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I'm wondering how to calculate, even approximately for the relatively mature universe, what the CBR wavelength was as a function of age of the universe. I get that the CBR began during recombination, though have read different values for the black body temperature at that recombination time from H ionization value 13.6 eV, to about 3000K. Those give starting wavelength for CBR of 91nm and 965nm respectively, though have read it started from 400nm to 800nm elsewhere. Currently, the CBR is 1.8mm lambda. But, what was it a billion years ago, or 5 billion years ago???? Does it just vary linearly with age of universe such that the start was close to zero, so as a good (simple) approximation I could just take the current wavelength 1.83mm * (12.7 / 13.7) = 1.69 mm???? for the CBR value a billion years ago (12.7 and 13.7 being the age of universe a billion years ago, or today, respectively.

Thanks,

rt

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# How to calculate CBR lambda vs age of universe?

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