Editing Physical cosmology (section)

==Areas of study==
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 in ''[[Timeline of the Big Bang]].''

===Very early universe===
[[File:Big bang inflation vs standard genericchart.png|thumb|upright=1.2|The inflationary theory as an augmentation to the Big Bang theory was first proposed by Alan Guth of MIT. Inflation solves the 'horizon problem' by making the early universe much more compact than was assumed in the standard model. Given such smaller size, causal contact (i.e., thermal communication) would have been possible among all regions of the early universe. The image was an adaptation from various generic charts depicting the growth of the size of the observable universe, for both the standard model and inflationary model respectively, of the Big Bang theory.]]
The early, hot universe appears to be well explained by the Big Bang from roughly 10<sup>−33</sup> seconds onwards, but there are several [[Big Bang#Problems|problems]]. One is that there is no compelling reason, using current particle physics, for the universe to be [[shape of the universe|flat]], homogeneous, and [[isotropic]] ''(see the [[cosmological principle]])''. Moreover, [[grand unified theory|grand unified theories]] of particle physics suggest that there should be [[magnetic monopole]]s in the universe, which have not been found. These problems are resolved by a brief period of [[cosmic inflation]], which drives the universe to [[Flatness (cosmology)|flatness]], smooths out [[Anisotropy|anisotropies]] and inhomogeneities to the observed level, and exponentially dilutes the monopoles.<ref name=Guth1981>{{cite journal | title=Inflationary universe: A possible solution to the horizon and flatness problems | last1=Guth | first1=Alan H. | journal=Physical Review D | volume=23 | issue=2 | date=15 January 1981 | pages=347–356 | doi=10.1103/PhysRevD.23.347 | bibcode=1981PhRvD..23..347G | doi-access=free }}</ref> 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 and [[quantum field theory]].{{Vague|date=April 2018}} Some cosmologists think that [[string theory]] and [[brane cosmology]] will provide an alternative to inflation.<ref name=Pogosian2003>{{cite journal | title=Observational constraints on cosmic string production during brane inflation | last1=Pogosian | first1=Levon | last2=Tye | first2=S.-H. Henry | last3=Wasserman | first3=Ira | last4=Wyman | first4=Mark | journal=Physical Review D | volume=68 | issue=2 | page=023506 | year=2003 | doi=10.1103/PhysRevD.68.023506 | bibcode=2003PhRvD..68b3506P | arxiv=hep-th/0304188 }}</ref>

Another major problem in cosmology is what caused the universe to contain far more matter than [[antimatter]]. Cosmologists can observationally deduce that the universe is not split into regions of matter and antimatter. If it were, there would be [[X-ray]]s and [[gamma ray]]s produced as a result of [[annihilation]], 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 called ''[[baryogenesis]]''. Three required conditions for baryogenesis were derived by [[Andrei Sakharov]] in 1967, and requires a violation of the particle physics [[Symmetry#In physics|symmetry]], called [[CP-symmetry]], between matter and antimatter.<ref name=Canetti2012>{{cite journal | title=Matter and antimatter in the universe | display-authors=1 | first1=Laurent | last1=Canetti | first2=Marco | last2=Drewes | first3=Mikhail | last3=Shaposhnikov | journal=New Journal of Physics | volume=14 | issue=9 | pages=095012 | date=September 2012 | doi=10.1088/1367-2630/14/9/095012 | bibcode=2012NJPh...14i5012C | arxiv=1204.4186 | s2cid=119233888 }}</ref> 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 the [[baryon asymmetry]].<ref name=Pandolfi2017>{{cite web | title=New source of asymmetry between matter and antimatter | first1=Stefania | last1=Pandolfi | date=30 January 2017 | publisher=CERN | url=https://home.cern/about/updates/2017/01/new-source-asymmetry-between-matter-and-antimatter | access-date=2018-04-09 }}</ref>

Both the problems of baryogenesis and cosmic inflation are very closely related to particle physics, and their resolution might come from high energy theory and [[particle accelerator|experiment]], rather than through observations of the universe.{{speculation inline|date=April 2018}}<!-- This statement is both speculative and somewhat obvious. It will either be one, the other, or none. Is it needed? -->

===Big Bang Theory===
{{Main|Big Bang nucleosynthesis}}
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 its [[temperature]] dropped below that at which [[nuclear 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 hydrogen [[ion]]s ([[proton]]s), it principally produced [[deuterium]], [[helium|helium-4]], and [[lithium]]. Other elements were produced in only trace abundances. The basic theory of nucleosynthesis was developed in 1948 by George Gamow, [[Ralph Asher Alpher]], and [[Robert Herman]].<ref name=Peebles2014>{{cite journal | title=Discovery of the hot Big Bang: What happened in 1948 | last1=Peebles | first1=Phillip James Edwin | journal=The European Physical Journal H | volume=39 | issue=2 | pages=205–223 | date=April 2014 | doi=10.1140/epjh/e2014-50002-y | bibcode=2014EPJH...39..205P | arxiv=1310.2146 | s2cid=118539956 }}</ref> 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.<ref name=Burles2001>{{cite journal | title=Big Bang Nucleosynthesis Predictions for Precision Cosmology | last1=Burles | first1=Scott | last2=Nollett | first2=Kenneth M. | last3=Turner | first3=Michael S. | journal=The Astrophysical Journal | volume=552 | issue=1 | pages=L1–L5 | date=May 2001 | doi=10.1086/320251 | bibcode=2001ApJ...552L...1B | arxiv=astro-ph/0010171 | s2cid=118904816 }}</ref> Specifically, it can be used to test the [[equivalence principle]],<ref name=Boucher2004>{{cite journal | title=Cosmic microwave background constraints on the strong equivalence principle | last1=Boucher | first1=V. | last2=Gérard | first2=J.-M. | last3=Vandergheynst | first3=P. | last4=Wiaux | first4=Y. | journal=Physical Review D | volume=70 | issue=10 | page=103528 | date=November 2004 | doi=10.1103/PhysRevD.70.103528 | bibcode=2004PhRvD..70j3528B | arxiv=astro-ph/0407208 | s2cid=1197376 }}</ref> to probe [[dark matter]], and test [[neutrino]] physics.<ref name=Cyburt2016>{{cite journal | title=Big bang nucleosynthesis: Present status | last1=Cyburt | first1=Richard H. | last2=Fields | first2=Brian D. | last3=Olive | first3=Keith A. | last4=Yeh | first4=Tsung-Han | journal=Reviews of Modern Physics | volume=88 | issue=1 | page=015004 | date=January 2016 | doi=10.1103/RevModPhys.88.015004 | bibcode=2016RvMP...88a5004C | arxiv=1505.01076 | s2cid=118409603}}</ref> Some cosmologists have proposed that Big Bang nucleosynthesis suggests there is a fourth "sterile" species of neutrino.<ref name=Lucente2018>{{cite arXiv| title=Leptogenesis, dark matter and neutrino masses | last1=Lucente | first1=Michele | last2=Abada | first2=Asmaa | last3=Arcadi | first3=Giorgio | last4=Domcke | first4=Valerie  | eprint=1803.10826 | date=March 2018| class=hep-ph }}</ref>

====Standard model of Big Bang cosmology====
The '''ΛCDM''' ('''Lambda cold dark matter''') or '''[[Lambda-CDM]]''' model is a [[Parametric equation|parametrization]] of the Big Bang cosmological model in which the universe contains a cosmological constant, denoted by [[Lambda]] ([[Greek alphabet|Greek]] '''Λ'''), associated with dark energy, and [[cold dark matter]] (abbreviated '''CDM'''). It is frequently referred to as the '''standard model''' of Big Bang cosmology.<ref name=Planck2015>{{cite journal | title=Planck 2015 Results. XIII. Cosmological Parameters | arxiv=1502.01589 | last1=Collaboration | first1=Planck | last2=Ade | first2=P. A. R. | last3=Aghanim | first3=N.|author3-link=Nabila Aghanim | last4=Arnaud | first4=M. | last5=Ashdown | first5=M. |last6=Aumont  | first6=J. | last7=Baccigalupi | first7=C. | last8=Banday | first8=A. J. | last9=Barreiro | first9=R. B. | last10=Bartlett | first10=J. G. | last11=Bartolo | first11=N. | last12=Battaner | first12=E. | last13=Battye | first13=R. | last14=Benabed | first14=K. | last15=Benoit | first15=A. | last16=Benoit-Levy | first16=A. | last17=Bernard | first17=J. -P. | last18=Bersanelli | first18=M.  | last19=Bielewicz | first19=P. | last20=Bonaldi | first20=A. | last21=Bonavera | first21=L. | last22=Bond | first22=J. R. | last23=Borrill | first23=J. | last24=Bouchet | first24=F. R. | last25=Boulanger | first25=F. | last26=Bucher | first26=M. | last27=Burigana | first27=C. | last28=Butler | first28=R. C. | last29=Calabrese | first29=E. | last30=Cardoso | first30=J.-F. | display-authors=29 | year=2016 | doi=10.1051/0004-6361/201525830 | volume=594 | issue=13 | journal=Astronomy & Astrophysics | page=A13 | bibcode=2016A&A...594A..13P | s2cid=119262962 }}</ref><ref name=Carlisle>{{cite journal | title=Planck Upholds Standard Cosmology | first1=Camille M. | last1=Carlisle  | journal=Sky and Telescope | date=10 February 2015 | volume=130 | issue=1 | page=28 | publisher=Sky & Telescope Media | bibcode=2015S&T...130a..28C | url=http://www.skyandtelescope.com/astronomy-news/planck-upholds-standard-cosmology-0210201523/ | access-date=2018-04-09 }}</ref>

===Cosmic microwave background===
{{Main|Cosmic microwave background}}
The cosmic microwave background is radiation left over from [[Decoupling (cosmology)|decoupling]] after the epoch of [[recombination (cosmology)|recombination]] when neutral atoms first formed. At this point, radiation produced in the Big Bang stopped [[Thomson scattering]] from charged ions. The radiation, first observed in 1965 by [[Arno Penzias]] and [[Robert Woodrow Wilson]], has a perfect thermal [[black body|black-body]] spectrum. It has a temperature of 2.7 [[kelvin]]s today and is isotropic to one part in 10<sup>5</sup>. [[Cosmological perturbation theory]], which describes the evolution of slight inhomogeneities in the early universe, has allowed cosmologists to precisely calculate the angular [[power spectrum]] of the radiation, and it has been measured by the recent satellite experiments ([[Cosmic Background Explorer|COBE]] and [[WMAP]])<ref name=Lamarre2010>{{cite book | chapter=The Cosmic Microwave Background | last1=Lamarre | first1=Jean-Michel | title=Observing Photons in Space | volume=9 | editor1-first=M. C. E. | editor1-last=Huber | editor2-first=A. | editor2-last=Pauluhn | editor3-first=J. L. | editor3-last=Culhane | editor4-first=J. G. | editor4-last=Timothy | editor5-first=K. | editor5-last=Wilhelm | editor6-first=A. | editor6-last=Zehnder | series=ISSI Scientific Reports Series | year=2010 | pages=149–162 | bibcode=2010ISSIR...9..149L }}</ref> and many ground and balloon-based experiments (such as [[Degree Angular Scale Interferometer]], [[Cosmic Background Imager]], and [[BOOMERanG experiment|Boomerang]]).<ref name=Sievers2003>{{cite journal | title=Cosmological Parameters from Cosmic Background Imager Observations and Comparisons with BOOMERANG, DASI, and MAXIMA | display-authors=1 | last1=Sievers | first1=J. L. | last2=Bond | first2=J. R. | last3=Cartwright | first3=J. K. | last4=Contaldi | first4=C. R. | last5=Mason | first5=B. S. | last6=Myers | first6=S. T. | last7=Padin | first7=S. | last8=Pearson | first8=T. J. | last9=Pen | first9=U.-L. | last10=Pogosyan | first10=D. | last11=Prunet | first11=S. | last12=Readhead | first12=A. C. S. | last13=Shepherd | first13=M. C. | last14=Udomprasert | first14=P. S. | last15=Bronfman | first15=L. | last16=Holzapfel | first16=W. L. | last17=May | first17=J. | journal=The Astrophysical Journal | volume=591 | issue=2 | pages=599–622 | year=2003 | doi=10.1086/375510 | bibcode=2003ApJ...591..599S | arxiv=astro-ph/0205387 | s2cid=14939106 }}</ref> 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.<ref name=Hinshaw2013>{{cite journal | title=Nine-year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results | display-authors=1 | last1=Hinshaw | first1=G. | last2=Larson | first2=D. | last3=Komatsu | first3=E. | last4=Spergel | first4=D. N. | last5=Bennett | first5=C. L. | last6=Dunkley | first6=J. | last7=Nolta | first7=M. R. | last8=Halpern | first8=M. | last9=Hill | first9=R. S. | last10=Odegard | first10=N. | last11=Page | first11=L. | last12=Smith | first12=K. M. | last13=Weiland | first13=J. L. | last14=Gold | first14=B. | last15=Jarosik | first15=N. | last16=Kogut | first16=A. | last17=Limon | first17=M. | last18=Meyer | first18=S. S. | last19=Tucker | first19=G. S. | last20=Wollack | first20=E. | last21=Wright | first21=E. L. | journal=The Astrophysical Journal Supplement | volume=208 | issue=2 | page=19 | date=October 2013 | doi=10.1088/0067-0049/208/2/19 | bibcode=2013ApJS..208...19H | arxiv=1212.5226 | s2cid=37132863 }}</ref>

Newer experiments, such as [[QUIET]] and the [[Atacama Cosmology Telescope]], are trying to measure the [[polarization (waves)|polarization]] of the cosmic microwave background.<ref name=Naess2014>{{cite journal | title=The Atacama Cosmology Telescope: CMB polarization at 200 < l < 9000 | display-authors=4 | last1=Naess | first1=Sigurd | last2=Hasselfield | first2=Matthew | last3=McMahon | first3=Jeff | last4=Niemack | first4=Michael D. | last5=Addison | first5=Graeme E. | last6=Ade | first6=Peter A. R. | last7=Allison | first7=Rupert | last8=Amiri | first8=Mandana | last9=Battaglia | first9=Nick | last10=Beall | first10=James A. | last11=de Bernardis | first11=Francesco | last12=Bond | first12=J. Richard | last13=Britton | first13=Joe | last14=Calabrese | first14=Erminia | last15=Cho | first15=Hsiao-mei | last16=Coughlin | first16=Kevin | last17=Crichton | first17=Devin | last18=Das | first18=Sudeep | last19=Datta | first19=Rahul | last20=Devlin | first20=Mark J. | last21=Dicker | first21=Simon R. | last22=Dunkley | first22=Joanna | last23=Dünner | first23=Rolando | last24=Fowler | first24=Joseph W. | last25=Fox | first25=Anna E. | last26=Gallardo | first26=Patricio | last27=Grace | first27=Emily | last28=Gralla | first28=Megan | last29=Hajian | first29=Amir | last30=Halpern | first30=Mark | last31=Henderson | first31=Shawn | last32=Hill | first32=J. Colin | last33=Hilton | first33=Gene C. | last34=Hilton | first34=Matt | last35=Hincks | first35=Adam D. | last36=Hlozek | first36=Renée | last37=Ho | first37=Patty | last38=Hubmayr | first38=Johannes | last39=Huffenberger | first39=Kevin M. | last40=Hughes | first40=John P. | last41=Infante | first41=Leopoldo | last42=Irwin | first42=Kent | last43=Jackson | first43=Rebecca | last44=Muya Kasanda | first44=Simon | last45=Klein | first45=Jeff | last46=Koopman | first46=Brian | last47=Kosowsky | first47=Arthur | last48=Li | first48=Dale | last49=Louis | first49=Thibaut | last50=Lungu | first50=Marius | last51=Madhavacheril | first51=Mathew | last52=Marriage | first52=Tobias A. | last53=Maurin | first53=Loïc | last54=Menanteau | first54=Felipe | last55=Moodley | first55=Kavilan | last56=Munson | first56=Charles | last57=Newburgh | first57=Laura | last58=Nibarger | first58=John | last59=Nolta | first59=Michael R. | last60=Page | first60=Lyman A. | last61=Pappas | first61=Christine | last62=Partridge | first62=Bruce | last63=Rojas | first63=Felipe | last64=Schmitt | first64=Benjamin L. | last65=Sehgal | first65=Neelima | last66=Sherwin | first66=Blake D. | last67=Sievers | first67=Jon | last68=Simon | first68=Sara | last69=Spergel | first69=David N. | last70=Staggs | first70=Suzanne T. | last71=Switzer | first71=Eric R. | last72=Thornton | first72=Robert | last73=Trac | first73=Hy | last74=Tucker | first74=Carole | last75=Uehara | first75=Masao | last76=Van Engelen | first76=Alexander | last77=Ward | first77=Jonathan T. | last78=Wollack | first78=Edward J. | journal=Journal of Cosmology and Astroparticle Physics | volume=2014 | issue=10 | pages=007 | date=October 2014 | doi=10.1088/1475-7516/2014/10/007 | bibcode=2014JCAP...10..007N | arxiv=1405.5524 | s2cid=118593572 }}</ref> These measurements are expected to provide further confirmation of the theory as well as information about cosmic inflation, and the so-called secondary anisotropies,<ref name=Jackson2009>{{Cite conference |title=Probing Inflation with CMB Polarization | display-authors=1 | last1=Baumann | first1=Daniel | last2=Jackson | first2=Mark G. | last3=Adshead | first3=Peter | last4=Amblard | first4=Alexandre | last5=Ashoorioon | first5=Amjad | last6=Bartolo | first6=Nicola | last7=Bean | first7=Rachel | last8=Beltrán | first8=Maria | last9=de Bernardis | first9=Francesco | last10=Bird | first10=Simeon | last11=Chen | first11=Xingang | last12=Chung | first12=Daniel J. H. | last13=Colombo | first13=Loris | last14=Cooray | first14=Asantha | last15=Creminelli | first15=Paolo | last16=Dodelson | first16=Scott | last17=Dunkley | first17=Joanna | last18=Dvorkin | first18=Cora | last19=Easther | first19=Richard | last20=Finelli | first20=Fabio | last21=Flauger | first21=Raphael | last22=Hertzberg | first22=Mark P. | last23=Jones-Smith | first23=Katherine | last24=Kachru | first24=Shamit | last25=Kadota | first25=Kenji | last26=Khoury | first26=Justin | last27=Kinney | first27=William H. | last28=Komatsu | first28=Eiichiro | last29=Krauss | first29=Lawrence M. | last30=Lesgourgues | first30=Julien | last31=Liddle | first31=Andrew | last32=Liguori | first32=Michele | last33=Lim | first33=Eugene | last34=Linde | first34=Andrei | last35=Matarrese | first35=Sabino | last36=Mathur | first36=Harsh | last37=McAllister | first37=Liam | last38=Melchiorri | first38=Alessandro | last39=Nicolis | first39=Alberto | last40=Pagano | first40=Luca | last41=Peiris | first41=Hiranya V. | last42=Peloso | first42=Marco | last43=Pogosian | first43=Levon | last44=Pierpaoli | first44=Elena | last45=Riotto | first45=Antonio | last46=Seljak | first46=Uroš | last47=Senatore | first47=Leonardo | last48=Shandera | first48=Sarah | last49=Silverstein | first49=Eva | last50=Smith | first50=Tristan | last51=Vaudrevange | first51=Pascal | last52=Verde | first52=Licia | last53=Wandelt | first53=Ben | last54=Wands | first54=David | last55=Watson | first55=Scott | last56=Wyman | first56=Mark | last57=Yadav | first57=Amit | last58=Valkenburg | first58=Wessel | last59=Zaldarriaga | first59=Matias |book-title=CMB Polarization Workshop: Theory and Foregrounds: CMBPol Mission Concept Study | volume=1141 | pages=10–120 | year=2009 | doi=10.1063/1.3160885 | bibcode=2009AIPC.1141...10B | arxiv=0811.3919 }}</ref> such as the [[Sunyaev-Zel'dovich effect]] and [[Sachs-Wolfe effect]], which are caused by interaction between [[galaxy|galaxies]] and [[galaxy cluster|clusters]] with the cosmic microwave background.<ref name=Scranton2003>{{cite arXiv | title=Physical Evidence for Dark Energy | display-authors=6 | last1=Scranton | first1=R. | last2=Connolly | first2=A. J. | last3=Nichol | first3=R. C. | last4=Stebbins | first4=A. | last5=Szapudi | first5=I. | last6=Eisenstein | first6=D. J. | last7=Afshordi | first7=N. | last8=Budavari | first8=T. | last9=Csabai | first9=I. | last10=Frieman | first10=J. A. | last11=Gunn | first11=J. E. | last12=Johnston | first12=D. | last13=Loh | first13=Y. | last14=Lupton | first14=R. H. | last15=Miller | first15=C. J. | last16=Sheldon | first16=E. S. | last17=Sheth | first17=R. S. | last18=Szalay | first18=A. S. | last19=Tegmark | first19=M. | last20=Xu | first20=Y. | eprint=astro-ph/0307335 | date=July 2003}}</ref><ref name=Refregier1999>{{cite book | chapter=Overview of Secondary Anisotropies of the CMB | last1=Refregier | first1=A. | title=Microwave Foregrounds | series=ASP Conference Series | volume=181 | editor1-first=A. | editor1-last=de Oliveira-Costa | editor2-first=M. | editor2-last=Tegmark | isbn=978-1-58381-006-4 | page=219 | year=1999 | bibcode=1999ASPC..181..219R | arxiv=astro-ph/9904235 }}</ref>

On 17 March 2014, astronomers of the [[BICEP and Keck Array#BICEP2|BICEP2 Collaboration]] announced the apparent detection of [[B-modes|''B''-mode]] polarization of the CMB, considered to be evidence of [[primordial gravitational wave]]s that are predicted by the theory of [[inflation (cosmology)|inflation]] to occur during the earliest phase of the Big Bang.<ref name="BICEP2-2014">{{cite web |title=BICEP2 2014 Results Release |url=http://bicepkeck.org/bicep2_2014_release.html |date=17 March 2014 |website=The BICEP / Keck CMB Experiments |access-date=18 March 2014 }}</ref><ref name="NASA-20140317">{{cite web |last=Clavin |first=Whitney |title=NASA Technology Views Birth of the Universe |url=http://www.jpl.nasa.gov/news/news.php?release=2014-082 |date=17 March 2014 |website=NASA |access-date=17 March 2014 }}</ref><ref name="NYT-20140317">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye
|title=Detection of Waves in Space Buttresses Landmark Theory of Big Bang
|url=https://www.nytimes.com/2014/03/18/science/space/detection-of-waves-in-space-buttresses-landmark-theory-of-big-bang.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2014/03/18/science/space/detection-of-waves-in-space-buttresses-landmark-theory-of-big-bang.html |archive-date=2022-01-01 |url-access=limited |date=17 March 2014 |work=[[The New York Times]] |access-date=17 March 2014 }}{{cbignore}}</ref><ref name="NYT-20140324">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Ripples From the Big Bang |url=https://www.nytimes.com/2014/03/25/science/space/ripples-from-the-big-bang.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2014/03/25/science/space/ripples-from-the-big-bang.html |archive-date=2022-01-01 |url-access=limited |date=25 March 2014 |work=[[The New York Times]] |access-date=24 March 2014 }}{{cbignore}}</ref> However, later that year the [[Planck spacecraft|Planck]] collaboration provided a more accurate measurement of [[cosmic dust]], concluding that the B-mode signal from dust is the same strength as that reported from BICEP2.<ref name="AXV-20140919">{{cite journal
 |author=Planck Collaboration
 |year=2016
 |title=Planck intermediate results. XXX. The angular power spectrum of polarized dust emission at intermediate and high Galactic latitudes
 |arxiv=1409.5738
 |doi=10.1051/0004-6361/201425034
 |volume=586
 |issue=133
 |journal=Astronomy & Astrophysics
 |page=A133
|bibcode = 2016A&A...586A.133P |s2cid=9857299
 }}</ref><ref name="NYT-20140922">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |date=22 September 2014 |title=Study Confirms Criticism of Big Bang Finding |url=https://www.nytimes.com/2014/09/23/science/space/study-confirms-criticism-of-big-bang-finding.html |url-access=limited |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2014/09/23/science/space/study-confirms-criticism-of-big-bang-finding.html |archive-date=2022-01-01 |access-date=2014-09-22 |work=[[The New York Times]]}}{{cbignore}}</ref> On 30 January 2015, a joint analysis of BICEP2 and [[Planck (spacecraft)|Planck]] data was published and the [[European Space Agency]] announced that the signal can be entirely attributed to interstellar dust in the Milky Way.<ref name="nature-20150130">{{cite news|last=Cowen|first=Ron|title=Gravitational waves discovery now officially dead|date=30 January 2015|newspaper=nature|doi=10.1038/nature.2015.16830}}<!--|access-date=2015-10-26--></ref>

===Formation and evolution of large-scale structure===
{{Main|Large-scale structure of the cosmos|Structure formation|Galaxy formation and evolution}}
Understanding the formation and evolution of the largest and earliest structures (i.e., quasars, galaxies, [[galaxy groups and clusters|clusters]] and [[supercluster]]s) is one of the largest efforts in cosmology. Cosmologists study a model of '''hierarchical 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.<ref name=Hess2013>{{cite journal | title=Simulating structure formation of the Local Universe | last1=Heß | first1=Steffen | last2=Kitaura | first2=Francisco-Shu | last3=Gottlöber | first3=Stefan | journal=Monthly Notices of the Royal Astronomical Society | volume=435 | issue=3 | pages=2065–2076 | date=November 2013 | doi=10.1093/mnras/stt1428 | doi-access=free | bibcode=2013MNRAS.435.2065H | arxiv=1304.6565 | s2cid=119198359 }}</ref> 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 matter [[power spectrum]]. This is the approach of the ''[[Sloan Digital Sky Survey]]'' and the [[2dF Galaxy Redshift Survey]].<ref name=Cole2005>{{cite journal | title=The 2dF Galaxy Redshift Survey: power-spectrum analysis of the final data set and cosmological implications | display-authors=6 | last1=Cole | first1=Shaun | last2=Percival | first2=Will J. | last3=Peacock | first3=John A. | last4=Norberg | first4=Peder | last5=Baugh | first5=Carlton M. | last6=Frenk | first6=Carlos S. | last7=Baldry | first7=Ivan | last8=Bland-Hawthorn | first8=Joss | last9=Bridges | first9=Terry | last10=Cannon | first10=Russell | last11=Colless | first11=Matthew | last12=Collins | first12=Chris | last13=Couch | first13=Warrick | last14=Cross | first14=Nicholas J. G. | last15=Dalton | first15=Gavin | last16=Eke | first16=Vincent R. | last17=De Propris | first17=Roberto | last18=Driver | first18=Simon P. | last19=Efstathiou | first19=George | last20=Ellis | first20=Richard S. | last21=Glazebrook | first21=Karl | last22=Jackson | first22=Carole | last23=Jenkins | first23=Adrian | last24=Lahav | first24=Ofer | last25=Lewis | first25=Ian | last26=Lumsden | first26=Stuart | last27=Maddox | first27=Steve | last28=Madgwick | first28=Darren | last29=Peterson | first29=Bruce A. | last30=Sutherland | first30=Will | last31=Taylor | first31=Keith | journal=Monthly Notices of the Royal Astronomical Society | volume=362 | issue=2 | pages=505–534 | year=2005 | doi=10.1111/j.1365-2966.2005.09318.x | doi-access=free | bibcode=2005MNRAS.362..505C | arxiv=astro-ph/0501174 | s2cid=6906627 }}</ref><ref name=Nichol2007>{{cite journal | title=The Shape of the Sloan Digital Sky Survey Data Release 5 Galaxy Power Spectrum | display-authors=1 | last1=Percival | first1=Will J. | last2=Nichol | first2=Robert C. | last3=Eisenstein | first3=Daniel J. | last4=Frieman | first4=Joshua A. | last5=Fukugita | first5=Masataka | last6=Loveday | first6=Jon | last7=Pope | first7=Adrian C. | last8=Schneider | first8=Donald P. | last9=Szalay | first9=Alex S. | last10=Tegmark | first10=Max | last11=Vogeley | first11=Michael S. | last12=Weinberg | first12=David H. | last13=Zehavi | first13=Idit | last14=Bahcall | first14=Neta A. | last15=Brinkmann | first15=Jon | last16=Connolly | first16=Andrew J. | last17=Meiksin | first17=Avery | journal=The Astrophysical Journal | volume=657 | issue=2 | pages=645–663 | year=2007 | doi=10.1086/510615 | bibcode=2007ApJ...657..645P | arxiv=astro-ph/0608636 | s2cid=53141475 }}</ref>

Another tool for understanding structure formation is simulations, which cosmologists use to study the gravitational aggregation of matter in the universe, as it clusters into [[Galaxy filament|filaments]], superclusters and [[void (astronomy)|voids]]. Most simulations contain only non-baryonic [[cold 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.<ref name=Kuhlen2012>{{cite journal | title=Numerical simulations of the dark universe: State of the art and the next decade | journal=Physics of the Dark Universe | volume=1 | issue=1–2 | date=November 2012 | pages=50–93 | first1=Michael | last1=Kuhlen | first2=Mark | last2=Vogelsberger | first3=Raul | last3=Angulo | doi=10.1016/j.dark.2012.10.002 | arxiv=1209.5745 | bibcode=2012PDU.....1...50K | s2cid=119232040 }}</ref>
[[File:Gravitational lens found in the DESI Legacy Surveys data (noirlab2104c).jpg|thumb|upright=1.2|An example of a gravitational lens found in the DESI Legacy Surveys data. There are four sets of lensed images in DESI-090.9854-35.9683, corresponding to four distinct background galaxies—from the outermost giant red arc to the innermost bright blue arc, arranged in four concentric circles. All of them are gravitationally warped—or lensed—by the orange galaxy at the very center. Dark matter is expected to produce gravitational lensing also.]]
Other, complementary observations to measure the distribution of matter in the distant universe and to probe [[reionization]] include:
* The [[Lyman-alpha forest]], which allows cosmologists to measure the distribution of neutral atomic hydrogen gas in the early universe, by measuring the absorption of light from distant quasars by the gas.<ref name=Weinberg2003>{{Cite book | last1=Weinberg | first1=David H. | last2=Davé | first2=Romeel | last3=Katz | first3=Neal | last4=Kollmeier | first4=Juna A. | chapter=The Lyman-α Forest as a Cosmological Tool | title=AIP Conference Proceedings: The Emergence of Cosmic Structure | date=May 2003 | series=AIP Conference Series | volume=666 | issue=2003 | editor1-first=S.H. | editor1-last=Holt | editor2-first=C. S. | editor2-last=Reynolds | pages=157–169 | arxiv=astro-ph/0301186 | doi=10.1063/1.1581786 | bibcode=2003AIPC..666..157W | citeseerx=10.1.1.256.1928 | s2cid=118868536 }}</ref>
* The [[Hydrogen line|21-centimeter]] [[Absorption (electromagnetic radiation)|absorption]] line of neutral atomic hydrogen also provides a sensitive test of cosmology.<ref name=Furlanetto2006>{{cite journal | title=Cosmology at low frequencies: The 21 cm transition and the high-redshift Universe | last1=Furlanetto | first1=Steven R. | last2=Oh | first2=S. Peng | last3=Briggs | first3=Frank H. | journal=Physics Reports | volume=433 | issue=4–6 | pages=181–301 | date=October 2006 | doi=10.1016/j.physrep.2006.08.002 | bibcode=2006PhR...433..181F | arxiv=astro-ph/0608032 | citeseerx=10.1.1.256.8319 | s2cid=118985424 }}</ref>
* [[Weak lensing]], the distortion of a distant image by [[gravitational lensing]] due to dark matter.<ref name=Munshi2008>{{cite journal | title=Cosmology with weak lensing surveys | last1=Munshi | first1=Dipak | last2=Valageas | first2=Patrick | last3=van Waerbeke | first3=Ludovic | last4=Heavens | first4=Alan | journal=Physics Reports | volume=462 | issue=3 | pages=67–121 | year=2008 | doi=10.1016/j.physrep.2008.02.003 | pmid=16286284 | bibcode=2008PhR...462...67M | arxiv=astro-ph/0612667 | citeseerx=10.1.1.337.3760 | s2cid=9279637 }}</ref>

These will help cosmologists settle the question of when and how structure formed in the universe.

===Dark matter===
{{Main|Dark matter}}
Evidence from [[Big Bang nucleosynthesis]], the [[cosmic microwave background]], structure formation, and [[galaxy rotation curve]]s 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, [[Radioactive decay|non-radiative]] fluid that forms [[galactic halo|haloes]] 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 stable [[supersymmetry|supersymmetric]] particle, a [[weakly interacting massive particle]], a gravitationally-interacting massive particle, an [[axion]], and a [[massive 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.<ref name=Klasen2015>{{cite journal | title=Indirect and direct search for dark matter | last1=Klasen | first1=M. | last2=Pohl | first2=M. | last3=Sigl | first3=G. | journal=Progress in Particle and Nuclear Physics | volume=85 | pages=1–32 | date=November 2015 | doi=10.1016/j.ppnp.2015.07.001 | arxiv=1507.03800 | bibcode=2015PrPNP..85....1K | s2cid=118359390 }}</ref>

===Dark energy===
{{Main|Dark energy}}
If the universe is [[Flatness (cosmology)|flat]], 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.<ref name=Perlmutter1999>{{cite journal | title=Constraining Dark Energy with Type Ia Supernovae and Large-Scale Structure | last1=Perlmutter | first1=Saul | last2=Turner | first2=Michael S. | last3=White | first3=Martin | journal=Physical Review Letters | volume=83 | issue=4 | year=1999 | pages=670–673 | doi=10.1103/PhysRevLett.83.670 | bibcode=1999PhRvL..83..670P | arxiv=astro-ph/9901052 | s2cid=119427069 | url=https://zenodo.org/record/1233929 }}</ref>

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 120 [[orders of magnitude]] larger than that observed.<ref name=Adler1995>{{cite journal | title=Vacuum catastrophe: An elementary exposition of the cosmological constant problem | last1=Adler | first1=Ronald J. | last2=Casey | first2=Brendan | last3=Jacob | first3=Ovid C. | journal=American Journal of Physics | volume=63 | issue=7 | pages=620–626 | date=July 1995 | doi=10.1119/1.17850 | bibcode=1995AmJPh..63..620A | doi-access=free }}</ref> [[Steven Weinberg]] and a number of string theorists ''(see [[string landscape]])'' have invoked the 'weak [[anthropic 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.<ref name=Siegfried2006>{{cite journal | title=A 'Landscape' Too Far? | first1=Tom | last1=Siegfried | journal=Science | date=11 August 2006 | volume=313 | issue=5788 | pages=750–753 | doi=10.1126/science.313.5788.750 | pmid=16902104 | s2cid=118891996 }}</ref> For example, the weak anthropic principle alone does not distinguish between:
* Only one universe will ever exist and there is some underlying principle that constrains the CC to the value we observe.
* Only one universe will ever exist and although there is no underlying principle fixing the CC, we got lucky.
* Lots of universes exist (simultaneously or serially) with a range of CC values, and of course ours is one of the life-supporting ones.

Other possible explanations for dark energy include [[quintessence (physics)|quintessence]]<ref name=Sahni2002>{{cite journal | title=The cosmological constant problem and quintessence | last1=Sahni | first1=Varun | journal=Classical and Quantum Gravity | volume=19 | issue=13 | pages=3435–3448 | year=2002 | doi=10.1088/0264-9381/19/13/304 | bibcode=2002CQGra..19.3435S | arxiv=astro-ph/0202076 | s2cid=13532332 }}</ref> or a modification of gravity on the largest scales.<ref name=Nojiri2007>{{cite journal | title=Introduction to Modified Gravity and Gravitational Alternative for Dark Energy | last1=Nojiri | first1=S. | last2=Odintsov | first2=S. D. | journal=International Journal of Geometric Methods in Modern Physics | volume=04 | issue=1 | pages=115–146 | year=2006  | doi=10.1142/S0219887807001928 | arxiv=hep-th/0601213 | bibcode=2007IJGMM..04..115N | s2cid=119458605 }}</ref> The effect on cosmology of the dark energy that these models describe is given by the dark energy's [[equation of state (cosmology)|equation 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 the [[ultimate fate of the universe]]. In the current cosmological epoch, the accelerated expansion due to dark energy is preventing structures larger than [[supercluster]]s from forming. It is not known whether the acceleration will continue indefinitely, perhaps even increasing until a [[big rip]], or whether it will eventually reverse, lead to a [[Heat death of the universe|Big Freeze]], or follow some other scenario.<ref name=Jambrina2014>{{cite journal | title=Grand rip and grand bang/crunch cosmological singularities | last1=Fernández-Jambrina | first1=L. | journal=Physical Review D | volume=90 | issue=6 | page=064014 | date=September 2014 | doi=10.1103/PhysRevD.90.064014 | bibcode=2014PhRvD..90f4014F | arxiv=1408.6997 | s2cid=118328824 }}</ref>

===Gravitational waves===
[[Gravitational wave]]s are ripples in the [[curvature]] of [[spacetime]] that propagate as [[wave]]s at the speed of light, generated in certain gravitational interactions that propagate outward from their source. [[Gravitational-wave astronomy]] is an emerging branch of [[observational astronomy]] which aims to use gravitational waves to collect observational data about sources of detectable gravitational waves such as [[binary star]] systems composed of [[white dwarf]]s, [[neutron star]]s, and [[black hole]]s; and events such as [[supernova]]e, and the formation of the [[chronology of the universe|early universe]] shortly after the Big Bang.<ref name=Colpi2017>{{cite book | chapter=Gravitational Wave Sources in the Era of Multi-Band Gravitational Wave Astronomy | last1=Colpi | first1=Monica | last2=Sesana | first2=Alberto | title=An Overview of Gravitational Waves: Theory, Sources and Detection | editor1-first=Augar | editor1-last=Gerard | editor2-first=Plagnol | editor2-last=Eric | isbn=978-981-314-176-6 | pages=43–140 | year=2017 | doi=10.1142/9789813141766_0002 | bibcode=2017ogw..book...43C | arxiv=1610.05309 | s2cid=119292265 }}</ref>

In 2016, the [[LIGO]] Scientific Collaboration and [[Virgo interferometer|Virgo]] Collaboration teams announced that they had made the [[first observation of gravitational waves]], originating from a [[Binary black hole|pair]] of [[Stellar collision|merging]] black holes using the Advanced LIGO detectors.<ref name="Discovery 2016">{{cite journal |title=Einstein's gravitational waves found at last |journal=Nature News| url=http://www.nature.com/news/einstein-s-gravitational-waves-found-at-last-1.19361 |date=11 February 2016 |last1=Castelvecchi |first1=Davide |last2=Witze |first2=Witze |doi=10.1038/nature.2016.19361  |s2cid=182916902|access-date=2016-02-11 }}</ref><ref name="Abbot">{{cite journal |author=Abbott |first=B. P. |display-authors=etal |year=2016 |title=Observation of Gravitational Waves from a Binary Black Hole Merger |journal=Physical Review Letters |volume=116 |issue=6 |pages=061102 |arxiv=1602.03837 |bibcode=2016PhRvL.116f1102A |doi=10.1103/PhysRevLett.116.061102 |pmid=26918975 |s2cid=124959784 |collaboration=LIGO Scientific Collaboration and Virgo Collaboration}}</ref><ref name='NSF'>{{cite web|title = Gravitational waves detected 100 years after Einstein's prediction |publisher=National Science Foundation|url = https://www.nsf.gov/news/news_summ.jsp?cntn_id=137628|website = www.nsf.gov|access-date = 11 February 2016}}</ref> On 15 June 2016, a [[GW151226|second detection]] of gravitational waves from coalescing black holes was announced.<ref name="NYT-20160615">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Scientists Hear a Second Chirp From Colliding Black Holes |url=https://www.nytimes.com/2016/06/16/science/ligo-gravitational-waves-einstein.html |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2016/06/16/science/ligo-gravitational-waves-einstein.html |archive-date=2022-01-01 |url-access=limited |date=15 June 2016 |work=[[The New York Times]] |access-date=15 June 2016 }}{{cbignore}}</ref> Besides LIGO, many other [[Gravitational-wave observatory|gravitational-wave observatories (detectors)]] are under construction.<ref>{{cite web|title=The Newest Search for Gravitational Waves has Begun|url=https://ligo.caltech.edu/news/ligo20150918|website=LIGO Caltech|publisher=[[LIGO]]|access-date=29 November 2015|date=18 September 2015}}</ref>

===Other areas of inquiry===
Cosmologists also study:
* Whether [[primordial black hole]]s were formed in our universe, and what happened to them.<ref name=Kovetz2017>{{cite journal | title=Probing Primordial Black Hole Dark Matter with Gravitational Waves | last1=Kovetz | first1=Ely D. | journal=Physical Review Letters | volume=119 | issue=13 | page=131301 | year=2017 | doi=10.1103/PhysRevLett.119.131301 | pmid=29341709 | bibcode=2017PhRvL.119m1301K | arxiv=1705.09182 | s2cid=37823911 }}</ref>
* Detection of cosmic rays with energies above the [[GZK cutoff]],<ref name=Takeda1998>{{cite journal | title=Extension of the Cosmic-Ray Energy Spectrum beyond the Predicted Greisen-Zatsepin-Kuz'min Cutoff | display-authors=1 | last1=Takeda | first1=M. | last2=Hayashida | first2=N. | last3=Honda | first3=K. | last4=Inoue | first4=N. | last5=Kadota | first5=K. | last6=Kakimoto | first6=F. | last7=Kamata | first7=K. | last8=Kawaguchi | first8=S. | last9=Kawasaki | first9=Y. | last10=Kawasumi | first10=N. | last11=Kitamura | first11=H. | last12=Kusano | first12=E. | last13=Matsubara | first13=Y. | last14=Murakami | first14=K. | last15=Nagano | first15=M. | last16=Nishikawa | first16=D. | last17=Ohoka | first17=H. | last18=Sakaki | first18=N. | last19=Sasaki | first19=M. | last20=Shinozaki | first20=K. | last21=Souma | first21=N. | last22=Teshima | first22=M. | last23=Torii | first23=R. | last24=Tsushima | first24=I. | last25=Uchihori | first25=Y. | last26=Yamamoto | first26=T. | last27=Yoshida | first27=S. | last28=Yoshii | first28=H. | journal=Physical Review Letters | volume=81 | issue=6 | date=10 August 1998 | pages=1163–1166 | doi=10.1103/PhysRevLett.81.1163 | bibcode=1998PhRvL..81.1163T | arxiv=astro-ph/9807193 | s2cid=14864921 }}</ref> and whether it signals a failure of [[special relativity]] at high energies.
* The [[equivalence principle]],<ref name=Boucher2004/> whether or not Einstein's general theory of relativity is the correct theory of [[gravitation]],<ref name=Turyshev2008>{{cite journal | title=Experimental Tests of General Relativity | last1=Turyshev | first1=Slava G. | journal=[[Annual Review of Nuclear and Particle Science]] | volume=58 | issue=1 | pages=207–248 | year=2008 | doi=10.1146/annurev.nucl.58.020807.111839| doi-access=free | bibcode=2008ARNPS..58..207T | arxiv=0806.1731 | s2cid=119199160 }}</ref> and if the fundamental [[laws of physics]] are the same everywhere in the universe.<ref name=Uzan2011>{{cite journal | title=Varying Constants, Gravitation and Cosmology | last1=Uzan | first1=Jean-Philippe | journal=Living Reviews in Relativity | volume=14 | issue=1 | pages=2 | date=March 2011 | doi=10.12942/lrr-2011-2 | doi-access=free | pmid=28179829 | pmc=5256069 | bibcode=2011LRR....14....2U | arxiv=1009.5514 }}</ref>
*{{anchor|Biophysical cosmology}}'''Biophysical cosmology''': a type of physical cosmology that studies and understands [[life]] as part or an inherent part of physical cosmology. It stresses that [[Universe#Habitability|life is inherent to the universe]] and therefore [[Life#In the universe|frequent]].<ref name="v237">{{cite book | last=Dick | first=Steven J. | title=Space, Time, and Aliens | chapter=The Biophysical Cosmology: The Place of Bioastronomy in the History of Science | publisher=Springer International Publishing | publication-place=Cham | date=2020 | isbn=978-3-030-41613-3 | doi=10.1007/978-3-030-41614-0_4 | pages=53–58}}</ref>