cosmological model containing a dark energy component with equation of statew < −1. This phantom energy density would become infinite in finite time, causing such a huge gravitational repulsion that the universe would lose all structure and end in aBig Rip.[26] For example, forw = −3/2 andH0 =70 km·s−1·Mpc−1, the time remaining before the universe ends in this Big Rip is 22 billion years.[27]
There are many alternative explanations for the accelerating universe. Some examples arequintessence, a proposed form of dark energy with a non-constant state equation, whose density decreases with time. Anegative mass cosmology does not assume that the mass density of the universe is positive (as is done in supernova observations), and instead finds a negative cosmological constant.Occam's razor also suggests that this is the 'more parsimonious hypothesis'.[28][29]Dark fluid is an alternative explanation for accelerating expansion which attempts to unite dark matter and dark energy into a single framework.[30] Alternatively, some authors have argued that the accelerated expansion of the universe could be due to a repulsivegravitational interaction of antimatter[31][32][33] or a deviation of the gravitational laws from general relativity, such asmassive gravity, meaning that gravitons themselves have mass.[34] The measurement of the speed of gravity with the gravitational wave eventGW170817 ruled out many modified gravity theories as alternative explanations to dark energy.[35][36][37] Another type of model, the backreaction conjecture,[38][39] was proposed by cosmologist Syksy Räsänen:[40] the rate of expansion is not homogenous, but Earth is in a region where expansion is faster than the background. Inhomogeneities in the early universe cause the formation of walls and bubbles, where the inside of a bubble has less matter than on average. According to general relativity, space is less curved than on the walls, and thus appears to have more volume and a higher expansion rate. In the denser regions, the expansion is slowed by a higher gravitational attraction. Therefore, the inward collapse of the denser regions looks the same as an accelerating expansion of the bubbles, leading us to conclude that the universe is undergoing an accelerated expansion.[41] The benefit is that it does not require any new physics such as dark energy. Räsänen does not consider the model likely, but without any falsification, it must remain a possibility. It would require rather large density fluctuations (20%) to work.[40]
Shockwave cosmology, proposed by Joel Smoller and Blake Temple in 2003, has the "big bang" as an explosion inside a black hole, producing the expanding volume of space and matter that includes the observable universe.[42] A related theory by Smoller, Temple, and Vogler proposes that this shockwave may have resulted in our part of the universe having a lower density than that surrounding it, causing the accelerated expansion normally attributed to dark energy.[43][44] They also propose that this related theory could be tested: a universe with dark energy should give a figure for the cubic correction to redshift versus luminosity C = −0.180 at a = a whereas for Smoller, Temple, and Vogler's alternative C should be positive rather than negative. They give a more precise calculation for their shockwave model alternative as: the cubic correction to redshift versus luminosity at a = a is C = 0.359.[44]
Although shockwave cosmology produces a universe that "looks essentially identical to the aftermath of the big bang",[45] cosmologists consider that it needs further development before it could be considered as a more advantageous model than the big bang theory (or standard model) in explaining the universe. In particular, and especially for the proposed alternative to dark energy, it would need to explain big bang nucleosynthesis, the quantitative details of the microwave background anisotropies, the Lyman-alpha forest, and galaxy surveys.[43]
A final possibility is that dark energy is an illusion caused by some bias in measurements. For example, if we are located in an emptier-than-average region of space, the observed cosmic expansion rate could be mistaken for a variation in time, or acceleration.[46][47][48][49] A different approach uses a cosmological extension of theequivalence principle to show how space might appear to be expanding more rapidly in the voids surrounding our local cluster. While weak, such effects considered cumulatively over billions of years could become significant, creating the illusion of cosmic acceleration, and making it appear as if we live in aHubble bubble.[50][51][52] Yet other possibilities are that the accelerated expansion of the universe is an illusion caused by the relative motion of us to the rest of the universe,[53][54] or that the supernova sample size used wasn't large enough.[55][56]
As the universe expands, the density of radiation and ordinarydark matter declines more quickly than the density ofdark energy (seeequation of state) and, eventually, dark energy dominates. Specifically, when the scale of the universe doubles, the density of matter is reduced by a factor of 8, but the density of dark energy is nearly unchanged (it is exactly constant if the dark energy is thecosmological constant).[12]
In models where dark energy is the cosmological constant, the universe will expand exponentially with time in the far future, coming closer and closer to ade Sitter universe. This will eventually lead to all evidence for the Big Bang disappearing, as the cosmic microwave background is redshifted to lower intensities and longer wavelengths. Eventually, its frequency will be low enough that it will be absorbed by theinterstellar medium, and so be screened from any observer within the galaxy. This will occur when the universe is less than 50 times its existing age, leading to the end of any life as the distant universe turns dark.[57]
A constantly expanding universe with a non-zero cosmological constant has mass density decreasing over time. Under such a scenario, it is understood that all matter will ionize and disintegrate into isolated stable particles such aselectrons andneutrinos, with all complex structures dissipating.[58] This is called "heat death of the universe" (or theBig Freeze).
Alternatives for theultimate fate of the universe include theBig Rip mentioned above, aBig Bounce, or aBig Crunch.
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| History of cosmological theories | |
| Past universe | |
| Present universe | |
| Future universe | |
| Components | |
| Structure formation | |
| Experiments | |
