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==Explanation==
 
==Explanation==
βˆ’
The solid line represents the theoretical {{w|blackbody radiation|radiation for a blackbody}} at 2.73 K according to {{w|Planck's Law}} (derived as early as 1900 by {{w|Max Planck}}). The formula, almost as written in the graph, can be found {{w|Black-body radiation#Planck's law of black-body radiation|here}}. The only changes are that on Wikipedia, the frequency ''f'' is represented by the Greek letter ''Ξ½'' (nu) and the temperature ''T'' is included as an independent variable, so ''I''(''f'') becomes ''I''(''v'',''T''). However, ''I''(''v'',''T'') still represents the {{w|Radiance#Spectral radiance|spectral radiance}} (similar to energy density). In this formula, ''h'' is the Planck constant, ''c'' is the speed of light in a vacuum, and ''k'' is the Boltzmann constant. The frequency (''f'' or ''v'') along the ''x''-axis is measured in {{w|gigahertz}}. The curve peaks at 160.4 GHz. There is no scale or unit on the {{w|energy density}} on the ''y''-axis.
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The solid line represents the theoretical {{w|blackbody radiation|radiation for a blackbody}} at 2.73 K according to {{w|Planck's Law}} (derived as early as 1900 by {{w|Max Planck}}). The formula, almost as written in the graph, can be found {{w|Black-body radiation#Planck's law of black-body radiation|here}}. The only changes are that on Wikipedia, the frequency f is represented by the Greek letter Ξ½ (nu) and the temperature T is included as an independent variable, so I(f) becomes I(v,T). However, I(v,T) still represents the {{w|Radiance#Spectral radiance|spectral radiance}} (similar to energy density). In this formula, h is the Planck constant, c is the speed of light in a vacuum, and k is the Boltzmann constant. The frequency (f or v) along the x-axis is measured in {{w|GHz}} (Giga (or billion) Hertz). The curve peaks at 160.4 GHz. There is no scale or unit on the {{w|energy density}} on the y-axis.
  
 
The theory is that the blackbody in question was the universe at the point when it had cooled down enough {{w|Decoupling (cosmology)|to allow photons to escape}}, {{w|Chronology of the universe|0.38 million years}} into its {{w|Big Bang|13.8 billion years}} history. The photons that reach us today are the ones that have been travelling to us at lightspeed since then. As the light from astronomical objects suffers from {{w|redshift}} due to the expansion of the universe, and this shift becomes more pronounced with distance from the observer, this light displays in the infrared range.
 
The theory is that the blackbody in question was the universe at the point when it had cooled down enough {{w|Decoupling (cosmology)|to allow photons to escape}}, {{w|Chronology of the universe|0.38 million years}} into its {{w|Big Bang|13.8 billion years}} history. The photons that reach us today are the ones that have been travelling to us at lightspeed since then. As the light from astronomical objects suffers from {{w|redshift}} due to the expansion of the universe, and this shift becomes more pronounced with distance from the observer, this light displays in the infrared range.

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