Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Dumb_Blonde on 28/03/2014 23:19:32
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Guys, please, I don't understand why CMB has so many different photon wavelenghts, how can all have the same average temperature? Shouldn't them be all microwaves?
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In theory, a "black body" at a certain temperature emits photons of all wavelengths, from radio waves to gamma rays. Two bodies at the same temperature will emit the same mix of wavelengths (ie the same "spectrum"); two bodies at different temperatures will each emit a different spectrum. This spectrum is called the "Black body radiation (http://en.wikipedia.org/wiki/Black-body_radiation)".
In practice, the energy to produce a gamma ray is so high that bodies at normal temperatures will almost never do it - there is effectively an upper limit on the energy of the photons emitted by a body of a given temperature.
The Cosmic Background radiation has a spectrum equivalent to an object at 2.7K (ie very cool by human standards). The peak of its radiation (http://en.wikipedia.org/wiki/Black-body_radiation#Wien.27s_displacement_law) is 2.7 x 58.8GHz, or around 160GHz in the microwave spectrum; this frequency is strongly absorbed by moisture in the atmosphere, but it would be detectable from satellites. The CMB also emits at higher and lower frequencies, such as the original discovery (http://en.wikipedia.org/wiki/Discovery_of_cosmic_microwave_background_radiation), at wavelengths around 7cm.
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Guys, please, I don't understand why CMB has so many different photon wavelenghts, how can all have the same average temperature? Shouldn't them be all microwaves?
The CMBR has a blackbody spectrum just like many objects in the universe such as the sun. I don't understand what you mean by "all have the same average temperature" though. "all" what? A photon doesn't have a temperature so I can't understand what you're trying to get to.