Editing Big Bang (section)

=={{anchor|Problems}}Problems and related issues in physics==
{{See also|List of unsolved problems in physics}}
As with any theory, a number of mysteries and problems have arisen as a result of the development of the Big Bang models. Some of these mysteries and problems have been resolved while others are still outstanding. Proposed solutions to some of the problems in the Big Bang model have revealed new mysteries of their own. For example, the [[horizon problem]], the [[Inflation (cosmology)#Magnetic-monopole problem|magnetic monopole problem]], and the [[flatness problem]] are most commonly resolved with inflation theory, but the details of the inflationary universe are still left unresolved and many, including some founders of the theory, say it has been disproven.<ref>{{cite journal |last1=Earman |first1=John |author1-link=John Earman |last2=Mosterín |first2=Jesús |author2-link=Jesús Mosterín |date=March 1999 |title=A Critical Look at Inflationary Cosmology |journal=[[Philosophy of Science (journal)|Philosophy of Science]] |volume=66 |issue=1 |pages=1–49 |doi=10.1086/392675 |jstor=188736|s2cid=120393154 }}</ref><ref>{{harvnb|Hawking|Israel|2010|pp=581–638|loc=chpt. 12: "Singularities and time-asymmetry" by [[Roger Penrose]].}}</ref><ref>{{harvnb|Penrose|1989}}</ref><ref>{{cite magazine |last=Steinhardt |first=Paul J. |author-link=Paul Steinhardt |date=April 2011 |title=The Inflation Debate: Is the theory at the heart of modern cosmology deeply flawed? |url=https://physics.princeton.edu/~steinh/0411036.pdf |url-status=live |magazine=[[Scientific American]] |doi=10.1038/scientificamerican0411-36 |volume=304 |issue=4 |pages=36–43 |archive-url=https://web.archive.org/web/20191101165817/https://physics.princeton.edu/~steinh/0411036.pdf |archive-date=1 November 2019 |access-date=23 December 2019}}</ref> What follows are a list of the mysterious aspects of the Big Bang concept still under intense investigation by cosmologists and [[Astrophysics|astrophysicists]].

===Baryon asymmetry===
{{Main|Baryon asymmetry}}
It is not yet understood why the universe has more matter than antimatter.<ref name="kolb_c6"/> It is generally assumed that when the universe was young and very hot it was in [[Statistical physics|statistical equilibrium]] and contained equal numbers of baryons and antibaryons. However, observations suggest that the universe, including its most distant parts, is made almost entirely of normal matter, rather than antimatter. A process called baryogenesis was hypothesized to account for the asymmetry. For baryogenesis to occur, the [[Sakharov conditions]] must be satisfied. These require that baryon number is not conserved, that [[C-symmetry]] and [[CP violation|CP-symmetry]] are violated and that the universe depart from [[thermodynamic equilibrium]].<ref name="sakharov">{{cite journal |last=Sakharov |first=Andrei D. |author-link=Andrei Sakharov |date=10 January 1967 |title=Нарушение ''СР''-инвариантности, ''С''-асимметрия и барионная асимметрия Вселенной |trans-title=Violation of ''CP''-invariance, ''C''-asymmetry and baryon asymmetry of the Universe |url=http://www.jetpletters.ac.ru/ps/808/article_12459.pdf |url-status=live |journal=[[Journal of Experimental and Theoretical Physics|Pis'ma v ZhETF]] |language=ru |volume=5 |issue=1 |pages=32–35 |archive-url=https://web.archive.org/web/20180728190714/http://www.jetpletters.ac.ru/ps/808/article_12459.pdf |archive-date=28 July 2018}}</ref><ref>{{cite journal |last=Sakharov |first=Andrei D. |author-link=Andrei Sakharov |date=10 January 1967 |title=Violation of CP Invariance, С Asymmetry, and Baryon Asymmetry of the Universe |url=http://www.jetpletters.ac.ru/ps/1643/article_25089.pdf |url-status=live |journal=[[Journal of Experimental and Theoretical Physics|JETP Letters]] |volume=5 |issue=1 |pages=24–27 |archive-url=https://web.archive.org/web/20191109163819/http://www.jetpletters.ac.ru/ps/1643/article_25089.pdf |archive-date=9 November 2019 |access-date=13 December 2019}}
* Sakharov (1967) translated into English. 
* Reprinted in: {{harvnb|Kolb|Turner|1988|pp=371–373}}.</ref> All these conditions occur in the Standard Model, but the effects are not strong enough to explain the present baryon asymmetry.

===Dark energy===
{{Main|Dark energy}}
Measurements of the redshift–[[apparent magnitude|magnitude]] relation for [[type Ia supernova]]e indicate that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, cosmological models require that much of the energy in the universe consists of a component with large negative pressure, dubbed "dark energy".<ref name="peebles" />

Dark energy, though speculative, solves numerous problems. Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the [[Friedmann equations#Density parameter|critical density]] of mass/energy. But the mass density of the universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density.<ref name="peebles" /> Since theory suggests that dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy also helps to explain two geometrical measures of the overall curvature of the universe, one using the frequency of [[gravitational lens]]es,<ref>{{cite journal | title=Constraining dark energy from the abundance of weak gravitational lenses  | first1=Nevin N. | last1=Weinberg | first2=Marc | last2=Kamionkowski | journal=Monthly Notices of the Royal Astronomical Society | volume=341 | issue=1 | date=May 2003 | pages=251–262 | bibcode=2003MNRAS.341..251W | arxiv=astro-ph/0210134 | doi=10.1046/j.1365-8711.2003.06421.x | doi-access=free | s2cid=1193946 }}</ref> and the other using the characteristic pattern of the large-scale structure--[[baryon acoustic oscillations]]--as a cosmic ruler.<ref>{{cite web |last1=White |first1=Martin |title=Baryon acoustic oscillations and dark energy |url=https://w.astro.berkeley.edu/~mwhite/bao/}}</ref><ref>
{{cite journal | title=Completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Cosmological implications from two decades of spectroscopic surveys at the Apache Point Observatory | first1=Shadab | last1=Alam | display-authors=etal | journal=Physical Review D | volume=103 | issue=8 | date=April 2021 | page=083533 | bibcode=2021PhRvD.103h3533A | arxiv=2007.08991 | doi=10.1103/PhysRevD.103.083533}}</ref>

Negative pressure is believed to be a property of [[vacuum energy]], but the exact nature and existence of dark energy remains one of the great mysteries of the Big Bang. Results from the WMAP team in 2008 are in accordance with a universe that consists of 73% dark energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos.<ref name="wmap7year" /> According to theory, the energy density in matter decreases with the expansion of the universe, but the dark energy density remains constant (or nearly so) as the universe expands. Therefore, matter made up a larger fraction of the total energy of the universe in the past than it does today, but its fractional contribution will fall in the [[far future]] as dark energy becomes even more dominant.{{citation needed|date=February 2023}}

The dark energy component of the universe has been explained by theorists using a variety of competing theories including Einstein's cosmological constant but also extending to more exotic forms of [[Quintessence (physics)|quintessence]] or other modified gravity schemes.<ref>{{harvnb|Tanabashi, M.|2018|pp=[http://pdg.lbl.gov/2018/reviews/rpp2018-rev-dark-energy.pdf 406–413]|loc=chpt. 27: "Dark Energy" (Revised September 2017) by David H. Weinberg and Martin White.}}

* {{harvnb|Olive|2014|pp=[http://pdg.lbl.gov/2014/reviews/rpp2014-rev-dark-energy.pdf 361–368]|loc=chpt. 26: "Dark Energy" (November 2013) by Michael J. Mortonson, David H. Weinberg, and Martin White.}} {{bibcode|2014arXiv1401.0046M}}</ref> A [[cosmological constant problem]], sometimes called the "most embarrassing problem in physics", results from the apparent discrepancy between the measured energy density of dark energy, and the one naively predicted from [[Planck units]].<ref>{{cite journal |last1=Rugh |first1=Svend E. |last2=Zinkernagel |first2=Henrik |title=The quantum vacuum and the cosmological constant problem |pages=663–705 |volume=33 |issue=4 |date=December 2002 |journal=[[Studies in History and Philosophy of Science Part B]] |arxiv=hep-th/0012253 |bibcode=2002SHPMP..33..663R |doi=10.1016/S1355-2198(02)00033-3 |s2cid=9007190 }}</ref>

===Dark matter===
{{Main|Dark matter}}
[[File:Cosmological Composition – Pie Chart.svg|thumb|right|upright=1.6|[[pie chart|Chart]] shows the proportion of different components of the universe {{spaced ndash}} about 95% is [[dark matter]] and [[dark energy]].]]
During the 1970s and the 1980s, various observations showed that there is not sufficient visible matter in the universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the universe is dark matter that does not emit light or interact with normal baryonic matter. In addition, the assumption that the universe is mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the universe today is far more lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter has always been controversial, it is inferred by various observations: the anisotropies in the CMB, the [[galaxy rotation problem]], [[galaxy cluster]] [[velocity dispersion]]s, large-scale structure distributions, [[gravitational lens]]ing studies, and [[X-ray astronomy|X-ray measurements]] of galaxy clusters.<ref>{{cite web |url=http://pages.astronomy.ua.edu/keel/galaxies/darkmatter.html |url-status=live |last=Keel |first=William C. |date=October 2009 |orig-date=Last changes: February 2015 |title=Dark Matter |website=Bill Keel's Lecture Notes – Galaxies and the Universe |archive-url=https://web.archive.org/web/20190503112916/http://pages.astronomy.ua.edu/keel/galaxies/darkmatter.html |archive-date=3 May 2019 |access-date=15 December 2019}}</ref>

Indirect evidence for dark matter comes from its gravitational influence on other matter, as no dark matter particles have been observed in laboratories. Many particle physics candidates for dark matter have been proposed, and several projects to detect them directly are underway.<ref name="pdg">{{harvnb|Tanabashi, M.|2018|pp=[http://pdg.lbl.gov/2018/reviews/rpp2018-rev-dark-matter.pdf 396–405]|loc=chpt. 26: "Dark Matter" (Revised September 2017) by Manuel Drees and Gilles Gerbier.}}
* {{harvnb|Yao, W.-M.|2006|pp=[http://pdg.lbl.gov/2006/reviews/darkmatrpp.pdf 233–237]|loc=chpt. 22: "Dark Matter" (September 2003) by Manuel Drees and Gilles Gerbier.}}</ref>

Additionally, there are outstanding problems associated with the currently favored cold dark matter model which include the [[dwarf galaxy problem]]<ref name="Martínez-Delgado">{{Cite book |arxiv= 1009.4505|last1 = Bullock|first1 = James S.|title = Local Group Cosmology|chapter= Notes on the Missing Satellites Problem |pages = 95–122|year = 2010 |doi=10.1017/CBO9781139152303.004|isbn = 9781139152303|s2cid = 119270708|editor1-last = Martinez-Delgado|editor1-first = David|editor2-last = Mediavilla|editor2-first = Evencio}}</ref> and the [[cuspy halo problem]].<ref name="Diemand2005">{{cite journal |last1=Diemand |first1=Jürg |last2=Zemp |first2=Marcel |last3=Moore |first3=Ben |last4=Stadel |first4=Joachim |last5=Carollo |first5=C. Marcella |author-link5=C. Marcella Carollo |date=December 2005 |title=Cusps in cold dark matter haloes |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=364 |issue=2 |pages=665–673 |arxiv=astro-ph/0504215 |bibcode=2005MNRAS.364..665D |doi=10.1111/j.1365-2966.2005.09601.x |doi-access=free |s2cid=117769706 }}</ref> Alternative theories have been proposed that do not require a large amount of undetected matter, but instead modify the laws of gravity established by Newton and Einstein; yet no alternative theory has been as successful as the cold dark matter proposal in explaining all extant observations.<ref>{{cite journal |last1=Dodelson |first1=Scott |date=31 December 2011 |title=The Real Problem with MOND |journal=[[International Journal of Modern Physics D]] |arxiv=1112.1320 |doi=10.1142/S0218271811020561 |volume=20 |issue=14 |pages=2749–2753 |bibcode=2011IJMPD..20.2749D |s2cid=119194106 }}</ref>

===Horizon problem===
{{Main|Horizon problem}}
The horizon problem results from the premise that information cannot travel [[Faster-than-light|faster than light]]. In a universe of finite age this sets a limit—the particle horizon—on the separation of any two regions of space that are in [[causality (physics)|causal]] contact.<ref name="kolb_c8">{{harvnb|Kolb|Turner|1988|loc=chpt. 8}}</ref> The observed isotropy of the CMB is problematic in this regard: if the universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause wider regions to have the same temperature.<ref name="Ryden2003">{{harvnb|Ryden|2003}}</ref>{{rp|191–202}}

A resolution to this apparent inconsistency is offered by inflation theory in which a homogeneous and isotropic [[Scalar field|scalar energy field]] dominates the universe at some very early period (before baryogenesis). During inflation, the universe undergoes [[Exponential growth|exponential]] expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable universe are well inside each other's particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation.<ref name="Guth1998" />{{rp|180–186}}

Heisenberg's uncertainty principle predicts that during the inflationary phase there would be [[primordial fluctuations|quantum thermal fluctuations]], which would be magnified to a cosmic scale. These fluctuations served as the seeds for all the current structures in the universe.<ref name="Ryden2003"/>{{rp|207}} Inflation predicts that the primordial fluctuations are nearly [[Scale invariance|scale invariant]] and [[Normal distribution|Gaussian]], which has been confirmed by measurements of the CMB.<ref name="wmap1year" />{{rp|sec 6}}

A related issue to the classic horizon problem arises because in most standard cosmological inflation models, inflation ceases well before [[Higgs mechanism|electroweak symmetry breaking]] occurs, so inflation should not be able to prevent large-scale discontinuities in the [[False vacuum|electroweak vacuum]] since distant parts of the observable universe were causally separate when the [[electroweak epoch]] ended.<ref>{{harvnb|Penrose|2007}}</ref>

===Magnetic monopoles===
The magnetic monopole objection was raised in the late 1970s. [[Grand Unified Theory|Grand unified theories]] (GUTs) predicted [[topological defect]]s in space that would manifest as [[magnetic monopole]]s. These objects would be produced efficiently in the hot early universe, resulting in a density much higher than is consistent with observations, given that no monopoles have been found. This problem is resolved by cosmic inflation, which removes all point defects from the observable universe, in the same way that it drives the geometry to flatness.<ref name="kolb_c8"/>

===Flatness problem===
[[File:End of universe.jpg|thumb|upright=1.5|The overall [[Shape of the universe|geometry of the universe]] is determined by whether the [[Friedmann equations#Density parameter|Omega cosmological parameter]] is less than, equal to or greater than 1. Shown from top to bottom are a [[Shape of the universe#Universe with positive curvature|closed universe]] with positive curvature, a [[Shape of the universe#Universe with negative curvature|hyperbolic universe]] with negative curvature and a [[Shape of the universe#Universe with zero curvature|flat universe]] with zero curvature.]]

The flatness problem (also known as the oldness problem) is an observational problem associated with a FLRW.<ref name="kolb_c8"/> The universe may have positive, negative, or zero spatial [[curvature]] depending on its total energy density. Curvature is negative if its density is less than the critical density; positive if greater; and zero at the critical density, in which case space is said to be ''flat''. Observations indicate the universe is consistent with being flat.<ref name=Filippenko2002>{{cite magazine |last1=Filippenko |first1=Alexei V. |author1-link=Alex Filippenko |last2=Pasachoff |first2=Jay M. |author2-link=Jay Pasachoff |date=March–April 2002 |title=A Universe from Nothing |url=http://www.astrosociety.org/pubs/mercury/31_02/nothing.html |url-status=dead |magazine=[[Mercury (magazine)|Mercury]] |volume=31 |issue=2 |page=15 |bibcode=2002Mercu..31b..15F |access-date=10 March 2010 |archive-url=https://web.archive.org/web/20131022135932/http://www.astrosociety.org/pubs/mercury/31_02/nothing.html |archive-date=22 October 2013}}</ref><ref name="Krauss2009">{{cite AV media |url=https://www.youtube.com/watch?v=7ImvlS8PLIo |title='A Universe From Nothing' by Lawrence Krauss, AAI 2009 |date=21 October 2009 |medium=Video |language=en-us |publisher=[[Richard Dawkins Foundation for Reason and Science]] |location=Washington, D.C. |access-date=17 October 2011 |archive-url=https://ghostarchive.org/varchive/youtube/20211123/7ImvlS8PLIo |archive-date=2021-11-23 |url-status=live |people=[[Lawrence M. Krauss]] (Speaker); R. Elisabeth Cornwell (Producer)}}{{cbignore}}</ref>

The problem is that any small departure from the critical density grows with time, and yet the universe today remains very close to flat.<ref group="notes">Strictly, dark energy in the form of a cosmological constant drives the universe towards a flat state; however, our universe remained close to flat for several billion years before the dark energy density became significant.</ref> Given that a natural timescale for departure from flatness might be the [[Planck time]], 10<sup>−43</sup> seconds,<ref name="HTUW"/> the fact that the universe has reached neither a [[heat death of the universe|heat death]] nor a [[Big Crunch]] after billions of years requires an explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the density of the universe must have been within one part in 10<sup>14</sup> of its critical value, or it would not exist as it does today.<ref>{{harvnb|Hawking|Israel|2010|pp=504–517|loc=chpt. 9: "The big bang cosmology — enigmas and nostrums" by [[Robert H. Dicke]] and [[Jim Peebles|Phillip J.E. Peebles]].}}</ref>