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Accelerating expansion of the universe
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{{cosmology}}The accelerating expansion of the universe is the observation that the universe appears to be expanding at an increasing rate,NEWS, Overbye, Dennis, Dennis Overbye, Cosmos Controversy: The Universe Is Expanding, but How Fast?,weblink 20 February 2017, The New York Times, 21 February 2017, WEB, Scharping, Nathaniel, Gravitational Waves Show How Fast The Universe is Expanding,weblink 18 October 2017, Astronomy (magazine), Astronomy, 18 October 2017, WEB, Weaver, Donna, Villard, Ray, Measuring universe expansion reveals mystery – Is something unpredicted going on in the depths of space?,weblink 11 March 2018, Earth & Sky, 11 March 2018, so that the velocity at which a distant galaxy is receding from the observer is continuously increasing with time.WEB,weblink Is the universe expanding faster than the speed of light?, The accelerated expansion was discovered in 1998, by two independent projects, the Supernova Cosmology Project and the High-Z Supernova Search Team, which both used distant type Ia supernovae to measure the acceleration.NEWS,weblink Nobel physics prize honours accelerating universe find, BBC News, 2011-10-04, WEB,weblink The Nobel Prize in Physics 2011, Nobelprize.org, 2011-10-06, JOURNAL, Peebles, P. J. E., Ratra, Bharat, The cosmological constant and dark energy, 2003, Reviews of Modern Physics, astro-ph/0207347, 75, 2, 559–606, 10.1103/RevModPhys.75.559, 2003RvMP...75..559P, The idea was that these type 1a supernovae all have almost the same intrinsic brightness (a standard candle). Since objects that are further away appear dimmer, we can use the observed brightness of these supernovae to measure the distance to them. The distance can then be compared to the supernovae's cosmological redshift, which measures how fast the supernovae are receding from us.See also Hubble law, which established that the further an object is from us, the faster it is receding. The unexpected result was that the universe seems to be expanding at an accelerating rate. Cosmologists at the time expected that the expansion would be decelerating due to the gravitational attraction of the matter in the universe. Three members of these two groups have subsequently been awarded Nobel Prizes for their discovery.BOOK, Cosmology, Steven, Weinberg, Oxford University Press, 2008, 9780198526827, Confirmatory evidence has been found in baryon acoustic oscillations, and in analyses of the clustering of galaxies.The expansion of the universe is thought to have been accelerating since the universe entered its dark-energy-dominated era roughly 5 billion years ago.JOURNAL, Frieman, Joshua A., Turner, Michael S., Huterer, Dragan, 2008, Dark Energy and the Accelerating Universe, Annual Review of Astronomy and Astrophysics, 46, 1, 385–432, 0803.0982, 2008ARA&A..46..385F, 10.1146/annurev.astro.46.060407.145243, {{refn |1= Frieman, Turner & Huterer (2008) p. 6: "The Universe has gone through three distinct eras: radiation-dominated, {{math|z ≳ 3000}}; matter-dominated, {{math|3000 ≳ z ≳ 0.5}}; and dark-energy-dominated, {{math|z ≲ 0.5}}. The evolution of the scale factor is controlled by the dominant energy form: {{math|a(t) ∝ t2/3(1 + w)}} (for constant {{mvar|w}}). During the radiation-dominated era, {{math|a(t) ∝ t1/2}}; during the matter-dominated era, {{math|a(t) ∝ t2/3}}; and for the dark energy-dominated era, assuming {{math|w {{=}} −1}}, asymptotically {{math|a(t) ∝ exp(Ht)}}."p. 44: "Taken together, all the current data provide strong evidence for the existence of dark energy; they constrain the fraction of critical density contributed by dark energy, 0.76 Â± 0.02, and the equation-of-state parameter, {{mvar|w}} â‰ˆ −1 Â± 0.1 (stat) ± 0.1 (sys), assuming that {{mvar|w}} is constant. This implies that the Universe began accelerating at redshift {{math|z ∼}} 0.4 and age {{math|t ∼}} 10 Gyr. These results are robust – data from any one method can be removed without compromising the constraints – and they are not substantially weakened by dropping the assumption of spatial flatness."|group="notes"}}Within the framework of general relativity, an accelerating expansion can be accounted for by a positive value of the cosmological constant {{mvar|Λ}}, equivalent to the presence of a positive vacuum energy, dubbed "dark energy". While there are alternative possible explanations, the description assuming dark energy (positive {{mvar|Λ}}) is used in the current standard model of cosmology, which also includes cold dark matter (CDM) and is known as the Lambda-CDM model.

Background

{{Nature timeline}}{{further|Cosmological constant|Lambda-CDM model|Hubble's law|Friedmann–Lemaître–Robertson–Walker metric|Friedmann equations}}In the decades since the detection of cosmic microwave background (CMB) in 1965,JOURNAL, Penzias, A. A., Wilson, R. W., 1965, A Measurement of Excess Antenna Temperature at 4080 Mc/s, The Astrophysical Journal, 142, 1, 419–421, 1965ApJ...142..419P, 10.1086/148307, the Big Bang model has become the most accepted model explaining the evolution of our universe. The Friedmann equation defines how the energy in the universe drives its expansion.
H^2={left ( frac{dot{a}}{a} right )}^2=frac{8{pi}G}{3}rho-frac{hide}Kappa}c^2}{a^2}
where {{mvar|Κ{edih} represents the curvature of the universe, {{math|a(t)}} is the scale factor, {{mvar|ρ}} is the total energy density of the universe, and {{mvar|H}} is the Hubble parameter.JOURNAL, Nemiroff, Robert J., Robert J. Nemiroff, Patla, Bijunath, Adventures in Friedmann cosmology: A detailed expansion of the cosmological Friedmann equations, American Journal of Physics, 76, 3, 265–276, 10.1119/1.2830536, astro-ph/0703739, 2008AmJPh..76..265N, 2008, We define a critical density
rho_c=frac{3H^2}{8{pi}G}
and the density parameter
Omega=frac{rho}{rho_c}
We can then rewrite the Hubble parameter as
H(a)=H_0 sqrt{{Omega_ka^{-2} + Omega}_ma^{-3} + Omega_ra^{-4} + Omega_mathrm{DE}a^{-3(1+w)}}
where the four currently hypothesized contributors to the energy density of the universe are curvature, matter, radiation and dark energy.BOOK, Lapuente, P., Baryon Acoustic Oscillations, Dark Energy: Observational and Theoretical Approaches, Cambridge, UK, Cambridge University Press, 2010, 978-0521518888, 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.The acceleration equation describes the evolution of the scale factor with time
frac{ddot{a}}{a}=-frac{4{pi}G}{3}left( rho + frac{3P}{c^2} right)
where the pressure {{mvar|P}} is defined by the cosmological model chosen. (see explanatory models below)Physicists at one time were so assured of the deceleration of the universe's expansion that they introduced a so-called deceleration parameter {{math|q0}}.BOOK, Introduction to Cosmology, Ryden, Barbara, San Francisco, CA, Addison Wesley, 2003, 978-0-8053-8912-8, {{Page needed |date=September 2016}} Current observations point towards this deceleration parameter being negative.

Relation to inflation

According to the theory of cosmic 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 current expansion, this was a period of accelerated expansion with some similarities to the current epoch.

Evidence for acceleration

To learn about the rate of expansion of the universe we look at the magnitude-redshift relationship of astronomical objects using standard candles, or their distance-redshift relationship using standard rulers. We can also look at the growth of large-scale structure, and find that the observed values of the cosmological parameters are best described by models which include an accelerating expansion.

Supernova observation

(File:Asymmetric Ashes (artist's impression).jpg|thumb|left|200px|Artist's impression of a Type Ia supernova, as revealed by spectro-polarimetry observations)The first evidence for acceleration came from the observation of Type Ia supernovae, which are exploding white dwarfs that have exceeded their stability limit. Because they all have similar masses, their intrinsic luminosity is standardizable. 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 (see distance measures in cosmology for details).ARXIV, Albrecht, Andreas, etal, Report of the Dark Energy Task Force, 2006, astro-ph/0609591, Spectral lines of their light can be used to determine their redshift.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 to Hubble'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 redshift {{mvar|z}} directly gives the cosmic scale factor at the time the supernova exploded.
a(t)=frac{1}{1+z}
So a supernova with a measured redshift {{math|z {{=}} 0.5}} implies the universe was {{sfrac|1|1 + 0.5}} = {{sfrac|2|3}} of its present size when the supernova exploded. In an accelerating universe, the universe was expanding more slowly in the past than it is today, which means it took a longer time to expand from two thirds its present size to its present size, compared to a non-accelerating universe with 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 Riess et al. found that "the distances of the high-redshift SNe Ia were, on average, 10% to 15% farther than expected in a low mass density {{math|ΩM {{=}} 0.2}} universe without a cosmological constant".JOURNAL, Riess, Adam G., etal, Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant, The Astronomical Journal, 116, 3, 1009–1038, 1998, 10.1086/300499, 1998AJ....116.1009R, astro-ph/9805201, This means that the measured high-redshift distances were too large, compared to nearby ones, for a decelerating universe.JOURNAL, Pain, Reynald, Astier, Pierre, Observational evidence of the accelerated expansion of the Universe, Comptes Rendus Physique, 13, 6, 521–538, 1204.5493, 10.1016/j.crhy.2012.04.009, 2012, 2012CRPhy..13..521A, 10.1.1.747.3792,

Baryon acoustic oscillations

In the early universe before recombination and decoupling took place, photons and matter existed in a primordial 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.{{Page needed|date=September 2016}} This contraction and expansion created vibrations in the plasma analogous to sound waves. Since dark matter only interacts gravitationally 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,JOURNAL, Hinshaw, G., 2009, Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Data Processing, Sky Maps, and Basic Results, 10.1088/0067-0049/180/2/225, Astrophysical Journal Supplement, 180, 2, 225–245, 0803.0732, 2009ApJS..180..225H, photons separated from matter and were able to stream freely through the universe, creating the cosmic microwave background as we know it. This left shells of baryonic 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 anisotropies 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 standard angular 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) at {{nowrap|100 h−1 Mpc}}, 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 that the expansion of the universe is accelerating.JOURNAL, Eisenstein, Daniel J., etal, Detection of the Baryon Acoustic Peak in the Large‐Scale Correlation Function of SDSS Luminous Red Galaxies, The Astrophysical Journal, 2005, 633, 2, 560–574, 10.1086/466512, 2005ApJ...633..560E, astro-ph/0501171,

Clusters of galaxies

Measuring the mass functions of galaxy clusters, which describe the number density of the clusters above a threshold mass, also provides evidence for dark energy {{explain|date=March 2018}}.BOOK, Dekel, Avishai, Formation of Structure in the Universe, New York, NY, Cambridge University Press, 1999, 9780521586320, By comparing these mass functions at high and low redshifts to those predicted by different cosmological models, values for {{mvar|w}} and {{mvar|Ωm}} are obtained which confirm a low matter density and a non zero amount of dark energy.

Age of the universe

{{see also|Age of the universe}}Given a cosmological model with certain values of the cosmological density parameters, it is possible to integrate the Friedmann equations and derive the age of the universe.
t_0=int_{0}^{1}frac{da}{dot{a}}
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.

Gravitational waves as standard sirens

Recent discoveries of gravitational waves through LIGO and VIRGO JOURNAL, The LIGO Scientific Collaboration and The Virgo Collaboration, The 1M2H Collaboration, The Dark Energy Camera GW-EM Collaboration and the DES Collaboration, The DLT40 Collaboration, The Las Cumbres Observatory Collaboration, The VINROUGE Collaboration, The MASTER Collaboration, 2017-11-02, A gravitational-wave standard siren measurement of the Hubble constant,weblink Nature, 551, 7678, 85–88, 10.1038/nature24471, 29094696, 0028-0836, 1710.05835, 2017Natur.551...85A, JOURNAL, Abbott, B. P., LIGO Scientific Collaboration and Virgo Collaboration, 2016-02-11, Observation of Gravitational Waves from a Binary Black Hole Merger, Physical Review Letters, 116, 6, 061102, 10.1103/PhysRevLett.116.061102, 26918975, 1602.03837, 2016PhRvL.116f1102A, JOURNAL, ur Rahman, Syed Faisal, 2018-04-01, Where next for the expanding universe?,weblink Astronomy & Geophysics, en, 59, 2, 2.39–2.42, 10.1093/astrogeo/aty088, 1366-8781, 2018A&G....59b2.39F, not only confirmed Einstein's predictions but also opened a new window into the universe. These gravitational waves can work as sort of standard 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. 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 and will be dependent on other cosmological parameters like the dark energy density, matter density, etc. for distant sources.JOURNAL, Rosado, Pablo A., Lasky, Paul D., Thrane, Eric, Zhu, Xingjiang, Mandel, Ilya, Sesana, Alberto, 2016, Detectability of Gravitational Waves from High-Redshift Binaries, Physical Review Letters, 116, 10, 101102, 10.1103/PhysRevLett.116.101102, 27015470, 1512.04950, 2016PhRvL.116j1102R,

Explanatory models

(File:Dark Energy.jpg|thumb|right|400px|The expansion of the Universe accelerating. Time flows from bottom to top)

Dark energy

The most important property of dark energy is that it has negative pressure which is distributed relatively homogeneously in space.
P=wc^2rho
where {{mvar|c}} is the speed of light and {{mvar|ρ}} is the energy density. Different theories of dark energy suggest different values of {{mvar|w}}, with {{math|w < −{{sfrac|1|3}}}} for cosmic acceleration (this leads to a positive value of {{mvar|ä}} in the acceleration equation above).The simplest explanation for dark energy is that it is a cosmological constant or vacuum energy; in this case {{math|w {{=}} −1}}. This leads to the Lambda-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. Riess et al. found that their results from supernovae observations favoured expanding models with positive cosmological constant ({{math|Ωλ > 0}}) and a current acceleration of the expansion ({{math|q0 < 0}}).

Phantom energy

Current observations allow the possibility of a cosmological model containing a dark energy component with equation of state {{math|w < −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 a Big Rip.JOURNAL, Caldwell, Robert, Kamionkowski, Marc, Weinberg, Nevin, Phantom Energy: Dark Energy with {{math, w < −1, Causes a Cosmic Doomsday |journal=Physical Review Letters |volume=91 |issue=7 |doi=10.1103/PhysRevLett.91.071301 |bibcode=2003PhRvL..91g1301C |pmid=12935004 |date=August 2003 |pages=071301 |arxiv=astro-ph/0302506 }} For example, for {{math|w {{=}} −{{sfrac|3|2}}}} and {{math|H0}} =70 km·s−1·Mpc−1, the time remaining before the universe ends in this Big Rip is 22 billion years.JOURNAL, Caldwell, R. R., A phantom menace? Cosmological consequences of a dark energy component with super-negative equation of state, Physics Letters B, 545, 1–2, 2002, 23–29, 10.1016/S0370-2693(02)02589-3, astro-ph/9908168, 2002PhLB..545...23C,

Alternative theories

{{see also|Dark energy#Theories of dark energy}}There are many alternative explanations for the accelerating universe. Some examples are quintessence, a proposed form of dark energy with a non-constant state equation, whose density decreases with time. Dark fluid is an alternative explanation for accelerating expansion which attempts to unite dark matter and dark energy into a single framework.JOURNAL, Anaelle, Halle, Hongsheng, Zhao, Baojiu, Li, 2008, Perturbations in a non-uniform dark energy fluid: equations reveal effects of modified gravity and dark matter, 0711.0958, 10.1086/587744, Astrophysical Journal Supplement Series, 177, 1, 1, 2008ApJS..177....1H, Alternatively, some authors have argued that the universe expansion acceleration could be due to a repulsive gravitational interaction of antimatterJOURNAL, A., Benoit-Lévy, G., Chardin,weblink Introducing the Dirac–Milne universe, Astronomy and Astrophysics, 537, 78, A78, 2012, 10.1051/0004-6361/201016103, 1110.3054, 2012A&A...537A..78B, {{open access}}JOURNAL, D. S., Hajduković, 10.1007/s10509-012-0992-y, Quantum vacuum and virtual gravitational dipoles: the solution to the dark energy problem?, Astrophysics and Space Science, 339, 1, 1–5, 2012, 1201.4594, 2012Ap&SS.339....1H,weblink JOURNAL, M., Villata, 10.1007/s10509-013-1388-3, On the nature of dark energy: the lattice Universe, 2013, Astrophysics and Space Science, 345, 345, 1–9, 1302.3515, 2013Ap&SS.345....1V, or a deviation of the gravitational laws from general relativity. The measurement of the speed of gravity with the gravitational wave event GW170817 ruled out many modified gravity theories as alternative explanation to dark energy.JOURNAL, Challenges to Self-Acceleration in Modified Gravity from Gravitational Waves and Large-Scale Structure, Physics Letters B, 765, 382, 382–385, Lucas, Lombriser, Nelson, Lima, 1602.07670, 2017, 10.1016/j.physletb.2016.12.048, NEWS,weblink Quest to settle riddle over Einstein's theory may soon be over, February 10, 2017, October 29, 2017, phys.org, NEWS,weblink Theoretical battle: Dark energy vs. modified gravity, February 25, 2017, October 27, 2017, Ars Technica, Another type of model, the backreaction conjecture,JOURNAL, 10.1088/0264-9381/28/16/164008, 28, 16, Backreaction: directions of progress, Classical and Quantum Gravity, 164008, 1102.0408, 2011CQGra..28p4008R, Räsänen, Syksy, Ratra, Bharat, 2011, JOURNAL, 10.1146/annurev.nucl.012809.104435, 62, 1, Backreaction in Late-Time Cosmology, Annual Review of Nuclear and Particle Science, 57–79, 1112.5335, 2012ARNPS..62...57B, Buchert, Thomas, Räsänen, Syksy, 2012, was proposed by cosmologist Syksy Räsänen:NEWS,weblink Is dark energy an illusion?, 2007, New Scientist, the rate of expansion is not homogenous, but we are 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 retarded 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 expanding at an accelerating rate.WEB,weblink A Cosmic 'Tardis': What the Universe Has In Common with 'Doctor Who', Space.com, 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.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.JOURNAL, Wiltshire, David L., 2007, Exact Solution to the Averaging Problem in Cosmology, Physical Review Letters, 99, 25, 251101, 10.1103/PhysRevLett.99.251101, 18233512, 2007PhRvL..99y1101W, 0709.0732, JOURNAL, Ishak, Mustapha, Richardson, James, Garred, David, Whittington, Delilah, Nwankwo, Anthony, Sussman, Roberto, 10.1103/PhysRevD.78.123531, Physical Review D, Dark Energy or Apparent Acceleration Due to a Relativistic Cosmological Model More Complex than FLRW?, 78, 12, 123531, 2008, 0708.2943, 2008PhRvD..78l3531I, JOURNAL, Mattsson, Teppo, 10.1007/s10714-009-0873-z, General Relativity and Gravitation, 42, Dark energy as a mirage, 3, 567–599, 2010, 0711.4264, 2010GReGr..42..567M, JOURNAL, Clifton, Timothy, Ferreira, Pedro, April 2009, Does Dark Energy Really Exist?, Scientific American, 300, 4, 48–55, 10.1038/scientificamerican0409-48,weblink April 30, 2009, 19363920, 2009SciAm.300d..48C, A different approach uses a cosmological extension of the equivalence 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 a Hubble bubble.JOURNAL, 10.1103/PhysRevD.78.084032, 0809.1183, Cosmological equivalence principle and the weak-field limit, Physical Review D, 78, 8, 084032, 2008, Wiltshire, D., 2008PhRvD..78h4032W, WEB, Gray, Stuart, Dark questions remain over dark energy,weblink ABC Science Australia, 27 January 2013, NEWS, Merali, Zeeya, Is Einstein's Greatest Work All Wrong—Because He Didn't Go Far Enough?,weblink 27 January 2013, Discover magazine, March 2012, 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,Wolchover, Natalie (27 September 2011) 'Accelerating universe' could be just an illusion, MSNBCJOURNAL, Tsagas, Christos G., Peculiar motions, accelerated expansion, and the cosmological axis, Physical Review D, 2011, 84, 6, 063503, 10.1103/PhysRevD.84.063503, 2011PhRvD..84f3503T, 1107.4045, or that the supernovae sample size used wasn't large enough.JOURNAL,weblink J. T. Nielsen, A. Guffanti, S. Sarkar, Marginal evidence for cosmic acceleration from Type Ia supernovae, Scientific Reports, 2016, 35596, 6, 35596, 10.1038/srep35596, 27767125, 5073293, 2016NatSR...635596N, 1506.01354, WEB,weblink The universe is expanding at an accelerating rate – or is it?, Stuart Gillespie, University of Oxford – News & Events – Science Blog (), 21 October 2016,

Theories for the consequences to the universe

{{See also|Future of an expanding universe}}As the universe expands, the density of radiation and ordinary dark matter declines more quickly than the density of dark energy (see equation 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 a cosmological constant).{{Page needed|date=September 2016}}In models where dark energy is a cosmological constant, the universe will expand exponentially with time from now on, coming closer and closer to a de Sitter spacetime. 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 the interstellar medium, and so be screened from any observer within the galaxy. This will occur when the universe is less than 50 times its current age, leading to the end of cosmology as we know it as the distant universe turns dark.JOURNAL, Krauss, Lawrence M., Scherrer, Robert J., The return of a static universe and the end of cosmology, General Relativity and Gravitation, 2007, 39, 10, 1545–1550, 10.1007/s10714-007-0472-9, 0704.0221, 2007GReGr..39.1545K, A constantly expanding universe with non-zero cosmological constant has mass density decreasing over time, to an undetermined point when zero matter density is reached. All matter (electrons, protons and neutrons) would ionize and disintegrate, with objects dissipating away.John Baez, "The End of the Universe", 7 February 2016.weblink for the ultimate fate of the universe include the Big Rip mentioned above, a Big Bounce, Big Freeze, Big Crunch or possible proton decay.

See also

{{Div col|colwidth=30em}} {{div col end}}

Notes

{{reflist|group=notes}}

References

{{Reflist|30em}}{{Cosmology topics}}{{Breakthrough of the Year}}

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