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{{About|scientific estimates of the age of the universe|religious and other non-scientific estimates|Dating creation}}
This is a preview for the new '''MathML rendering mode''' (with SVG fallback), which is availble in production for registered users.
{{Cosmology}}


In [[physical cosmology]], the '''age of the universe''' is the [[cosmological time|time]] elapsed since the [[Big Bang]]. The [[Planck (spacecraft)#2013 data release|best measurement]] of the age of the universe is {{val|13.798|0.037}} billion years ({{val|13.798|0.037|e=9}} years or {{val|4.354|0.012|e=17}} seconds) within the [[Lambda-CDM model|Lambda-CDM concordance model]].<ref name='planck_overview'>
If you would like use the '''MathML''' rendering mode, you need a wikipedia user account that can be registered here [[https://en.wikipedia.org/wiki/Special:UserLogin/signup]]
{{cite arXiv
* Only registered users will be able to execute this rendering mode.
|author=Planck Collaboration
* Note: you need not enter a email address (nor any other private information). Please do not use a password that you use elsewhere.
|year=2013
|title=Planck 2013 results. I. Overview of products and scientific results
|class=astro-ph.CO
|eprint=1303.5062
}}</ref><ref name="arxiv-20121220">
{{cite arXiv
|last=Bennett |first=C.L.
|author2=''et al.''
|year=2013
|title=Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Final Maps and Results
|eprint=1212.5225
|class=astro-ph.CO
}}</ref> The [[measurement uncertainty|uncertainty]] of 37 million years has been obtained by the agreement of a number of scientific research projects, such as [[microwave background radiation]] [[measurement]]s by the [[Planck satellite]], the [[Wilkinson Microwave Anisotropy Probe]] and other probes. Measurements of the cosmic background radiation give the cooling time of the [[universe]] since the Big Bang,<ref name="arxiv-20121220" /> and measurements of the [[Red shift#Extragalactic observations|expansion rate]] of the universe can be used to calculate its approximate age by extrapolating backwards in time.


== Explanation ==
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The [[Lambda-CDM model|Lambda-CDM concordance model]] describes the evolution of the universe from a very uniform, hot, dense primordial state to its present state over a span of about 13.8 billion years<ref>{{cite web
|date=2 April 2013
|title=Cosmic Detectives
|url=http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives
|publisher=[[European Space Agency]]
|accessdate=2013-04-15
}}</ref> of [[cosmological time]]. This model is well understood theoretically and strongly supported by recent high-precision astronomical observations such as [[WMAP]]. In contrast, theories of the origin of the primordial state remain very speculative. If one extrapolates the Lambda-CDM model backward from the earliest well-understood state, it quickly (within a small fraction of a second) reaches a [[Gravitational singularity|singularity]] called the "Big Bang singularity." This singularity is not understood as having a physical significance in the usual sense, but it is convenient to quote times measured "since the Big Bang" even though they do not correspond to a physically measurable time. For example, "10<sup>−6</sup> seconds after the Big Bang" is a well-defined era in the universe's evolution. If one referred to the same era as "13.8 billion years minus 10<sup>−6</sup> seconds ago," the precision of the meaning would be lost because the minuscule latter time interval is swamped by uncertainty in the former.


Though the universe might in theory have a longer history, the [[International Astronomical Union]]<ref>
'''MathML'''
{{cite news
:<math forcemathmode="mathml">E=mc^2</math>
|last=Chang |first=K.
|date=9 March 2008
|title=Gauging Age of Universe Becomes More Precise
|url=http://www.nytimes.com/2008/03/09/science/space/09cosmos.html?_r=1&oref=slogin
|work=[[The New York Times]]
|accessdate=
}}</ref> presently use "age of the universe" to mean the duration of the Lambda-CDM expansion, or equivalently the elapsed time since the Big Bang in the current [[observable universe]].


==Observational limits==
<!--'''PNG'''  (currently default in production)
Since the universe must be at least as old as the oldest thing in it, there are a number of observations which put a lower limit on the age of the universe; these include the temperature of the coolest [[white dwarf]]s, which gradually cool as they age, and the dimmest [[turnoff point]] of [[main sequence]] [[stars]] in clusters (lower-mass stars spend a greater amount of time on the main sequence, so the lowest-mass stars that have evolved off of the main sequence set a minimum age).
:<math forcemathmode="png">E=mc^2</math>


==Cosmological parameters==
'''source'''
[[Image:Universe.svg|thumb|400px|The age of the universe can be determined by measuring the [[Hubble constant]] today and extrapolating back in time with the observed value of density parameters (Ω). Before the discovery of [[dark energy]], it was believed that the universe was matter-dominated, and so Ω on this graph corresponds to Ω<sub>''m''</sub>. Note that the [[accelerating universe]] has the greatest age, while the [[Big Crunch]] universe has the least age.]]
:<math forcemathmode="source">E=mc^2</math> -->
[[File:Age Universe Planck 2013.png|thumb|400px|The value of the age correction factor, ''F'', is shown as a function of two [[cosmology|cosmological parameters]]: the current fractional matter density Ω<sub>''m''</sub> and cosmological constant density Ω<sub>''Λ''</sub>.  The [[Lambda-CDM model|best-fit values]] of these parameters are shown by the box in the upper left; the matter-dominated universe is shown by the star in the lower right.]]


The problem of determining the age of the universe is closely tied to the problem of determining the values of the cosmological parameters. Today this is largely carried out in the context of the [[Lambda CDM model|ΛCDM]] model, where the universe is assumed to contain normal (baryonic) matter, cold [[dark matter]], radiation (including both [[photon]]s and [[neutrino]]s), and a [[cosmological constant]].  The fractional contribution of each to the current energy density of the universe is given by the [[density parameter]]s Ω<sub>''m''</sub>, Ω<sub>''r''</sub>, and Ω<sub>Λ</sub>.  The full ΛCDM model is described by a number of other parameters, but for the purpose of computing its age these three, along with the [[Hubble constant|Hubble parameter]] <math>H_0</math>, are the most important.
<span style="color: red">Follow this [https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-rendering link] to change your Math rendering settings.</span> You can also add a [https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-rendering-skin Custom CSS] to force the MathML/SVG rendering or select different font families. See [https://www.mediawiki.org/wiki/Extension:Math#CSS_for_the_MathML_with_SVG_fallback_mode these examples].


If one has accurate measurements of these parameters, then the age of the universe can be determined by using the [[Friedmann equations|Friedmann equation]].  This equation relates the rate of change in the [[scale factor (cosmology)|scale factor]] ''a''(''t'') to the matter content of the universe.  Turning this relation around, we can calculate the change in time per change in scale factor and thus calculate the total age of the universe by [[Integral|integrating]] this formula. The age ''t''<sub>0</sub> is then given by an expression of the form
==Demos==
:<math>t_0 = \frac{1}{H_0} F(\Omega_r,\Omega_m,\Omega_\Lambda,\dots) </math>
where <math>H_0</math> is the [[Hubble's law|Hubble parameter]] and the function ''F'' depends only on the fractional contribution to the universe's energy content that comes from various components. The first observation that one can make from this formula is that it is the Hubble parameter that controls that age of the universe, with a correction arising from the matter and energy content. So a rough estimate of the age of the universe comes from the [[Hubble time]], the inverse of the Hubble parameter. With a value for <math>H_0</math> around {{val|68|u=km/s/Mpc}}, the Hubble time evaluates to <math>1/H_0</math> = {{val|14.4}} billion years.<ref>
{{Cite book
|last=Liddle |first=A. R.
|year=2003
|title=An Introduction to Modern Cosmology
|edition=2nd |page=57
|publisher=[[John Wiley & Sons|Wiley]]
|isbn=0-470-84835-9
}}</ref>


To get a more accurate number, the correction factor ''F'' must be computed. In general this must be done numerically, and the results for a range of cosmological parameter values are shown in the figure. For the [[Lambda CDM model|Planck values]] (Ω<sub>''m''</sub>, Ω<sub>''Λ''</sub>) = (0.3086, 0.6914), shown by the box in the upper left corner of the figure, this correction factor is about ''F'' = 0.956. For a flat universe without any cosmological constant, shown by the star in the lower right corner, ''F'' = {{Frac|2|3}} is much smaller and thus the universe is younger for a fixed value of the Hubble parameter. To make this figure, Ω<sub>''r''</sub> is held constant (roughly equivalent to holding the [[Cosmic Microwave Background|CMB]] temperature constant) and the curvature density parameter is fixed by the value of the other three.
Here are some [https://commons.wikimedia.org/w/index.php?title=Special:ListFiles/Frederic.wang demos]:


Apart from the Planck satellite, the Wilkinson Microwave Anisotropy Probe ([[WMAP]]) was instrumental in establishing an accurate age of the universe, though other measurements must be folded in to gain an accurate number.  [[CMB]] measurements are very good at constraining the matter content Ω<sub>''m''</sub><ref>
{{cite web
|last=Hu |first=W.
|title=Animation: Matter Content Sensitivity. The matter-radiation ratio is raised while keeping all other parameters fixed.
|url=http://background.uchicago.edu/%7Ewhu/physics/anim2.html
|publisher=[[University of Chicago]]
|accessdate=2008-02-23
|archiveurl=http://web.archive.org/web/20080223184613/http://background.uchicago.edu/%7Ewhu/physics/anim2.html
|archivedate=23 February 2008 <!--DASHBot-->
|deadurl=no
}}</ref> and curvature parameter Ω<sub>''k''</sub>.<ref name="anim3">
{{cite web
|last=Hu |first=W.
|title=Animation: Angular diameter distance scaling with curvature and lambda
|url=http://background.uchicago.edu/%7Ewhu/physics/anim3.html
|publisher=[[University of Chicago]]
|accessdate=2008-02-23
|archiveurl=http://web.archive.org/web/20080223184618/http://background.uchicago.edu/%7Ewhu/physics/anim3.html
|archivedate=23 February 2008 <!--DASHBot-->
|deadurl=no
}}</ref>  It is not as sensitive to Ω<sub>Λ</sub> directly,<ref name="anim3"/> partly because the cosmological constant becomes important only at low redshift.  The most accurate determinations of the Hubble parameter ''H''<sub>0</sub> come from [[Type Ia supernova]]e.  Combining these measurements leads to the generally accepted value for the age of the universe quoted above.


The cosmological constant makes the universe "older" for fixed values of the other parameters. This is significant, since before the cosmological constant became generally accepted, the Big Bang model had difficulty explaining why [[globular cluster]]s in the Milky Way appeared to be far older than the age of the universe as calculated from the Hubble parameter and a matter-only universe.<ref>
* accessibility:
{{cite web
** Safari + VoiceOver: [https://commons.wikimedia.org/wiki/File:VoiceOver-Mac-Safari.ogv video only], [[File:Voiceover-mathml-example-1.wav|thumb|Voiceover-mathml-example-1]], [[File:Voiceover-mathml-example-2.wav|thumb|Voiceover-mathml-example-2]], [[File:Voiceover-mathml-example-3.wav|thumb|Voiceover-mathml-example-3]], [[File:Voiceover-mathml-example-4.wav|thumb|Voiceover-mathml-example-4]], [[File:Voiceover-mathml-example-5.wav|thumb|Voiceover-mathml-example-5]], [[File:Voiceover-mathml-example-6.wav|thumb|Voiceover-mathml-example-6]], [[File:Voiceover-mathml-example-7.wav|thumb|Voiceover-mathml-example-7]]
|date=1 July 2011
** [https://commons.wikimedia.org/wiki/File:MathPlayer-Audio-Windows7-InternetExplorer.ogg Internet Explorer + MathPlayer (audio)]
|title=Globular Star Clusters
** [https://commons.wikimedia.org/wiki/File:MathPlayer-SynchronizedHighlighting-WIndows7-InternetExplorer.png Internet Explorer + MathPlayer (synchronized highlighting)]
|url=http://www.seds.org/messier/glob.html
** [https://commons.wikimedia.org/wiki/File:MathPlayer-Braille-Windows7-InternetExplorer.png Internet Explorer + MathPlayer (braille)]
|publisher=[[SEDS]]
** NVDA+MathPlayer: [[File:Nvda-mathml-example-1.wav|thumb|Nvda-mathml-example-1]], [[File:Nvda-mathml-example-2.wav|thumb|Nvda-mathml-example-2]], [[File:Nvda-mathml-example-3.wav|thumb|Nvda-mathml-example-3]], [[File:Nvda-mathml-example-4.wav|thumb|Nvda-mathml-example-4]], [[File:Nvda-mathml-example-5.wav|thumb|Nvda-mathml-example-5]], [[File:Nvda-mathml-example-6.wav|thumb|Nvda-mathml-example-6]], [[File:Nvda-mathml-example-7.wav|thumb|Nvda-mathml-example-7]].
|accessdate=2013-07-19
** Orca: There is ongoing work, but no support at all at the moment [[File:Orca-mathml-example-1.wav|thumb|Orca-mathml-example-1]], [[File:Orca-mathml-example-2.wav|thumb|Orca-mathml-example-2]], [[File:Orca-mathml-example-3.wav|thumb|Orca-mathml-example-3]], [[File:Orca-mathml-example-4.wav|thumb|Orca-mathml-example-4]], [[File:Orca-mathml-example-5.wav|thumb|Orca-mathml-example-5]], [[File:Orca-mathml-example-6.wav|thumb|Orca-mathml-example-6]], [[File:Orca-mathml-example-7.wav|thumb|Orca-mathml-example-7]].
|archiveurl=http://web.archive.org/web/20080224064318/http://seds.org/messier/glob.html
** From our testing, ChromeVox and JAWS are not able to read the formulas generated by the MathML mode.
|archivedate=24 February 2008 <!--DASHBot-->
|deadurl=no
}}</ref><ref>
{{cite web
|last=Iskander |first=E.
|date=11 January 2006
|title=Independent age estimates
|url=http://www.astro.ubc.ca/people/scott/bbage.html
|publisher=[[University of British Columbia]]
|accessdate=2008-02-23
|archiveurl=http://web.archive.org/web/20080306024809/http://www.astro.ubc.ca/people/scott/bbage.html
|archivedate=6 March 2008 <!--DASHBot-->
|deadurl=no
}}</ref>  Introducing the cosmological constant allows the universe to be older than these clusters, as well as explaining other features that the matter-only cosmological model could not.<ref>
{{cite arXiv
|title=Cosmic Concordance
|last1=Ostriker |first1=J. P.
|last2=Steinhardt |first2=P. J.
|year=1995
|class=astro-ph
|eprint=astro-ph/9505066
}}</ref>


==WMAP==
==Test pages ==
[[NASA]]'s [[Wilkinson Microwave Anisotropy Probe]] (WMAP) project's [[Wilkinson Microwave Anisotropy Probe#Nine-year data release|nine-year data release]] in 2012 estimated the age of the universe to be {{val|13.772|0.059|e=9}} years (13.772 billion years, with an uncertainty of plus or minus 59 million years).<ref name="arxiv-20121220" />


However, this age is based on the assumption that the project's underlying model is correct; other methods of estimating the age of the universe could give different ages. Assuming an extra background of relativistic particles, for example, can enlarge the error bars of the WMAP constraint by one order of magnitude.<ref>
To test the '''MathML''', '''PNG''', and '''source''' rendering modes, please go to one of the following test pages:
{{cite journal
*[[Displaystyle]]
|last1=de Bernardis |first1=F.
*[[MathAxisAlignment]]
|last2=Melchiorri |first2=A.
*[[Styling]]
|last3=Verde |first3=L.
*[[Linebreaking]]
|last4=Jimenez |first4=R.
*[[Unique Ids]]
|year=2008
*[[Help:Formula]]
|title=The Cosmic Neutrino Background and the Age of the Universe
|journal=[[Journal of Cosmology and Astroparticle Physics]]
|volume=2008 |issue=3 |pages=20
|arxiv=0707.4170
|bibcode=2008JCAP...03..020D
|doi=10.1088/1475-7516/2008/03/020
}}</ref>


This measurement is made by using the location of the first acoustic peak in the [[cosmic microwave background radiation|microwave background]] power spectrum to determine the size of the decoupling surface (size of the universe at the time of recombination).  The light travel time to this surface (depending on the geometry used) yields a reliable age for the universe.  Assuming the validity of the models used to determine this age, the residual accuracy yields a margin of error near one percent.<ref name="wmap">
*[[Inputtypes|Inputtypes (private Wikis only)]]
{{cite journal
*[[Url2Image|Url2Image (private Wikis only)]]
|first=D. N. |last=Spergel
==Bug reporting==
|author2=''et al.''
If you find any bugs, please report them at [https://bugzilla.wikimedia.org/enter_bug.cgi?product=MediaWiki%20extensions&component=Math&version=master&short_desc=Math-preview%20rendering%20problem Bugzilla], or write an email to math_bugs (at) ckurs (dot) de .
|year=2003
|title=First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters
|journal=[[The Astrophysical Journal Supplement Series]]
|volume=148 |issue=1 |pages=175–194
|arxiv=astro-ph/0302209
|bibcode=2003ApJS..148..175S
|doi=10.1086/377226
}}</ref>
 
==Planck==
In 2013, the [[European Space Agency]]'s [[Planck (spacecraft)|Planck spacecraft]] team estimated the age of the universe to be {{val|13.82}} billion years,<ref name="ESA-20130321">
{{cite web
|author=Staff
|title=Planck Reveals An Almost Perfect Universe
|url=http://www.esa.int/Our_Activities/Space_Science/Planck/Planck_reveals_an_almost_perfect_Universe
|publisher=[[European Space Agency]]
|date=21 March 2013
|accessdate=2013-03-21
}}</ref><ref name="NASA-20130321">
{{cite web
|last1=Clavin |first1=W.
|last2=Harrington |first2=J. D.
|title=Planck Mission Brings Universe Into Sharp Focus
|url=http://www.jpl.nasa.gov/news/news.php?release=2013-109&rn=news.xml&rst=3739
|date=21 March 2013
|publisher=[[NASA]]
|accessdate=2013-03-21
}}</ref><ref name="NYT-20130321">
{{cite news
|last=Overbye |first=D.
|date=21 March 2013
|title=An Infant Universe, Born Before We Knew
|url=http://www.nytimes.com/2013/03/22/science/space/planck-satellite-shows-image-of-infant-universe.html
|work=[[New York Times]]
|accessdate=2013-03-21
}}</ref><ref name="NBC-20130321">
{{cite web
|last=Boyle |first=A.
|date=21 March 2013
|title=Planck probe's cosmic 'baby picture' revises universe's vital statistics
|url=http://cosmiclog.nbcnews.com/_news/2013/03/21/17397298-planck-probes-cosmic-baby-picture-revises-universes-vital-statistics
|work=[[NBC News]]
|accessdate=2013-03-21
}}</ref> slightly higher but within the uncertainties of the earlier number derived from the WMAP data. By combining the Planck data with previous missions, the best combined estimate of the age of the universe is [[Planck (spacecraft)#2013 data release|{{val|13.798|0.037|e=9|u=years}} old]].<ref name="planck_overview" />
 
<center>
{| border="2" cellpadding="4" cellspacing="0" style="margin: 1em 1em 1em 0; background: #f9f9f9; border: 1px #aaa solid; border-collapse: collapse; font-size: 70%; text-align:center;"
|- bgcolor="#B0C4DE" align="center"
|+ [[Lambda-CDM model|Cosmological parameters]] from 2013 Planck results<ref name="planck_overview" /><ref name='planck_cosmological_parameters'>
{{cite arXiv
|author=Planck collaboration
|year=2013
|title=Planck 2013 results. XVI. Cosmological parameters
|class=astro-ph.CO
|eprint=1303.5076
}}</ref>
! Parameter !! Symbol !! Planck<br> Best fit !! Planck<br> 68% limits !! Planck+lensing<br> Best fit !! Planck+lensing<br> 68% limits !! Planck+WP<br> Best fit !! Planck+WP<br> 68% limits !! Planck+WP<br> +HighL<br> Best fit !! Planck+WP<br> +HighL<br> 68% limits !! Planck+lensing<br> +WP+highL<br> Best fit !! Planck+lensing<br> +WP+highL<br> 68% limits !! Planck+[[WMAP|WP]]<br> +highL+[[baryon acoustic oscillations|BAO]]<br> Best fit !! Planck+[[WMAP|WP]]<br> +highL+[[baryon acoustic oscillations|BAO]]<br> 68% limits
|-
| Age of the universe<br> (Ga) || <math>t_0</math> || 13.819 || {{val|13.813|0.058}} || 13.784 || {{val|13.796|0.058}} || 13.8242 || {{val|13.817|0.048}} || 13.8170 || {{val|13.813|0.047}} || 13.7914 || {{val|13.794|0.044}} || 13.7965 || {{val|13.798|0.037}}
|-
| [[Hubble's constant]]<br> ( {{frac|km|Mpc·s}} ) || <math>H_0</math> || 67.11 || {{val|67.4|1.4}} || 68.14 || {{val|67.9|1.5}} || 67.04 || {{val|67.3|1.2}} || 67.15 || {{val|67.3|1.2}} || 67.94 || {{val|67.9|1.0}}|| 67.77 || {{val|67.80|0.77}}
|}
</center>
{{-}}
 
==Assumption of strong priors==
Calculating the age of the universe is accurate only if the assumptions built into the models being used to estimate it are also accurate. This is referred to as [[strong priors]] and essentially involves stripping the potential errors in other parts of the model to render the accuracy of actual observational data directly into the concluded result.  Although this is not a valid procedure in all contexts (as noted in the accompanying caveat: "based on the fact we have assumed the underlying model we used is correct"), the age given is thus accurate to the specified error (since this error represents the error in the instrument used to gather the raw data input into the model).
 
The age of the universe based on the best fit to [[Planck (spacecraft)#2013 data release|Planck 2013 data]] alone is {{val|13.813|0.058}} billion years (the other estimate of {{val|13.798|0.037}} billion years uses Gaussian [[Prior probability|prior]]s based on earlier estimates from other studies to determine the combined uncertainty). This number represents the first accurate "direct" measurement of the age of the universe (other methods typically involve [[Hubble's law]] and the age of the oldest stars in globular clusters, etc.). It is possible to use different methods for determining the same parameter (in this case – the age of the universe) and arrive at different answers with no overlap in the "errors". To best avoid the problem, it is common to show two sets of uncertainties; one related to the actual measurement and the other related to the systematic errors of the model being used.
 
An important component to the analysis of data used to determine the age of the universe (e.g. from [[Planck (spacecraft)|Planck]]) therefore is to use a [[Bayesian statistics|Bayesian statistical]] analysis, which normalizes the results based upon the priors (i.e. the model).<ref name="wmap" /> This quantifies any uncertainty in the accuracy of a measurement due to a particular model used.<ref>
{{cite conference
|last=Loredo |first=T. J.
|year=1992
|title=The Promise of Bayesian Inference for Astrophysics
|url=http://www.astro.cornell.edu/staff/loredo/bayes/promise.pdf
|editor1-last=Feigelson |editor1-first=E. D.
|editor2-last=Babu |editor2-first=G. J.
|booktitle=Statistical Challenges in Modern Astronomy
|pages=275–297
|publisher=[[Springer-Verlag]]
|bibcode=1992scma.conf..275L
|doi=10.1007/978-1-4613-9290-3_31
|isbn=978-1-4613-9292-7
}}</ref><ref>
{{cite journal
|last1=Colistete |first1=R.
|last2=Fabris |first2=J. C.
|last3=Concalves |first3=S. V. B.
|year=2005
|title=Bayesian Statistics and Parameter Constraints on the Generalized Chaplygin Gas Model Using SNe ia Data
|journal=[[International Journal of Modern Physics D]]
|volume=14 |issue=5 |pages=775–796
|arxiv=astro-ph/0409245
|bibcode=2005IJMPD..14..775C
|doi=10.1142/S0218271805006729
}}</ref>
 
==History==
{{Expand section|date=February 2014|1= The estimated age of the universe for certain crucial points in history, e.g.:
* before Einstein
* after Einstein
* before Hubble
* after Hubble
* etc}}
In the 18th century, the concept that the [[age of the Earth]] was millions, if not billions, of years began to appear. However, most scientists throughout the 19th century and into the first decades of the 20th century presumed that the universe itself was [[Steady State theory|Steady State]] and eternal, with maybe stars coming and going but no changes occurring at the largest scale known at the time.
 
The first scientific theories indicating that the age of the universe might be finite were the studies of [[thermodynamics]], formalized in the mid-19th century. The concept of [[entropy]] dictates that if the universe (or any other closed system) were infinitely old, then everything inside would be at the same temperature, and thus there would be no stars and no life. No scientific explanation for this contradiction was put forth at the time. In 1915 [[Albert Einstein]] published the theory of [[general relativity]].<ref>
{{cite journal
|last=Einstein |first=A.
|year=1915
|title=Zur allgemeinen Relativitätstheorie
|journal=[[Sitzungsberichte der Königlich Preußischen Akademie der Wissenschaften]]
|pages=778–786
|language=German
|bibcode=1915SPAW.......778E
}}</ref> Based on Einstein's theory, [[Georges Lemaître]]'s work showed that the universe cannot be static and must be either expanding or contracting. Einstein himself did not believe this result and so he added what he called a [[cosmological constant]] to his equations in an unsuccessful attempt to produce a theory consistent with a steady state universe.
 
The first direct observational evidence that the universe has a finite age came from the observations of astronomer [[Edwin Hubble]] published in 1929.<ref>
{{cite journal
|last=Hubble |first=E.
|year=1929
|title=A relation between distance and radial velocity among extra-galactic nebulae
|journal=[[Proceedings of the National Academy of Sciences]]
|volume=15 |issue=3 |pages=168&ndash;173
|url=http://www.pnas.org/cgi/reprint/15/3/168
|bibcode=1929PNAS...15..168H
|doi=10.1073/pnas.15.3.168
|pmid=16577160
|pmc=522427
}}</ref> Earlier in the 20th century, Hubble and others resolved individual stars within certain [[nebula]]e, thus determining that they were [[Galaxy|galaxies]], similar to, but external to, our [[Milky Way Galaxy]]. In addition, these galaxies were very large and very far away. [[electromagnetic spectrum|Spectra]] taken of these distant galaxies showed a [[red shift]] in their [[spectral lines]] presumably caused by the [[Doppler effect]], thus indicating that these galaxies were moving away from the Earth. In addition, the farther away these galaxies seemed to be, the greater the redshift and thus the faster they seemed to be moving away. This was the first direct evidence that the universe is not static but expanding. The first estimate of the age of the universe came from the calculation of when all of the objects must have started speeding out from the same point. Hubble's initial value for the universe's age was very low, as the galaxies were assumed to be much closer than later observations found them to be.
 
The first reasonably accurate measurement of the rate of expansion of the universe, a numerical value now known as the [[Hubble constant]], was made in 1958 by astronomer [[Allan Sandage]].<ref>
{{cite journal
|last1=Sandage |first1=A. R.
|title=Current Problems in the Extragalactic Distance Scale
|year=1958
|journal=[[The Astrophysical Journal]]
|volume=127 |issue=3 |pages=513–526
|bibcode=1958ApJ...127..513S
|doi=10.1086/146483
}}</ref> His measured value for the Hubble constant came very close to the value range generally accepted today.
 
However Sandage, like Einstein, did not believe his own results at the time of discovery. His value for the age of the universe{{Elucidate|date=February 2014|reason=What was his value for the age of the universe?}} was too short to reconcile with the 25-billion-year age estimated at that time for the oldest known [[star]]s. Sandage and other astronomers repeated these measurements numerous times, attempting to reduce the Hubble constant and thus increase the resulting age for the universe. Sandage even proposed new theories of [[cosmogony]] to explain this discrepancy. This issue was finally resolved by improvements in the theoretical models used for estimating the ages of stars. As of 2013, using the latest models for stellar evolution, the estimated age of the [[HD 140283|oldest known star]] is {{val|14.46|0.8}} billion years.<ref name=arxiv>
{{cite journal
|last1=Bond |first1=H. E.
|last2=Nelan |first2=E. P.
|last3=Vandenberg|first3=D. A.
|last4=Schaefer|first4=G. H.
|last5=Harmer |first5=D.
|title=HD 140283: A Star in the Solar Neighborhood that Formed Shortly After the Big Bang
|year=2013
|journal=[[The Astrophysical Journal]]
|volume=765 |pages=L12 |issue=12
|arxiv=1302.3180
|bibcode=2013ApJ...765L..12B
|doi=10.1088/2041-8205/765/1/L12
}}</ref>
 
The discovery of [[microwave]] [[cosmic background radiation]] announced in 1965<ref name="apj142:419">
{{cite journal
|last=Penzias |first=A. A.
|last2=Wilson |first2=R .W.
|year=1965
|title=A Measurement of Excess Antenna Temperature at 4080 Mc/s
|journal=[[The Astrophysical Journal]]
|volume=142 |pages=419–421
|doi=10.1086/148307
|bibcode=1965ApJ...142..419P
}}</ref> finally brought an effective end to the remaining scientific uncertainty over the expanding universe. The recently launched space probes WMAP, launched in 2001, and Planck, launched in 2009, produced data that determines the Hubble constant and the age of the universe independent of galaxy distances, removing the largest source of error.<ref name="wmap"/>
 
== See also ==
{{Portal|Astronomy}}
* [[Age crisis]]
* [[Age of the Earth]]
* [[Anthropic principle]]
* [[Cosmology]]
* [[Hubble Deep Field]]
* [[Metric expansion of space]]
* [[Multiverse]]
* [[Observable universe]]
* [[Red Shift#Observations in astronomy|Red shift observations in astronomy]]
* [[Static universe]]
* [[The First Three Minutes: A Modern View of the Origin of the Universe]] an essay written by [[Steven Weinberg]] and published in 1977
* [[Dark Ages Radio Explorer (DARE)]]
 
==References==
{{Reflist|30em}}
 
==External links==
*[http://www.astro.ucla.edu/~wright/cosmolog.htm Ned Wright's Cosmology Tutorial]
*{{cite web | url =http://www.astro.ucla.edu/~wright/age.html | first =Edward L. | last =Wright | title =Age of the Universe | date =2 July 2005}}
*Wayne Hu's [http://background.uchicago.edu/~whu/metaanim.html cosmological parameter animations]
*{{cite arXiv|eprint=astro-ph/9505066|author1=Ostriker|author2=Steinhardt|title=Cosmic Concordance|class=astro-ph|year=1995}}
*SEDS page on [http://www.seds.org/messier/glob.html "Globular Star Clusters"]
*Douglas Scott [http://www.astro.ubc.ca/people/scott/bbage.html "Independent Age Estimates"]
*KryssTal [http://www.krysstal.com/scale.html "The Scale of the Universe"] Space and Time scaled for the beginner.
*[http://icosmos.co.uk/ iCosmos: Cosmology Calculator (With Graph Generation )]
*[http://www.aip.org/history/cosmology/ideas/expanding.htm The Expanding Universe] (American Institute of Physics)
 
{{DEFAULTSORT:Age Of The Universe}}
[[Category:Universe]]
[[Category:Physical cosmology]]
[[Category:Big Bang]]

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