Universe Warning: You are not logged in. Your IP address will be publicly visible if you make any edits. If you log in or create an account, your edits will be attributed to your username, along with other benefits.Anti-spam check. Do not fill this in! == Composition == {{See also|Galaxy formation and evolution|Galaxy cluster|Nebula}} The universe is composed almost completely of dark energy, dark matter, and [[matter|ordinary matter]]. Other contents are [[electromagnetic radiation]] (estimated to constitute from 0.005% to close to 0.01% of the total [[mass–energy equivalence|mass–energy]] of the universe) and [[antimatter]].<ref>{{cite encyclopedia|title=electromagnetic radiation {{!}} physics|url=http://www.britannica.com/science/electromagnetic-radiation|access-date=July 26, 2015|encyclopedia=Encyclopædia Britannica|last=Fritzsche|first=Hellmut|page=1|archive-date=August 31, 2015|archive-url=https://web.archive.org/web/20150831050929/http://www.britannica.com/science/electromagnetic-radiation|url-status=live}}</ref><ref>{{Cite web|url=http://physics.ucr.edu/~wudka/Physics7/Notes_www/Pdf_downloads/8.pdf|title=Physics 7:Relativity, SpaceTime and Cosmology|access-date=July 26, 2015|website=Physics 7:Relativity, SpaceTime and Cosmology|publisher=University of California Riverside|archive-url=https://web.archive.org/web/20150905155421/http://physics.ucr.edu/~wudka/Physics7/Notes_www/Pdf_downloads/8.pdf|archive-date=September 5, 2015|url-status=dead}}</ref><ref>{{Cite web|title=Physics – for the 21st Century|url=http://www.learner.org/courses/physics/unit/text.html?unit=11&secNum=6|website=learner.org|access-date=July 27, 2015|publisher=Harvard-Smithsonian Center for Astrophysics Annenberg Learner|archive-url=https://web.archive.org/web/20150907212145/http://www.learner.org/courses/physics/unit/text.html?unit=11&secNum=6|archive-date=September 7, 2015|url-status=dead}}</ref> The proportions of all types of matter and energy have changed over the history of the universe.<ref>{{cite web|title=Dark matter – A history shapes by dark force|publisher=National Geographic|url=http://ngm.nationalgeographic.com/2015/01/hidden-cosmos/timeline-graphic|work=Timothy Ferris|year=2015|access-date=December 29, 2015|archive-date=March 4, 2016|archive-url=https://web.archive.org/web/20160304095337/http://ngm.nationalgeographic.com/2015/01/hidden-cosmos/timeline-graphic|url-status=dead}}</ref> The total amount of electromagnetic radiation generated within the universe has decreased by 1/2 in the past 2 billion years.<ref>{{Cite web|title=It's Official: The Universe Is Dying Slowly|url=http://www.scientificamerican.com/article/it-s-official-the-universe-is-dying-slowly/|access-date=August 11, 2015|first=Nola Taylor|last=Redd, SPACE.com|website=[[Scientific American]]|archive-date=August 12, 2015|archive-url=https://web.archive.org/web/20150812010821/http://www.scientificamerican.com/article/it-s-official-the-universe-is-dying-slowly/|url-status=live}}</ref><ref>{{Cite web |title=RIP Universe – Your Time Is Coming… Slowly {{!}} Video |url=http://www.space.com/30194-rip-universe-your-time-is-coming-slowly-video.html |publisher=Space.com |first=Will |last=Parr |display-authors=et al |access-date=August 20, 2015 |archive-date=August 13, 2015 |archive-url=https://web.archive.org/web/20150813221122/http://www.space.com/30194-rip-universe-your-time-is-coming-slowly-video.html |url-status=live }}</ref> Today, ordinary matter, which includes atoms, stars, galaxies, and [[life]], accounts for only 4.9% of the contents of the universe.<ref name="planck2013parameters" /> The present overall [[density]] of this type of matter is very low, roughly 4.5 × 10<sup>−31</sup> grams per cubic centimeter, corresponding to a density of the order of only one proton for every four cubic metres of volume.<ref name="wmap_universe_made_of" /> The nature of both dark energy and dark matter is unknown. Dark matter, a mysterious form of matter that has not yet been identified, accounts for 26.8% of the cosmic contents. Dark energy, which is the energy of empty space and is causing the expansion of the universe to accelerate, accounts for the remaining 68.3% of the contents.<ref name="planck2013parameters">{{cite web|title=First Planck results: the universe is still weird and interesting|url=https://arstechnica.com/science/2013/03/first-planck-results-the-universe-is-still-weird-and-interesting/|work=Matthew Francis|publisher=Ars technica|date=March 21, 2013|access-date=August 21, 2015|archive-date=May 2, 2019|archive-url=https://web.archive.org/web/20190502143413/https://arstechnica.com/science/2013/03/first-planck-results-the-universe-is-still-weird-and-interesting/|url-status=live}}</ref><ref name="DarkMatter" /><ref name="peebles">{{cite journal |author=Peebles |first1=P. J. E. |last2=Ratra |first2=Bharat |name-list-style=amp |date=2003 |title=The cosmological constant and dark energy |journal=Reviews of Modern Physics |volume=75 |issue=2 |pages=559–606 |arxiv=astro-ph/0207347 |bibcode=2003RvMP...75..559P |doi=10.1103/RevModPhys.75.559 |s2cid=118961123}}</ref> [[File:Formation of galactic clusters and filaments.jpg|thumb|upright=2.4|The formation of clusters and large-scale [[Galaxy filament|filaments]] in the [[cold dark matter]] model with [[dark energy]]. The frames show the evolution of structures in a 43 million parsecs (or 140 million light-years) box from redshift of 30 to the present epoch (upper left z=30 to lower right z=0).]] [[File:Nearsc.gif|thumb|upright=2.4|A map of the superclusters and [[void (astronomy)|voids]] nearest to Earth]] Matter, dark matter, and dark energy are distributed homogeneously throughout the universe over length scales longer than 300 million light-years (ly) or so.<ref>{{Cite journal |last1=Mandolesi |first1=N. |last2=Calzolari |first2=P. |last3=Cortiglioni |first3=S. |last4=Delpino |first4=F. |last5=Sironi |first5=G. |last6=Inzani |first6=P. |last7=Deamici |first7=G. |last8=Solheim |first8=J.-E. |last9=Berger |first9=L. |doi=10.1038/319751a0 |last10=Partridge |first10=R.B. |last11=Martenis |first11=P.L. |last12=Sangree |first12=C.H. |last13=Harvey |first13=R.C. |title=Large-scale homogeneity of the universe measured by the microwave background |journal=Nature |volume=319 |issue=6056 |pages=751–753 |year=1986 |bibcode=1986Natur.319..751M |s2cid=4349689 }}</ref> However, over shorter length-scales, matter tends to clump hierarchically; many [[atom]]s are condensed into [[star]]s, most stars into galaxies, most galaxies into [[galaxy groups and clusters|clusters, superclusters]] and, finally, large-scale [[Galaxy filament|galactic filaments]]. The observable universe contains as many as an estimated 2 trillion galaxies<ref name="BBC-20231129">{{cite news |last=Gunn |first=Alistair |title=How many galaxies are there in the universe? – Do astronomers know how many galaxies exist? How many can we see in the observable Universe? |url=https://www.skyatnightmagazine.com/space-science/how-many-galaxies-in-universe |date=November 29, 2023 |work=[[BBC Sky at Night]] |url-status=live |archiveurl=https://archive.today/20231203021645/https://www.skyatnightmagazine.com/space-science/how-many-galaxies-in-universe |archivedate=December 3, 2023 |accessdate=December 2, 2023 }}</ref><ref>{{cite journal |title=New Horizons spacecraft answers the question: How dark is space? |website=phys.org |url=https://phys.org/news/2021-01-horizons-spacecraft-dark-space.html |access-date=January 15, 2021 |language=en |archive-date=January 15, 2021 |archive-url=https://web.archive.org/web/20210115110710/https://phys.org/news/2021-01-horizons-spacecraft-dark-space.html |url-status=live }}</ref><ref>{{cite news |last1=Howell |first1=Elizabeth |title=How Many Galaxies Are There? |url=https://www.space.com/25303-how-many-galaxies-are-in-the-universe.html |website=Space.com |access-date=March 5, 2021 |date=March 20, 2018 |archive-date=February 28, 2021 |archive-url=https://web.archive.org/web/20210228013433/https://www.space.com/25303-how-many-galaxies-are-in-the-universe.html |url-status=live }}</ref> and, overall, as many as an estimated 10<sup>24</sup> stars<ref name="ESA-2019">{{cite web |author=Staff |title=How Many Stars Are There In The Universe? |url=https://www.esa.int/Our_Activities/Space_Science/Herschel/How_many_stars_are_there_in_the_Universe |date=2019 |work=[[European Space Agency]] |access-date=September 21, 2019 |archive-date=September 23, 2019 |archive-url=https://web.archive.org/web/20190923134902/http://www.esa.int/Our_Activities/Space_Science/Herschel/How_many_stars_are_there_in_the_Universe |url-status=live }}</ref><ref>{{Cite book|chapter=The Structure of the Universe|doi=10.1007/978-1-4614-8730-2_10|title=The Fundamentals of Modern Astrophysics|pages=279–294|year=2015|last1=Marov|first1=Mikhail Ya.|isbn=978-1-4614-8729-6}}</ref> – more stars (and earth-like planets) than all the [[Sand|grains of beach sand]] on planet [[Earth]];<ref name="SU-20020201">{{cite web |last=Mackie |first=Glen |title=To see the Universe in a Grain of Taranaki Sand |url=http://astronomy.swin.edu.au/~gmackie/billions.html |date=February 1, 2002 |work=[[Centre for Astrophysics and Supercomputing]] |access-date=January 28, 2017 |archive-date=August 11, 2011 |archive-url=https://www.webcitation.org/60r7Xm9UZ?url=http://astronomy.swin.edu.au/~gmackie/billions.html |url-status=live }}</ref><ref name="CNET-20150319">{{cite news |last=Mack |first=Eric |title=There may be more Earth-like planets than grains of sand on all our beaches – New research contends that the Milky Way alone is flush with billions of potentially habitable planets – and that's just one sliver of the universe. |url=https://www.cnet.com/science/the-milky-way-is-flush-with-habitable-planets-study-says/ |date=March 19, 2015 |work=[[CNET]] |url-status=live |archiveurl=https://archive.today/20231201144523/https://www.cnet.com/science/the-milky-way-is-flush-with-habitable-planets-study-says/ |archivedate=December 1, 2023 |accessdate=December 1, 2023 }}</ref><ref name="MNRAS-20150313">{{cite journal |last1=T. Bovaird |first1=T. |last2=Lineweaver |first2=C.H. |last3=Jacobsen |first3=S.K. |title=Using the inclinations of Kepler systems to prioritize new Titius–Bode-based exoplanet predictions |url=https://academic.oup.com/mnras/article/448/4/3608/970734 |date=March 13, 2015 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=448 |issue=4 |pages=3608–3627 |doi=10.1093/mnras/stv221 |url-status=live |archiveurl=https://archive.today/20231201151205/https://academic.oup.com/mnras/article/448/4/3608/970734 |archivedate=December 1, 2023 |accessdate=December 1, 2023 |doi-access=free |arxiv=1412.6230 }}</ref> but less than the total number of atoms estimated in the universe as 10<sup>82</sup>;<ref name="LS-20210711">{{cite news |last=Baker |first=Harry |title=How many atoms are in the observable universe? |url=https://www.livescience.com/how-many-atoms-in-universe.html |date=July 11, 2021 |work=[[Live Science]] |url-status=live |archiveurl=https://archive.today/20231201143640/https://www.livescience.com/how-many-atoms-in-universe.html |archivedate=December 1, 2023 |accessdate=December 1, 2023 }}</ref> and the estimated total number of stars in an [[Inflation (cosmology)|inflationary universe]] (observed and unobserved), as 10<sup>100</sup>.<ref name="SR-20200203">{{cite journal |last=Totani |first=Tomonori |title=Emergence of life in an inflationary universe |date=February 3, 2020 |journal=[[Scientific Reports]] |volume=10 |number=1671 |page=1671 |doi=10.1038/s41598-020-58060-0 |doi-access=free |pmid=32015390 |pmc=6997386 |arxiv=1911.08092 |bibcode=2020NatSR..10.1671T }}</ref> Typical galaxies range from [[dwarf galaxy|dwarfs]] with as few as ten million<ref>{{cite journal|date=May 3, 2000|url=http://www.eso.org/public/usa/news/eso0018/|title=Unveiling the Secret of a Virgo Dwarf Galaxy|journal=European Southern Observatory Press Release|pages=12|publisher=ESO|access-date=January 3, 2007|bibcode=2000eso..pres...12.|archive-date=July 13, 2015|archive-url=https://web.archive.org/web/20150713223811/http://www.eso.org/public/usa/news/eso0018/|url-status=live}}</ref> (10<sup>7</sup>) stars up to giants with one [[10^12|trillion]]<ref name="M101">{{cite web|date=February 28, 2006|url=http://www.nasa.gov/mission_pages/hubble/science/hst_spiral_m10.html|title=Hubble's Largest Galaxy Portrait Offers a New High-Definition View|publisher=NASA|access-date=January 3, 2007|archive-date=May 27, 2020|archive-url=https://web.archive.org/web/20200527063744/https://www.nasa.gov/mission_pages/hubble/science/hst_spiral_m10.html|url-status=live}}</ref> (10<sup>12</sup>) stars. Between the larger structures are [[void (astronomy)|voids]], which are typically 10–150 Mpc (33 million–490 million ly) in diameter. The [[Milky Way]] is in the [[Local Group]] of galaxies, which in turn is in the [[Laniakea Supercluster]].<ref name=":0">{{Cite journal|url=http://www.nature.com/news/earth-s-new-address-solar-system-milky-way-laniakea-1.15819|title=Earth's new address: 'Solar System, Milky Way, Laniakea'|journal=Nature|date=September 3, 2014|access-date=August 21, 2015|doi=10.1038/nature.2014.15819|last1=Gibney|first1=Elizabeth|author-link=Elizabeth Gibney|s2cid=124323774|archive-date=January 7, 2019|archive-url=https://web.archive.org/web/20190107010904/http://www.nature.com/news/earth-s-new-address-solar-system-milky-way-laniakea-1.15819?error=cookies_not_supported&code=81eb43f5-e92f-436d-9725-3b681615454d|url-status=live}}</ref> This supercluster spans over 500 million light-years, while the Local Group spans over 10 million light-years.<ref>{{cite web|url=http://www.universetoday.com/30286/local-group/|title=Local Group|publisher=Universe Today|work=Fraser Cain|date=May 4, 2009|access-date=August 21, 2015|archive-url=https://web.archive.org/web/20180621093042/https://www.universetoday.com/30286/local-group/|archive-date=June 21, 2018|url-status=dead}}</ref> The universe also has vast regions of relative emptiness; the largest known void measures 1.8 billion ly (550 Mpc) across.<ref>{{Cite news|url=https://www.theguardian.com/science/2015/apr/20/astronomers-discover-largest-known-structure-in-the-universe-is-a-big-hole|title=Astronomers discover largest known structure in the universe is ... a big hole|date=April 20, 2015|newspaper=The Guardian|last1=Devlin|first1=Hannah|author-link=Hannah Devlin|last2=Correspondent|first2=Science|access-date=December 18, 2016|archive-date=February 7, 2017|archive-url=https://web.archive.org/web/20170207131614/https://www.theguardian.com/science/2015/apr/20/astronomers-discover-largest-known-structure-in-the-universe-is-a-big-hole|url-status=live}}</ref> [[File:Universe content bar chart.svg|thumb|upright=1.5|Comparison of the contents of the universe today to 380,000 years after the Big Bang as measured with 5 year WMAP data (from 2008).<ref>{{Cite web|title=Content of the Universe – WMAP 9yr Pie Chart|url=http://wmap.gsfc.nasa.gov/media/080998/|website=wmap.gsfc.nasa.gov|access-date=July 26, 2015|archive-date=September 5, 2015|archive-url=https://web.archive.org/web/20150905184934/http://wmap.gsfc.nasa.gov/media/080998/|url-status=live}}</ref> Due to rounding errors, the sum of these numbers is not 100%. This reflects the 2008 limits of WMAP's ability to define dark matter and dark energy.]] The observable universe is [[isotropic]] on scales significantly larger than superclusters, meaning that the statistical properties of the universe are the same in all directions as observed from Earth. The universe is bathed in highly isotropic [[microwave]] [[electromagnetic radiation|radiation]] that corresponds to a [[thermal equilibrium]] [[blackbody spectrum]] of roughly 2.72548 [[kelvin]]s.<ref name="Fixsen" /> The hypothesis that the large-scale universe is homogeneous and isotropic is known as the [[cosmological principle]].<ref>[[#Rindler|Rindler]], p. 202.</ref> A universe that is both homogeneous and isotropic looks the same from all vantage points<ref name=Liddle>{{cite book |title=An Introduction to Modern Cosmology |edition=2nd |first=Andrew |last=Liddle |isbn=978-0-470-84835-7 |year=2003 |publisher=John Wiley & Sons}}. p. 2.</ref> and has no center.<ref name="livio">{{cite book|title=The Accelerating Universe: Infinite Expansion, the Cosmological Constant, and the Beauty of the Cosmos|last=Livio|first=Mario|author-link=Mario Livio|date=2001|publisher=John Wiley and Sons|page=53|url=https://books.google.com/books?id=4EidS6_VVNYC&q=cosmological+principle+%22center+of+the+universe%22&pg=PA53|access-date=March 31, 2012|isbn=978-0-471-43714-7|archive-date=May 13, 2021|archive-url=https://web.archive.org/web/20210513224845/https://books.google.com/books?id=4EidS6_VVNYC&q=cosmological+principle+%22center+of+the+universe%22&pg=PA53|url-status=live}}</ref> === Dark energy === {{Main|Dark energy}} An explanation for why the expansion of the universe is accelerating remains elusive. It is often attributed to "dark energy", an unknown form of energy that is hypothesized to permeate space.<ref name="peebles(a)">{{cite journal|author1=Peebles, P.J.E. |author2=Ratra, Bharat |name-list-style=amp |title=The cosmological constant and dark energy|year=2003|journal=Reviews of Modern Physics|arxiv=astro-ph/0207347|volume=75|issue=2|pages=559–606|doi=10.1103/RevModPhys.75.559|bibcode=2003RvMP...75..559P|s2cid=118961123 }}</ref> On a [[mass–energy equivalence]] basis, the density of dark energy (~ 7 × 10<sup>−30</sup> g/cm<sup>3</sup>) is much less than the density of ordinary matter or dark matter within galaxies. However, in the present dark-energy era, it dominates the mass–energy of the universe because it is uniform across space.<ref>{{cite journal|title=Why the cosmological constant is small and positive |first1=Paul J. |last1=Steinhardt |first2=Neil|last2=Turok|journal=Science|volume=312|issue=5777|pages=1180–1183 |doi=10.1126/science.1126231 |arxiv=astro-ph/0605173 |year=2006 |bibcode=2006Sci...312.1180S |pmid=16675662|s2cid=14178620 }}</ref><ref>{{cite web |url=http://hyperphysics.phy-astr.gsu.edu/hbase/astro/dareng.html |title=Dark Energy |work=Hyperphysics |access-date=January 4, 2014 |archive-url=https://web.archive.org/web/20130527105518/http://hyperphysics.phy-astr.gsu.edu/HBASE/astro/dareng.html |archive-date=May 27, 2013 |url-status=dead }}</ref> Two proposed forms for dark energy are the [[cosmological constant]], a ''constant'' energy density filling space homogeneously,<ref name="carroll">{{cite journal|author=Carroll, Sean |year=2001 |url=http://relativity.livingreviews.org/Articles/lrr-2001-1/index.html |title=The cosmological constant |journal=Living Reviews in Relativity |volume=4 |issue=1 |page=1 |access-date=September 28, 2006 |doi=10.12942/lrr-2001-1 |pmid=28179856 |pmc=5256042 |url-status=dead |archive-url=https://web.archive.org/web/20061013042057/http://relativity.livingreviews.org/Articles/lrr-2001-1/index.html |archive-date=October 13, 2006 |arxiv=astro-ph/0004075 |bibcode=2001LRR.....4....1C |author-link=Sean M. Carroll }}</ref> and [[scalar field]]s such as [[quintessence (physics)|quintessence]] or [[moduli (physics)|moduli]], ''dynamic'' quantities whose energy density can vary in time and space while still permeating then enough to cause the observed rate of expansion. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to [[vacuum energy]]. Scalar fields having only a slight amount of spatial inhomogeneity would be difficult to distinguish from a cosmological constant. === Dark matter === {{Main|Dark matter}} Dark matter is a hypothetical kind of [[matter]] that is invisible to the entire [[electromagnetic spectrum]], but which accounts for most of the matter in the universe. The existence and properties of dark matter are inferred from its gravitational effects on visible matter, radiation, and the [[Observable universe#Large-scale structure|large-scale structure]] of the universe. Other than [[neutrinos]], a form of [[hot dark matter]], dark matter has not been detected directly, making it one of the greatest mysteries in modern [[astrophysics]]. Dark matter neither [[blackbody spectrum|emits]] nor absorbs light or any other [[electromagnetic radiation]] at any significant level. Dark matter is estimated to constitute 26.8% of the total mass–energy and 84.5% <!--26.8/(4.9 + 26.8)--> of the total matter in the universe.<ref name="DarkMatter">Sean Carroll, Ph.D., Caltech, 2007, The Teaching Company, ''Dark Matter, Dark Energy: The Dark Side of the Universe'', Guidebook Part 2. p. 46, Accessed October 7, 2013, "...dark matter: An invisible, essentially collisionless component of matter that makes up about 25 percent of the energy density of the universe... it's a different kind of particle... something not yet observed in the laboratory..."</ref><ref name=planckcam>{{cite web |url=http://www.cam.ac.uk/research/news/planck-captures-portrait-of-the-young-universe-revealing-earliest-light |title=Planck captures portrait of the young universe, revealing earliest light |date=March 21, 2013 |publisher=University of Cambridge |access-date=March 21, 2013 |archive-date=April 17, 2019 |archive-url=https://web.archive.org/web/20190417165900/https://www.cam.ac.uk/research/news/planck-captures-portrait-of-the-young-universe-revealing-earliest-light |url-status=live }}</ref> === Ordinary matter === {{Main|Matter}} The remaining 4.9% of the mass–energy of the universe is ordinary matter, that is, [[atom]]s, [[ion]]s, [[electron]]s and the objects they form. This matter includes [[star]]s, which produce nearly all of the light we see from galaxies, as well as interstellar gas in the [[interstellar medium|interstellar]] and [[intergalactic medium|intergalactic]] media, [[planet]]s, and all the objects from everyday life that we can bump into, touch or squeeze.<ref name="Davies2">{{cite book |author=Davies |first=P. |url=https://books.google.com/books?id=akb2FpZSGnMC&pg=PA1 |title=The New Physics: A Synthesis |date=1992 |publisher=[[Cambridge University Press]] |isbn=978-0-521-43831-5 |page=1 |language=en |access-date=May 17, 2020 |archive-url=https://web.archive.org/web/20210203103749/https://books.google.com/books?id=akb2FpZSGnMC&pg=PA1 |archive-date=February 3, 2021 |url-status=live}}</ref> The great majority of ordinary matter in the universe is unseen, since visible stars and gas inside galaxies and clusters account for less than 10 percent of the ordinary matter contribution to the mass–energy density of the universe.<ref>{{Cite journal | last1=Persic | first1=Massimo | last2=Salucci | first2=Paolo | date=September 1, 1992 | title=The baryon content of the universe | journal=Monthly Notices of the Royal Astronomical Society | language=en | volume=258 | issue=1 | pages=14P–18P | doi=10.1093/mnras/258.1.14P | issn=0035-8711 |arxiv=astro-ph/0502178 |bibcode=1992MNRAS.258P..14P |s2cid=17945298 }}</ref><ref>{{Cite journal |last1=Shull |first1=J. Michael |last2=Smith |first2=Britton D. |last3=Danforth |first3=Charles W. |date=November 1, 2012 |title=The Baryon Census in a Multiphase Intergalactic Medium: 30% of the Baryons May Still Be Missing |url=https://iopscience.iop.org/article/10.1088/0004-637X/759/1/23 |journal=The Astrophysical Journal |volume=759 |issue=1 |pages=23 |doi=10.1088/0004-637X/759/1/23 |arxiv=1112.2706 |bibcode=2012ApJ...759...23S |s2cid=119295243 |issn=0004-637X |quote=Galaxy surveys have found ~10% of these baryons in collapsed objects such as galaxies, groups, and clusters [...] Of the remaining 80%–90% of cosmological baryons, approximately half can be accounted for in the low-z [intergalactic medium] |access-date=February 27, 2023 |archive-date=September 21, 2023 |archive-url=https://web.archive.org/web/20230921160249/https://iopscience.iop.org/article/10.1088/0004-637X/759/1/23 |url-status=live }}</ref><ref>{{Cite journal |last1=Macquart |first1=J.-P. |last2=Prochaska |first2=J. X. |last3=McQuinn |first3=M. |last4=Bannister |first4=K. W. |last5=Bhandari |first5=S. |last6=Day |first6=C. K. |last7=Deller |first7=A. T. |last8=Ekers |first8=R. D. |last9=James |first9=C. W. |last10=Marnoch |first10=L. |last11=Osłowski |first11=S. |last12=Phillips |first12=C. |last13=Ryder |first13=S. D. |last14=Scott |first14=D. R. |last15=Shannon |first15=R. M. |date=May 28, 2020 |title=A census of baryons in the Universe from localized fast radio bursts |url=http://www.nature.com/articles/s41586-020-2300-2 |journal=Nature |language=en |volume=581 |issue=7809 |pages=391–395 |doi=10.1038/s41586-020-2300-2 |pmid=32461651 |arxiv=2005.13161 |bibcode=2020Natur.581..391M |s2cid=256821489 |issn=0028-0836 |access-date=February 27, 2023 |archive-date=November 5, 2023 |archive-url=https://web.archive.org/web/20231105012727/https://www.nature.com/articles/s41586-020-2300-2 |url-status=live }}</ref> Ordinary matter commonly exists in four [[state of matter|states]] (or [[phase (matter)|phases]]): [[solid]], [[liquid]], [[gas]], and [[plasma (physics)|plasma]].<ref>{{cite book |url=https://openstax.org/books/chemistry-2e/pages/1-2-phases-and-classification-of-matter |title=Chemistry 2e |publisher=OpenStax |first1=Paul |last1=Flowers |display-authors=etal |year=2019 |isbn=978-1-947-17262-3 |page=14 |access-date=February 17, 2023 |archive-date=February 17, 2023 |archive-url=https://web.archive.org/web/20230217173041/https://openstax.org/books/chemistry-2e/pages/1-2-phases-and-classification-of-matter |url-status=live }}</ref> However, advances in experimental techniques have revealed other previously theoretical phases, such as [[Bose–Einstein condensate]]s and [[fermionic condensate]]s.<ref>{{Cite web |title=The Nobel Prize in Physics 2001 |url=https://www.nobelprize.org/prizes/physics/2001/popular-information/ |access-date=February 17, 2023 |website=NobelPrize.org |language=en-US |archive-date=February 17, 2023 |archive-url=https://web.archive.org/web/20230217172801/https://www.nobelprize.org/prizes/physics/2001/popular-information/ |url-status=live }}</ref><ref>{{Cite book |last1=Cohen-Tannoudji |first1=Claude |url=https://books.google.com/books?id=HT_ICgAAQBAJ |title=Advances In Atomic Physics: An Overview |last2=Guery-Odelin |first2=David |date=2011 |publisher=World Scientific |isbn=978-981-4390-58-3 |pages=684 |language=en |author-link=Claude Cohen-Tannoudji |access-date=February 17, 2023 |archive-date=June 4, 2023 |archive-url=https://web.archive.org/web/20230604212103/https://books.google.com/books?id=HT_ICgAAQBAJ |url-status=live }}</ref> Ordinary matter is composed of two types of [[elementary particle]]s: [[quark]]s and [[lepton]]s.<ref name="Hooft">{{cite book |author='t Hooft |first=G. |url=https://archive.org/details/insearchofultima0000hoof |title=In search of the ultimate building blocks |date=1997 |publisher=[[Cambridge University Press]] |isbn=978-0-521-57883-7 |page=[https://archive.org/details/insearchofultima0000hoof/page/6 6] |language=en |url-access=registration}}</ref> For example, the proton is formed of two [[up quarks]] and one [[down quark]]; the neutron is formed of two down quarks and one up quark; and the electron is a kind of lepton. An atom consists of an [[atomic nucleus]], made up of protons and neutrons (both of which are [[baryons]]), and electrons that orbit the nucleus.<ref name="OpenStax-college-physics">{{cite book |url=https://openstax.org/books/college-physics-2e/pages/33-4-particles-patterns-and-conservation-laws |title=College Physics 2e |publisher=OpenStax |first1=Paul Peter |last1=Urone |display-authors=etal |isbn=978-1-951-69360-2 |year=2022 |access-date=February 13, 2023 |archive-date=February 13, 2023 |archive-url=https://web.archive.org/web/20230213180410/https://openstax.org/books/college-physics-2e/pages/33-4-particles-patterns-and-conservation-laws |url-status=live }}</ref>{{rp|1476}} Because most of the mass of an atom is concentrated in its nucleus, which is made up of baryons, astronomers often use the term ''[[Baryon#Baryonic matter|baryonic matter]]'' to describe ordinary matter, although a small fraction of this "baryonic matter" is electrons. Soon after the [[Big Bang]], primordial protons and neutrons formed from the [[quark–gluon plasma]] of the early universe as it cooled below two trillion degrees. A few minutes later, in a process known as [[Big Bang nucleosynthesis]], nuclei formed from the primordial protons and neutrons. This nucleosynthesis formed lighter elements, those with small atomic numbers up to [[lithium]] and [[beryllium]], but the abundance of heavier elements dropped off sharply with increasing atomic number. Some [[boron]] may have been formed at this time, but the next heavier element, [[carbon]], was not formed in significant amounts. Big Bang nucleosynthesis shut down after about 20 minutes due to the rapid drop in temperature and density of the expanding universe. Subsequent formation of [[metallicity|heavier elements]] resulted from [[stellar nucleosynthesis]] and [[supernova nucleosynthesis]].<ref name=Clayton1983>{{cite book|last1=Clayton|first1=Donald D.|title=Principles of Stellar Evolution and Nucleosynthesis|url=https://archive.org/details/principlesofstel0000clay|url-access=registration|date=1983|publisher=The University of Chicago Press|isbn=978-0-226-10953-4|pages=[https://archive.org/details/principlesofstel0000clay/page/362 362–435]}}</ref> === Particles === [[File:Standard Model of Elementary Particles.svg|thumb|upright=2.2|Standard model of elementary particles: the 12 fundamental fermions and 4 fundamental bosons. Brown loops indicate which bosons (red) couple to which fermions (purple and green). Columns are three generations of matter (fermions) and one of forces (bosons). In the first three columns, two rows contain quarks and two leptons. The top two rows' columns contain up (u) and down (d) quarks, charm (c) and strange (s) quarks, top (t) and bottom (b) quarks, and photon (γ) and gluon (g), respectively. The bottom two rows' columns contain electron neutrino (ν<sub>e</sub>) and electron (e), muon neutrino (ν<sub>μ</sub>) and muon (μ), tau neutrino (ν<sub>τ</sub>) and tau (τ), and the Z<sup>0</sup> and W<sup>±</sup> carriers of the weak force. Mass, charge, and spin are listed for each particle.|alt=A four-by-four table of particles. Columns are three generations of matter (fermions) and one of forces (bosons). In the first three columns, two rows contain quarks and two leptons. The top two rows' columns contain up (u) and down (d) quarks, charm (c) and strange (s) quarks, top (t) and bottom (b) quarks, and photon (γ) and gluon (g), respectively. The bottom two rows' columns contain electron neutrino (ν sub e) and electron (e), muon neutrino (ν sub μ) and muon (μ), and tau neutrino (ν sub τ) and tau (τ), and Z sup 0 and W sup ± weak force. Mass, charge, and spin are listed for each particle.]] {{Main|Particle physics}} Ordinary matter and the forces that act on matter can be described in terms of [[elementary particle]]s.<ref>{{cite book |author=Veltman, Martinus |title=Facts and Mysteries in Elementary Particle Physics |url=https://archive.org/details/factsmysteriesin0000velt |url-access=registration |publisher=World Scientific |year=2003 |isbn=978-981-238-149-1}}</ref> These particles are sometimes described as being fundamental, since they have an unknown substructure, and it is unknown whether or not they are composed of smaller and even more fundamental particles.<ref name=PFIp1-3>{{cite book |first1=Sylvie |last1=Braibant |first2=Giorgio |last2=Giacomelli |first3=Maurizio |last3=Spurio |year=2012 |title=Particles and Fundamental Interactions: An Introduction to Particle Physics |url=https://books.google.com/books?id=e8YUUG2pGeIC&pg=PA1 |edition=2nd |pages=1–3 |publisher=[[Springer (publisher)|Springer]] |isbn=978-94-007-2463-1 |access-date=January 27, 2016 |archive-date=August 26, 2016 |archive-url=https://web.archive.org/web/20160826133823/https://books.google.com/books?id=e8YUUG2pGeIC&pg=PA1 |url-status=live }}</ref><ref name=Close>{{cite book |author-last=Close |author-first=Frank |year=2012 |title=Particle Physics: A Very Short Introduction |publisher=Oxford University Press |isbn=978-0-19-280434-1 }}</ref> In most contemporary models they are thought of as points in space.<ref>{{Cite web |last=Mann |first=Adam |date=August 20, 2022 |title=What Are Elementary Particles? |url=https://www.livescience.com/65427-fundamental-elementary-particles.html |access-date=August 17, 2023 |website=Live Science |archive-date=August 17, 2023 |archive-url=https://web.archive.org/web/20230817161504/https://www.livescience.com/65427-fundamental-elementary-particles.html |url-status=live }}</ref> All elementary particles are currently best explained by [[quantum mechanics]] and exhibit [[wave–particle duality]]: their behavior has both particle-like and [[wave]]-like aspects, with different features dominating under different circumstances.<ref>{{cite book |last=Zwiebach |first=Barton |title=Mastering Quantum Mechanics: Essentials, Theory, and Applications |publisher=MIT Press |year=2022 |isbn=978-0-262-04613-8 |page=31 |author-link=Barton Zwiebach}}</ref> Of central importance is the [[Standard Model]], a theory that is concerned with [[Electromagnetism|electromagnetic]] interactions and the [[Weak interaction|weak]] and [[Strong interaction|strong]] nuclear interactions.<ref name="Oerter2006">{{cite book |author=Oerter |first=R. |url=https://archive.org/details/theoryofalmostev0000oert |title=The Theory of Almost Everything: The Standard Model, the Unsung Triumph of Modern Physics |publisher=[[Penguin Group]] |year=2006 |isbn=978-0-13-236678-6 |page=[https://archive.org/details/theoryofalmostev0000oert/page/2 2] |format=Kindle |url-access=registration}}</ref> The Standard Model is supported by the experimental confirmation of the existence of particles that compose matter: [[quark]]s and [[lepton]]s, and their corresponding "[[antimatter]]" duals, as well as the force particles that mediate [[fundamental interactions|interactions]]: the [[photon]], the [[W and Z bosons]], and the [[gluon]].<ref name=PFIp1-3 /> The Standard Model predicted the existence of the recently discovered [[Higgs boson]], a particle that is a manifestation of a field within the universe that can endow particles with mass.<ref name="OnyisiFAQ">{{cite web |last=Onyisi |first=P. |date=October 23, 2012 |title=Higgs boson FAQ |url=https://wikis.utexas.edu/display/utatlas/Higgs+boson+FAQ |publisher=[[University of Texas]] ATLAS group |access-date=January 8, 2013 |archive-date=October 12, 2013 |archive-url=https://web.archive.org/web/20131012130340/https://wikis.utexas.edu/display/utatlas/Higgs+boson+FAQ |url-status=live }}</ref><ref name="strasslerFAQ2">{{cite web |last=Strassler |first=M. |date=October 12, 2012 |title=The Higgs FAQ 2.0 |url=http://profmattstrassler.com/articles-and-posts/the-higgs-particle/the-higgs-faq-2-0/ |work=ProfMattStrassler.com |access-date=January 8, 2013 |quote=[Q] Why do particle physicists care so much about the Higgs particle?<br />[A] Well, actually, they don't. What they really care about is the Higgs ''field'', because it is ''so'' important. [emphasis in original] |archive-date=October 12, 2013 |archive-url=https://web.archive.org/web/20131012042637/http://profmattstrassler.com/articles-and-posts/the-higgs-particle/the-higgs-faq-2-0/ |url-status=live }}</ref> Because of its success in explaining a wide variety of experimental results, the Standard Model is sometimes regarded as a "theory of almost everything".<ref name=Oerter2006 /> The Standard Model does not, however, accommodate gravity. A true force–particle "theory of everything" has not been attained.<ref name="Weinberg2011">{{cite book|first=Steven|last=Weinberg|title=Dreams of a Final Theory: The Scientist's Search for the Ultimate Laws of Nature|publisher=Knopf Doubleday Publishing Group|isbn=978-0-307-78786-6|date=2011}}</ref> ==== Hadrons ==== {{Main|Hadron}} A hadron is a [[composite particle]] made of [[quark]]s [[bound state|held together]] by the [[strong force]]. Hadrons are categorized into two families: [[baryon]]s (such as [[proton]]s and [[neutron]]s) made of three quarks, and [[meson]]s (such as [[pion]]s) made of one quark and one [[antiparticle|antiquark]]. Of the hadrons, protons are stable, and neutrons bound within atomic nuclei are stable. Other hadrons are unstable under ordinary conditions and are thus insignificant constituents of the modern universe.<ref name=Allday2002/>{{rp|118–123}} From approximately 10<sup>−6</sup> seconds after the [[Big Bang]], during a period known as the [[hadron epoch]], the temperature of the universe had fallen sufficiently to allow quarks to bind together into hadrons, and the mass of the universe was dominated by [[hadron]]s. Initially, the temperature was high enough to allow the formation of hadron–anti-hadron pairs, which kept matter and antimatter in [[thermal equilibrium]]. However, as the temperature of the universe continued to fall, hadron–anti-hadron pairs were no longer produced. Most of the hadrons and anti-hadrons were then eliminated in particle–antiparticle [[annihilation]] reactions, leaving a small residual of hadrons by the time the universe was about one second old.<ref name=Allday2002>{{cite book|last1=Allday|first1=Jonathan|title=Quarks, Leptons and the Big Bang|date=2002|publisher=IOP Publishing|isbn=978-0-7503-0806-9|edition=2nd}}</ref>{{rp|244–266}} ==== Leptons ==== {{Main|Lepton}} A lepton is an [[elementary particle|elementary]], [[half-integer spin]] particle that does not undergo strong interactions but is subject to the [[Pauli exclusion principle]]; no two leptons of the same species can be in exactly the same state at the same time.<ref>{{cite encyclopedia |title=Lepton (physics) |url=http://www.britannica.com/EBchecked/topic/336940/lepton |encyclopedia=[[Encyclopædia Britannica]] |access-date=September 29, 2010 |archive-date=May 11, 2015 |archive-url=https://web.archive.org/web/20150511203531/http://www.britannica.com/EBchecked/topic/336940/lepton |url-status=live }}</ref> Two main classes of leptons exist: [[electric charge|charged]] leptons (also known as the ''electron-like'' leptons), and neutral leptons (better known as [[neutrino]]s). Electrons are stable and the most common charged lepton in the universe, whereas [[muon]]s and [[tau (particle)|taus]] are unstable particles that quickly decay after being produced in [[high energy physics|high energy]] collisions, such as those involving [[cosmic ray]]s or carried out in [[particle accelerator]]s.<ref>{{cite book | last=Harari | first=H. | year=1977 | chapter=Beyond charm | title=Weak and Electromagnetic Interactions at High Energy, Les Houches, France, Jul 5 – Aug 14, 1976 | editor1-last=Balian | editor1-first=R. | editor2-last=Llewellyn-Smith | editor2-first=C.H. | series=Les Houches Summer School Proceedings | volume=29 | page=613 | publisher=[[North-Holland Publishing Company|North-Holland]] }}</ref><ref>{{cite conference |author=Harari H. |title=Three generations of quarks and leptons |url=https://www.slac.stanford.edu/cgi-bin/getdoc/slac-pub-1974.pdf |book-title=Proceedings of the XII Rencontre de Moriond |editor1=E. van Goeler |editor2=Weinstein R. |page=170 |year=1977 |id=SLAC-PUB-1974 |conference= |access-date=May 29, 2020 |archive-date=May 13, 2020 |archive-url=https://web.archive.org/web/20200513180308/https://www.slac.stanford.edu/cgi-bin/getdoc/slac-pub-1974.pdf |url-status=live }}</ref> Charged leptons can combine with other particles to form various [[composite particle]]s such as [[atom]]s and [[positronium]]. The [[electron]] governs nearly all of [[chemistry]], as it is found in [[atom]]s and is directly tied to all [[chemical property|chemical properties]]. Neutrinos rarely interact with anything, and are consequently rarely observed. Neutrinos stream throughout the universe but rarely interact with normal matter.<ref>{{cite press release |publisher=[[Massachusetts Institute of Technology|MIT News Office]] |date=April 18, 2007 |title=Experiment confirms famous physics model |url=http://web.mit.edu/newsoffice/2007/neutrino.html |access-date=June 2, 2015 |archive-date=July 5, 2013 |archive-url=https://web.archive.org/web/20130705100832/http://web.mit.edu/newsoffice/2007/neutrino.html |url-status=live }}</ref> The [[lepton epoch]] was the period in the evolution of the early universe in which the [[lepton]]s dominated the mass of the universe. It started roughly 1 second after the [[Big Bang]], after the majority of hadrons and anti-hadrons annihilated each other at the end of the [[hadron epoch]]. During the lepton epoch the temperature of the universe was still high enough to create lepton–anti-lepton pairs, so leptons and anti-leptons were in thermal equilibrium. Approximately 10 seconds after the Big Bang, the temperature of the universe had fallen to the point where lepton–anti-lepton pairs were no longer created.<ref>{{cite web|title=Thermal history of the universe and early growth of density fluctuations|url=http://wwwmpa.mpa-garching.mpg.de/~gamk/TUM_Lectures/Lecture4.pdf|work=Guinevere Kauffmann|publisher=[[Max Planck Institute for Astrophysics]]|access-date=January 6, 2016|archive-date=August 21, 2016|archive-url=https://web.archive.org/web/20160821041542/http://wwwmpa.mpa-garching.mpg.de/~gamk/TUM_Lectures/Lecture4.pdf|url-status=live}}</ref> Most leptons and anti-leptons were then eliminated in [[annihilation]] reactions, leaving a small residue of leptons. The mass of the universe was then dominated by [[photon]]s as it entered the following [[photon epoch]].<ref>{{cite web|title=First few minutes|work=Eric Chaisson|publisher=Harvard Smithsonian Center for Astrophysics|url=https://www.cfa.harvard.edu/~ejchaisson/cosmic_evolution/docs/fr_1/fr_1_part3.html|access-date=January 6, 2016|archive-date=December 4, 2013|archive-url=https://web.archive.org/web/20131204050252/https://www.cfa.harvard.edu/~ejchaisson/cosmic_evolution/docs/fr_1/fr_1_part3.html|url-status=live}}</ref><ref>{{cite web|title=Timeline of the Big Bang|work=The physics of the Universe|url=https://www.physicsoftheuniverse.com/topics_bigbang_timeline.html|access-date=January 6, 2016|archive-date=March 30, 2020|archive-url=https://web.archive.org/web/20200330140345/https://www.physicsoftheuniverse.com/topics_bigbang_timeline.html|url-status=live}}</ref> ==== Photons ==== {{Main|Photon epoch}} {{See also|Photino}} A photon is the [[quantum]] of [[light]] and all other forms of [[electromagnetic radiation]]. It is the [[force carrier|carrier]] for the [[electromagnetic force]]. The effects of this [[force]] are easily observable at the [[microscopic scale|microscopic]] and at the [[macroscopic scale|macroscopic]] level because the photon has zero [[rest mass]]; this allows long distance [[fundamental interaction|interactions]].<ref name="OpenStax-college-physics"/>{{rp|1470}} The photon epoch started after most leptons and anti-leptons were [[annihilation|annihilated]] at the end of the lepton epoch, about 10 seconds after the Big Bang. Atomic nuclei were created in the process of nucleosynthesis which occurred during the first few minutes of the photon epoch. For the remainder of the photon epoch the universe contained a hot dense [[plasma (physics)|plasma]] of nuclei, electrons and photons. About 380,000 years after the Big Bang, the temperature of the universe fell to the point where nuclei could combine with electrons to create neutral atoms. As a result, photons no longer interacted frequently with matter and the universe became transparent. The highly redshifted photons from this period form the cosmic microwave background. Tiny variations in temperature and density detectable in the CMB were the early "seeds" from which all subsequent [[structure formation]] took place.<ref name=Allday2002 />{{rp|244–266}} {{Big Bang timeline|state=collapsed}} Summary: Please note that all contributions to Christianpedia may be edited, altered, or removed by other contributors. If you do not want your writing to be edited mercilessly, then do not submit it here. You are also promising us that you wrote this yourself, or copied it from a public domain or similar free resource (see Christianpedia:Copyrights for details). Do not submit copyrighted work without permission! Cancel Editing help (opens in new window) Discuss this page