Editing Big Bang (section)

==Concept history==
{{Main|History of the Big Bang theory}}
{{See also|Timeline of cosmological theories}}

===Etymology===
[[English people|English]] [[astronomer]] [[Fred Hoyle]] is credited with coining the term "Big Bang" during a talk for a March 1949 [[BBC Radio]] broadcast,<ref>{{cite news |author=<!--Staff writer(s); no by-line.--> |date=22 August 2001 |title='Big bang' astronomer dies |url=http://news.bbc.co.uk/2/hi/uk_news/1503721.stm |url-status=live |department=Sci/Tech |work=[[BBC News]] |location=London |publisher=[[BBC]] |access-date=2 December 2019 |archive-url=https://web.archive.org/web/20190903152416/http://news.bbc.co.uk/2/hi/uk_news/1503721.stm |archive-date=3 September 2019}}</ref> saying: "These theories were based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past."<ref>{{cite web |url=https://www.joh.cam.ac.uk/library/special_collections/hoyle/exhibition/radio/ |url-status=live |title=Hoyle on the Radio: Creating the 'Big Bang' |author=|website=Fred Hoyle: An Online Exhibition |location=Cambridge |publisher=[[St John's College, Cambridge|St John's College]] |archive-url=https://web.archive.org/web/20140526084945/https://www.joh.cam.ac.uk/library/special_collections/hoyle/exhibition/radio/ |archive-date=26 May 2014 |access-date=2 December 2019}}</ref><ref name="Kragh2013">{{cite journal |last=Kragh |first=Helge |author-link=Helge Kragh |date=April 2013 |title=Big Bang: the etymology of a name |journal=[[Astronomy & Geophysics]] |volume=54 |issue=2 |pages=2.28–2.30 |doi=10.1093/astrogeo/att035 |bibcode=2013A&G....54b2.28K|doi-access=free }}</ref> However, it did not catch on until the 1970s.<ref name="Kragh2013"/>

It is popularly reported that Hoyle, who favored an alternative "[[Steady-state model|steady-state]]" cosmological model, intended this to be pejorative,<ref>{{cite web |last=Mattson |first=Barbara (Project Leader) |date=8 December 2017 |title=Hoyle Scoffs at 'Big Bang' Universe Theory |url=https://imagine.gsfc.nasa.gov/educators/programs/cosmictimes/online_edition/1955/hoyle.html |url-status=live |archive-url=https://web.archive.org/web/20180310172435/https://imagine.gsfc.nasa.gov/educators/programs/cosmictimes/online_edition/1955/hoyle.html |archive-date=10 March 2018 |access-date=2 December 2019 |website=Cosmic Times (hosted by Imagine the Universe!) |publisher=[[NASA]]: [[Goddard Space Flight Center#High Energy Astrophysics Science Archive Research Center|High Energy Astrophysics Science Archive Research Center]] |oclc=227004453 |location=Greenbelt, Maryland}}
</ref><ref name="Mathew2013">{{cite book |last1=Mathew |first1=Santhosh |title=Essays on the Frontiers of Modern Astrophysics and Cosmology |date=2013 |publisher=Springer Science & Business Media |isbn=978-3-319-01887-4 |page=13 |url=https://books.google.com/books?id=1--3BAAAQBAJ&pg=PA13}}
</ref><ref name="Ferris"/> but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models.<ref name="Croswell1995_chpt9">{{harvnb|Croswell|1995|loc=chapter 9|p=113}}</ref><ref name="Mitton2011_p129">{{harvnb|Mitton|2011|p=[https://books.google.com/books?id=MWKQhXo2eaIC&pg=PA129 129]}}: "To create a picture in the mind of the listener, Hoyle had likened the explosive theory of the universe's origin to a 'big bang'."</ref>{{refn|Hoyle stated:
"I was constantly striving over the radio – where I had no visual aids, nothing except the spoken word – for visual images. And that seemed to be one way of distinguishing between the steady-state and the explosive big bang. And so that was the language I used."<ref name="Kragh2013"/><ref>{{cite book |last1=Kragh |first1=Helge |title=Masters of the Universe: Conversations with Cosmologists of the Past |date=2014 |publisher=Oxford University Press |isbn=978-0-19-103442-8 |page=210n30 |url=https://books.google.com/books?id=ZripBAAAQBAJ&pg=PT210}}</ref>}} [[Helge Kragh]] writes that the evidence for the claim that it was meant as a pejorative is "unconvincing", and mentions a number of indications that it was not a pejorative.<ref name="Kragh2013"/>

A primordial singularity is sometimes called "the Big Bang",<ref>{{harvnb|Roos|2012|p=216}}: "This singularity is termed the Big Bang."</ref> but the term can also refer to a more generic early hot, dense phase.<ref>{{harvnb|Drees|1990|pp=[https://archive.org/details/beyondbigbangqua0000dree/page/223 223–224]}}</ref> The term itself has been argued to be a misnomer because it evokes an explosion.<ref name="Kragh2013"/><ref>{{cite book |last1=Kaler |first1=James B. |title=The Little Book of Stars |date=2013 |publisher=Springer Science & Business Media |isbn=978-0-387-21621-8 |page=3 |url=https://books.google.com/books?id=l8S9BwAAQBAJ&pg=PA4}}</ref> The argument is that whereas an explosion suggests expansion into a surrounding space, the Big Bang only describes the intrinsic expansion of the contents of the universe.<ref>
{{cite book |last1=Emam |first1=Moataz |url=https://books.google.com/books?id=wX4fEAAAQBAJ&pg=PA208 |title=Covariant Physics: From Classical Mechanics to General Relativity and Beyond |date=2021 |publisher=Oxford University Press |isbn=978-0-19-886489-9 |pages=208–246 |quote=The term "Big Bang" is an unfortunate misnomer. It implies an "explosion," and explosions are events that happen ''in'' space. This is incorrect; the term describes the first instant in the expansion ''of'' space itself. Some would even interpret it as the very beginning of the universe, evolving from "nothing." It is hard to imagine exactly what it was, but an explosion it most definitely wasn't.}}
</ref><ref>
{{cite web |last1=Moskowitz |first1=Clara |title=Was the Big Bang Really an Explosion? |url=https://www.livescience.com/32278-was-the-big-bang-really-an-explosion.html |website=Live Science |date=2010}}
</ref> Another issue pointed out by Santhosh Mathew is that bang implies sound, which is not an important feature of the model.<ref name="Mathew2013"/> However, an attempt to find a more suitable alternative was not successful.<ref name="Kragh2013"/> According to [[Timothy Ferris]]:<ref name="Ferris">{{cite book |last1=Ferris |first1=Timothy |title=The Whole Shebang: A State of the Universe Report |date=1998 |publisher=Simon and Schuster |isbn=978-0-684-83861-8 |page=323n10 |url=https://books.google.com/books?id=qjYbQ7EBAKwC&pg=PA323}}</ref><ref>{{cite book |last1=Gaither |first1=Carl C. |last2=Cavazos-Gaither |first2=Alma E. |title=Gaither's Dictionary of Scientific Quotations |date=2012 |publisher=Springer Science & Business Media |isbn=978-1-4614-1114-7 |edition=2nd |url=https://books.google.com/books?id=zQaCSlEM-OEC&pg=PA272}} (quoting Ferris).</ref>
<blockquote>The term 'big bang' was coined with derisive intent by Fred Hoyle, and its endurance testifies to Sir Fred's creativity and wit. Indeed, the term survived an international competition in which three judges — the television science reporter [[Hugh Downs]], the astronomer [[Carl Sagan]], and myself — sifted through 13,099 entries from 41 countries and concluded that none was apt enough to replace it. No winner was declared, and like it or not, we are stuck with 'big bang'.</blockquote>

===Before the name===
{{Multiple image |direction=vertical |align=right |width=400|image1=XDF-scale.jpg|image2=The Hubble eXtreme Deep Field.jpg |image3=XDF-separated.jpg|caption1=''[[Hubble Ultra-Deep Field#Hubble eXtreme Deep Field|XDF]]'' size compared to the size of the [[Moon]] (''XDF'' is the small box to the left of, and nearly below, the Moon)  – several thousand galaxies, each consisting of billions of stars, are in this small view. |caption2=''[[Hubble Ultra-Deep Field#Hubble eXtreme Deep Field|XDF]]'' (2012) view – each light speck is a galaxy – some of these are as old as 13.2 billion years<ref name="Space-20120925">{{cite web |url=https://www.space.com/17755-farthest-universe-view-hubble-space-telescope.html |url-status=live |last=Moskowitz |first=Clara |date=25 September 2012 |title=Hubble Telescope Reveals Farthest View Into Universe Ever |website=[[Space.com]] |location=New York |publisher=[[Future plc]] |archive-url=https://web.archive.org/web/20191012164808/https://www.space.com/17755-farthest-universe-view-hubble-space-telescope.html |archive-date=12 October 2019 |access-date=3 December 2019}}</ref> – the universe is estimated to contain 200 billion galaxies. |caption3=''[[Hubble Ultra-Deep Field#Hubble eXtreme Deep Field|XDF]]'' image shows fully mature galaxies in the foreground plane – nearly mature galaxies from 5 to 9 billion years ago – [[Protogalaxy|protogalaxies]], blazing with [[young star]]s, beyond 9 billion years. |header=''[[Hubble Ultra-Deep Field#Hubble eXtreme Deep Field|Hubble eXtreme Deep Field (XDF)]]''}}
Early cosmological models developed from observations of the structure of the universe and from theoretical considerations. In 1912, [[Vesto Slipher]] measured the first [[Doppler effect|Doppler shift]] of a "[[Spiral galaxy#Spiral nebula|spiral nebula]]" (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was [[Great Debate (astronomy)|highly controversial]] whether or not these nebulae were "island universes" outside our [[Milky Way]].<ref>{{cite journal |last=Slipher |first=Vesto M. |author-link=Vesto Slipher |year=1913 |title=The Radial Velocity of the Andromeda Nebula |journal=[[Lowell Observatory Bulletin]] |volume=1 |issue=8 |pages=56–57 |bibcode=1913LowOB...2...56S}}</ref><ref>{{cite journal |last=Slipher |first=Vesto M. |author-link=Vesto Slipher |date=January 1915 |title=Spectrographic Observations of Nebulae |journal=[[Popular Astronomy (US magazine)|Popular Astronomy]] |volume=23 |pages=21–24 |bibcode=1915PA.....23...21S}}</ref> Ten years later, [[Alexander Friedmann]], a [[Russia]]n [[physical cosmology|cosmologist]] and [[mathematician]], derived the [[Friedmann equations]] from the Einstein field equations, showing that the universe might be expanding in contrast to the [[static universe]] model advocated by [[Albert Einstein]] at that time.<ref name=af1922>{{cite journal |last=Friedman |first=Alexander |author-link=Alexander Friedmann |date=December 1922 |title=Über die Krümmung des Raumes |journal=[[Zeitschrift für Physik]] |language=de |volume=10 |issue=1 |pages=377–386 |bibcode=1922ZPhy...10..377F |doi=10.1007/BF01332580 |s2cid=125190902}}</ref><ref>{{cite journal |last=Friedmann |first=Alexander |author-link=Alexander Friedmann |date=December 1999 |title=On the Curvature of Space |journal=[[General Relativity and Gravitation]] |language=en |volume=31 |issue=12 |pages=1991–2000 |bibcode=1999GReGr..31.1991F |doi=10.1023/A:1026751225741 |s2cid=122950995}}
* Friedmann (1922) translated into English.</ref>

In 1924, [[Americans|American]] astronomer [[Edwin Hubble]]'s measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Starting that same year, Hubble painstakingly developed a series of distance indicators, the forerunner of the [[cosmic distance ladder]], using the {{convert|100|in|m|adj=on}} [[100-inch Hooker telescope|Hooker telescope]] at [[Mount Wilson Observatory]]. This allowed him to estimate distances to galaxies whose [[redshift]]s had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and [[recessional velocity]]—now known as Hubble's law.<ref name="hubble">{{cite journal|last=Hubble|first=Edwin|author-link=Edwin Hubble|date=15 March 1929|title=A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae|url=https://apod.nasa.gov/debate/1996/hub_1929.html|url-status=live|journal=[[Proceedings of the National Academy of Sciences]]|volume=15|issue=3|pages=168–173|bibcode=1929PNAS...15..168H|doi=10.1073/pnas.15.3.168|pmc=522427|pmid=16577160|archive-url=https://web.archive.org/web/20061001060258/https://apod.nasa.gov/debate/1996/hub_1929.html|archive-date=1 October 2006|access-date=28 November 2019|doi-access=free}}</ref><ref name="christianson">{{harvnb|Christianson|1995}}</ref>

{{anchor|primeval atom}}Independently deriving Friedmann's equations in 1927, [[Georges Lemaître]], a [[Belgium|Belgian]] [[physicist]] and [[Roman Catholic priest]], proposed that the recession of the nebulae was due to the expansion of the universe.<ref name="gl1927">{{cite journal |last=Lemaître |first=Georges |author-link=Georges Lemaître |date=April 1927 |title=Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques |url=https://archive.org/details/B-001-004-204 |journal=Annales de la Société scientifique de Bruxelles |language=fr |volume=47 |pages=49–59 |bibcode=1927ASSB...47...49L }}</ref><ref>{{cite journal |last=Lemaître |first=Georges |author-link=Georges Lemaître |date=March 1931 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=91 |issue=5 |pages=483–490 |title=A Homogeneous Universe of Constant Mass and Increasing Radius accounting for the Radial Velocity of Extra-galactic Nebulæ |bibcode=1931MNRAS..91..483L |doi=10.1093/mnras/91.5.483 |doi-access=free}}
* Lemaître (1927) translated into English.</ref> He inferred the relation that Hubble would later observe, given the cosmological principle.<ref name="peebles" /> In 1931, Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in the past all the mass of the universe was concentrated into a single point, a "primeval atom" where and when the fabric of time and space came into existence.<ref>{{cite journal |last=Lemaître |first=Abbé Georges |author-link=Georges Lemaître |date=24 October 1931 |title=Contributions to a British Association Discussion on the Evolution of the Universe |journal=[[Nature (journal)|Nature]] |volume=128 |issue=3234 |pages=704–706 |bibcode=1931Natur.128..704L |doi=10.1038/128704a0 |s2cid=4028196}}</ref>

In the 1920s and 1930s, almost every major cosmologist preferred an eternal steady-state universe, and several complained that the beginning of time implied by an expanding universe imported religious concepts into physics; this objection was later repeated by supporters of the steady-state theory.<ref>{{harvnb|Kragh|1996}}</ref> This perception was enhanced by the fact that the originator of the expanding universe concept, Lemaître, was a Roman Catholic priest.<ref name="WGBH1927">{{cite web |author=<!--Staff writer(s); no by-line.--> |year=1998 |title=Big bang theory is introduced – 1927 |url=https://www.pbs.org/wgbh/aso/databank/entries/dp27bi.html |url-status=live |archive-url=https://web.archive.org/web/19990423033457/https://www.pbs.org/wgbh/aso/databank/entries/dp27bi.html |archive-date=23 April 1999 |access-date=31 July 2014 |website=A Science Odyssey |publisher=[[WGBH-TV|WGBH Boston]] |location=Boston, Massachusetts}}</ref> [[Arthur Eddington]] agreed with [[Aristotle]] that the universe did not have a beginning in time, ''viz''., that [[Eternity of the world|matter is eternal]]. A beginning in time was "repugnant" to him.<ref>{{cite journal |last=Eddington |first=Arthur S. |author-link=Arthur Eddington |date=21 March 1931 |title=The End of the World: from the Standpoint of Mathematical Physics |journal=[[Nature (journal)|Nature]] |volume=127 |issue=3203 |pages=447–453 |bibcode=1931Natur.127..447E |doi=10.1038/127447a0 |s2cid=4140648}}</ref><ref>{{cite journal |last=Appolloni |first=Simon |date=17 June 2011 |title='Repugnant', 'Not Repugnant at All': How the Respective Epistemic Attitudes of Georges Lemaitre and Sir Arthur Eddington Influenced How Each Approached the Idea of a Beginning of the Universe |url=https://journal.ibsu.edu.ge/index.php/ibsusj/article/view/180 |journal=IBSU Scientific Journal |volume=5 |issue=1 |pages=19–44}}</ref> Lemaître, however, disagreed:
{{blockquote|text=If the world has begun with a single [[quantum]], the notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time.<ref>{{cite journal |last=Lemaître |author-link=Georges Lemaître |first=Georges |date=9 May 1931 |title=The Beginning of the World from the Point of View of Quantum Theory |journal=[[Nature (journal)|Nature]] |volume=127 |issue=3210 |page=706 |bibcode=1931Natur.127..706L |doi=10.1038/127706b0 |s2cid=4089233 |issn=0028-0836|doi-access=free }}</ref>}}

During the 1930s, other ideas were proposed as [[Non-standard cosmology|non-standard cosmologies]] to explain Hubble's observations, including the [[Milne model]],<ref>{{harvnb|Milne|1935}}</ref> the [[Cyclic model|oscillatory universe]] (originally suggested by Friedmann, but advocated by Albert Einstein and [[Richard C. Tolman]])<ref>{{harvnb|Tolman|1934}}</ref> and [[Fritz Zwicky]]'s [[tired light]] hypothesis.<ref>{{cite journal |last=Zwicky |first=Fritz |author-link=Fritz Zwicky |date=15 October 1929 |title=On the Red Shift of Spectral Lines through Interstellar Space |journal=Proceedings of the National Academy of Sciences |volume=15 |issue=10 |pages=773–779 |bibcode=1929PNAS...15..773Z |doi=10.1073/pnas.15.10.773 |pmc=522555 |pmid=16577237|doi-access=free }}</ref>

After [[World War II]], two distinct possibilities emerged. One was Fred Hoyle's steady-state model, whereby new matter would be created as the universe seemed to expand. In this model the universe is roughly the same at any point in time.<ref>{{cite journal |last=Hoyle |first=Fred |author-link=Fred Hoyle |date=October 1948 |title=A New Model for the Expanding Universe |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=108 |issue=5 |pages=372–382 |bibcode=1948MNRAS.108..372H |doi=10.1093/mnras/108.5.372|doi-access=free }}</ref> The other was Lemaître's expanding universe theory, advocated and developed by [[George Gamow]], who used it to develop a theory for the abundance of chemical elements in the universe.<ref>{{cite journal |last1=Alpher |first1=Ralph A. |author1-link=Ralph Asher Alpher |last2=Bethe |first2=Hans |author2-link=Hans Bethe |last3=Gamow |first3=George |author3-link=George Gamow |date=1 April 1948 |title=The Origin of Chemical Elements |journal=[[Physical Review]] |volume=73 |issue=7 |pages=803–804 |bibcode=1948PhRv...73..803A |doi=10.1103/PhysRev.73.803 |pmid=18877094|doi-access=free }}</ref> and whose associates, [[Ralph Alpher]] and [[Robert Herman]], predicted the cosmic background radiation.<ref>{{cite journal |last1=Alpher |first1=Ralph A. |author1-link=Ralph Asher Alpher |last2=Herman |first2=Robert |author2-link=Robert Herman |date=13 November 1948 |title=Evolution of the Universe |journal=[[Nature (journal)|Nature]]  |volume=162 |issue=4124 |pages=774–775 |bibcode=1948Natur.162..774A |doi=10.1038/162774b0 |s2cid=4113488}}</ref>

=== As a named model ===
Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it as "this ''big bang'' idea" during a BBC Radio broadcast in March 1949.<ref name="Mitton2011_p129" /><ref name="Kragh2013" />{{refn|It is commonly reported that Hoyle intended this to be pejorative. However, Hoyle later denied that, saying that it was just a striking image meant to emphasize the difference between the two theories for radio listeners.<ref name="Croswell1995_chpt9" />|group="notes"}} For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio [[source counts]], began to favor Big Bang over steady state. The discovery and confirmation of the CMB in 1964 secured the Big Bang as the best theory of the origin and evolution of the universe.<ref name="penzias">{{cite journal |last1=Penzias |first1=Arno A. |author1-link=Arno Allan Penzias |last2=Wilson |first2=R. W. |author2-link=Robert Woodrow Wilson |date=July 1965 |title=A Measurement of Excess Antenna Temperature at 4080&nbsp;Mc/s |url=https://fermatslibrary.com/s/a-measurement-of-excess-antenna-temperature-at-4080-mc-s |url-status=live |journal=[[The Astrophysical Journal]] |volume=142 |pages=419–421 |bibcode=1965ApJ...142..419P |doi=10.1086/148307 |archive-url=https://web.archive.org/web/20191014185903/https://fermatslibrary.com/s/a-measurement-of-excess-antenna-temperature-at-4080-mc-s |archive-date=14 October 2019 |access-date=5 December 2019|doi-access=free }}</ref>

In 1968 and 1970, [[Roger Penrose]], [[Stephen Hawking]], and [[George F. R. Ellis]] published papers where they showed that [[Singularity (mathematics)|mathematical singularities]] were an inevitable initial condition of relativistic models of the Big Bang.<ref>{{cite journal |last1=Hawking |first1=Stephen W. |author1-link=Stephen Hawking |last2=Ellis |first2=George F. R. |author2-link=George F. R. Ellis |date=April 1968 |title=The Cosmic Black-Body Radiation and the Existence of Singularities in our Universe |journal=[[The Astrophysical Journal]] |volume=152 |page=25 |bibcode=1968ApJ...152...25H |doi=10.1086/149520}}</ref><ref>{{cite journal |last1=Hawking |first1=Stephen W. |author1-link=Stephen Hawking |last2=Penrose |first2=Roger |author2-link=Roger Penrose |date=27 January 1970 |title=The Singularities of Gravitational Collapse and Cosmology |volume=314 |issue=1519 |pages=529–548 |journal=[[Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences]] |bibcode=1970RSPSA.314..529H |doi=10.1098/rspa.1970.0021|s2cid=120208756 |doi-access= }}</ref> Then, from the 1970s to the 1990s, cosmologists worked on characterizing the features of the Big Bang universe and resolving outstanding problems. In 1981, [[Alan Guth]] made a breakthrough in theoretical work on resolving certain outstanding theoretical problems in the Big Bang models with the introduction of an epoch of rapid expansion in the early universe he called "inflation".<ref>{{cite journal |last=Guth |first=Alan |author-link=Alan Guth |date=15 January 1981 |title=Inflationary universe: A possible solution to the horizon and flatness problems |journal=[[Physical Review D]] |volume=23 |issue=2 |pages=347–356 |bibcode=1981PhRvD..23..347G |doi=10.1103/PhysRevD.23.347 |doi-access=free }}</ref> Meanwhile, during these decades, two questions in [[observational cosmology]] that generated much discussion and disagreement were over the precise values of the Hubble Constant<ref>{{cite journal |url=https://www.cfa.harvard.edu/~dfabricant/huchra/hubble/ |title=The Hubble Constant |last1=Huchra |first1=John P. |volume=256 |issue=5055 |pages=321–5 |author-link=John Huchra |year=2008 |journal=Science |archive-url=https://web.archive.org/web/20190930124013/https://www.cfa.harvard.edu/~dfabricant/huchra/hubble/ |archive-date=30 September 2019 |access-date=5 December 2019|pmid=17743107 |doi=10.1126/science.256.5055.321 |s2cid=206574821 }}</ref> and the matter-density of the universe (before the discovery of dark energy, thought to be the key predictor for the eventual [[Ultimate fate of the universe|fate of the universe]]).<ref>{{harvnb|Livio|2000|p=160}}</ref>

Significant progress in Big Bang cosmology has been made since the late 1990s as a result of advances in [[telescope]] technology as well as the analysis of data from satellites such as the [[Cosmic Background Explorer]] (COBE),<ref name=cobe>{{cite journal |last1=Boggess |first1=Nancy W. | last2=Mather | first2=John C. |author2-link=John C. Mather |author3-last=Weiss |author3-first=Rainer |author3-link=Rainer Weiss |last4=Bennett |first4=C. L. |last5=Cheng |first5=E. S. |last6=Dwek |first6=E. |last7=Gulkis |first7=S. |last8=Hauser |first8=M. G. |last9=Janssen |first9=M. A. |last10=Kelsall |first10=T. |last11=Meyer |first11=S. S. |last12=Moseley |first12=S. H. |last13=Murdock |first13=T. L. |last14=Shafer |first14=R. A. |last15=Silverberg |first15=R. F. |last16=Smoot |first16=G. F. |last17=Wilkinson |first17=D. T. |last18=Wright |first18=E. L. |display-authors=3 |date=1 October 1992 |title=The COBE Mission: Its Design and Performance Two Years after the launch |journal=[[The Astrophysical Journal]] |volume=397 |pages=420–429 |bibcode=1992ApJ...397..420B |doi=10.1086/171797|doi-access=free }}</ref> the [[Hubble Space Telescope]] and WMAP.<ref name="wmap1year">{{cite journal |last1=Spergel |first1=David N. |author1-link=David Spergel |last2=Bean |first2=Rachel |author2-link=Rachel Bean |last3=Doré |first3=Olivier |author-link3=Olivier Doré |display-authors=etal |date=June 2007 |title=Three-Year ''Wilkinson Microwave Anisotropy Probe (WMAP)'' Observations: Implications for Cosmology |journal=[[The Astrophysical Journal Supplement Series]] |volume=170 |issue=2 |pages=377–408 |arxiv=astro-ph/0603449 |bibcode=2007ApJS..170..377S |doi=10.1086/513700 |s2cid=1386346}}</ref> Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating.<ref>{{cite journal |last1=Reiss |first1=Adam G. |last2=Filippenko |first2=Alexei V. |last3=Challis |first3=Peter |last4=Clocchiatti |first4=Alejandro |last5=Diercks |first5=Alan |last6=Garnavich |first6=Peter M. |last7=Gilliland |first7=Ron L. |last8=Hogan |first8=Craig J. |last9=Jha |first9=Saurabh |last10=Kirshner |first10=Robert P. |last11=Leibundgut |first11=B. |last12=Phillips |first12=M. M. |last13=Reiss |first13=David |last14=Schmidt |first14=Brian P. |last15=Schommer |first15=Robert A. |last16=Smith |first16=R. Chris |last17=Spyromilio |first17=J. |last18=Stubbs |first18=Christopher |last19=Suntzeff |first19=Nicholas B. |last20=Tonry |first20=John |title=Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant |date=1998 |journal=[[The Astronomical Journal]] |volume=116 |issue=3 |pages=1009–1038 |doi=10.1086/300499 |arxiv=astro-ph/9805201|bibcode=1998AJ....116.1009R |s2cid=15640044 }}</ref><ref>{{cite journal |last1=Perlmutter |first1=S. |last2=Aldering |first2=G. |last3=Goldhaber |first3=G. |last4=Knop |first4=R.A. |last5=Nugent |first5=P. |last6=Castro |first6=P.G. |last7=Deustua |first7=S. |last8=Fabbro |first8=S. |last9=Goobar |first9=A. |last10=Groom |first10=D.E. |last11=Hook |first11=I.M. |last12=Kim |first12=A.G. |last13=Kim |first13=M.Y. |last14=Lee |first14=J.C. |last15=Nunes |first15=N.J. |last16=Pain |first16=R. |last17=Pennypacker |first17=C.R. |last18=Quimby |first18=R. |last19=Lidman |first19=C. |last20=Ellis |first20=R.S. |last21=Irwin |first21=M. |last22=McMahon |first22=R.G.  |last23=Ruiz-Lapuente |first23=P.  |last24=Walton  |first24=N.  |last25=Schaefer  |first25=B. |last26=Boyle |first26=B.J. |last27=Filippenko |first27=A.V. |last28=Matheson |first28=T. |last29=Fruchter |first29=A.S. |last30=Panagia |first30=N. |last31=Newberg |first31=H.J.M. |last32=Couch |first32=W.J. |date=1999 |title=Measurements of Omega and Lambda from 42 High-Redshift Supernovae |volume=517 |issue=2 |journal=[[The Astrophysical Journal]] |pages=565–586 |arxiv=astro-ph/9812133 |doi=10.1086/307221|bibcode=1999ApJ...517..565P |s2cid=118910636 }}</ref>