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[[Image:WMAP 2010.png|thumb|right|300px|[[WMAP]] image of the CMB temperature anisotropy.]]
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Measurements of the CMB have made the inflationary Big Bang theory the standard model of the earliest eras of the universe.<ref>{{cite journal|last=Scott|first=D.|year=2005|title=The Standard Cosmological Model|bibcode=2005astro.ph.10731S|id={{arxiv|astro-ph/0510731}}}}</ref> This theory predicts that the initial conditions for the universe are originally random in nature, and follow a roughly [[normal distribution|Gaussian probability distribution]], which when graphed in cross-section forms bell-shaped curves. By analyzing this distribution at different frequencies, a [[spectral density]] or power spectrum is generated. The power spectrum of these fluctuations has been calculated, and agrees with the observations, although certain observables, for example the overall amplitude of the fluctuations, are more or less free parameters of the [[cosmic inflation]] model.<ref>{{cite arxiv|last=Turner|first=M. S.|year=2002|title=The New Cosmology: Mid-term Report Card for Inflation|class=astro-ph|eprint=astro-ph/0212281}}</ref> Therefore, meaningful statements about the inhomogeneities in the universe need to be [[statistics|statistical]] in nature. This leads to [[cosmic variance]] in which the uncertainties in the variance of the largest scale fluctuations observed in the universe are difficult to accurately compare to theory. The model uses a [[Gaussian random field]] with a nearly [[scale invariant]] or [[Harrison-Zel'dovich]] spectrum to represent the primeval inhomogeneities.<ref>
{{cite arxiv|last=Torres|first=S.|year=1993|title=Topological Analysis of COBE-DMR CMB Maps|class=astro-ph|eprint=astro-ph/9311067}}</ref><ref>{{cite book|last=Thompson|first=J. M. T.|year=2005|title=Advances in Astronomy|pages=24–25|publisher=[[Imperial College Press]]|isbn=1860945775 }}</ref>
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The cosmic microwave background radiation and the cosmological [[red shift]] are together regarded as the best available evidence for the [[Big Bang]] theory. The discovery of the CMB in the mid-1960s curtailed interest in [[non-standard cosmology|alternatives]] such as the [[steady state theory]].<ref>{{cite book|author=Durham, Frank; Purrington, Robert D.|title=Frame of the universe: a history of physical cosmology|publisher=Columbia University Press|year=1983|isbn=0231053932|pages=193–209}}</ref> The CMB gives a snapshot of the [[Universe]] when, according to standard cosmology, the temperature dropped enough to allow [[electron]]s and [[proton]]s to form [[hydrogen]] atoms, thus making the universe transparent to radiation. When it originated some 380,000 years after the Big Bang—this time is generally known as the "time of last scattering" or the period of [[recombination (cosmology)|recombination]] or [[decoupling]]—the temperature of the Universe was about 4,000 K. This corresponds to an energy of about 0.25 [[electronvolt|eV]], which is much less than the 13.6 eV ionization energy of hydrogen.<ref>{{cite web|last=Brandenberger|first=Robert H.|year=1995|title=Formation of Structure in the Universe|url=http://adsabs.harvard.edu/abs/1995astro.ph..8159B|accessdate=2009-09-01}}</ref>
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For details about the reasoning that the radiation is evidence for the Big Bang, see [[Big Bang#Cosmic microwave background radiation|Cosmic background radiation of the Big Bang]].
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[[Image:PowerSpectrumExt.svg|thumb|right|300px|The power spectrum of the cosmic microwave background radiation temperature anisotropy in terms of the angular scale (or [[multipole moment]]). The data shown come from the [[WMAP]] (2006), [[Arcminute Cosmology Bolometer Array Receiver|Acbar]] (2004) [[BOOMERanG experiment|Boomerang]] (2005), [[Cosmic Background Imager|CBI]] (2004), and [[Very Small Array|VSA]] (2004) instruments. Also shown is a theoretical model (solid line).]]
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The maximum of the PVF (the time where it is most likely that a given CMB photon last scattered) is known quite precisely. The first-year [[Wilkinson Microwave Anisotropy Probe|WMAP]] results put the time at which P(t) is maximum as {{val|372|14|u=kyr}}.<ref name="WMAP_1_cosmo_params">{{cite journal|author=WMAP Collaboration|year=2003|title=First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters|journal=[[Astrophysical Journal]] (Supplement Series)|volume=148|issue=1|pages=175–194|doi=10.1086/377226|id={{arXiv|astro-ph/0302209}}|last2=Verde|first2=L.|last3=Peiris|first3=H. V.|last4=Komatsu|first4=E.|last5=Nolta|first5=M. R.|last6=Bennett|first6=C. L.|last7=Halpern|first7=M.|last8=Hinshaw|first8=G.|last9=Jarosik|first9=N.}}</ref> This is often taken as the "time" at which the CMB formed. However, to figure out how ''long'' it took the photons and baryons to decouple, we need a measure of the width of the PVF. The WMAP team finds that the PVF is greater than half of its maximum value (the "full width at half maximum", or FWHM) over an interval of {{val|115|5|u=kyr}}. By this measure, decoupling took place over roughly 115,000 years, and when it was complete, the universe was roughly 487,000 years old.
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Since the CMB came into existence, it has apparently been modified by several subsequent physical processes, which are collectively referred to as late-time anisotropy, or secondary anisotropy. When the CMB photons became free to travel unimpeded, ordinary matter in the universe was mostly in the form of neutral hydrogen and helium atoms. However, observations of galaxies today seem to indicate that most of the volume of the [[intergalactic medium]] (IGM) consists of ionized material (since there are few absorption lines due to hydrogen atoms). This implies a period of [[reionization]] during which some of the material of the universe was broken into hydrogen ions.
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[[Image:WMAP 3yr EE.png|thumb|right|300px|E polarization measurements as of March 2006 in terms of angular scale (or [[multipole moment]]). The polarization is much more poorly measured than the temperature anisotropy.]]
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{{main|Polarization in astronomy}}
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