My question is. If the cosmic background radiation is red shifted how would you calculate the wavelength at the time it was emitted ?


1 Answer 1


When talking of cosmology one needs a model, and the standard model for cosmology at present is the Big Bang Model. This developed fitting observations and taking into account General Relativity ( which includes gravitational redshift.) . In this model the universe started from a small spacetime singularity which is not explorable with the physics we know now and is continuously expanding (with varying rates).

The expansion has been measured by measuring the galactic clusters receding from us and the model has been established. Expanding space leads to a redshift, because the wavelength becomes larger:

The photons that existed at the time of photon decoupling have been propagating ever since, though growing fainter and less energetic, since the expansion of space causes their wavelength to increase over time (and wavelength is inversely proportional to energy according to Planck's relation).

The wavelength at the time it was emitted was from photons with the average energy at the decoupling stage when neutral hydrogen could form, and start the matter stage as we know it, order of electron volts.

It is all consistent, the expansion rate since last decoupling, the temperature/ (wavelength) the CMB is now and what it was at decoupling, which is why the model is successful.

  • $\begingroup$ Thank you ! I read the links you attached now I have another question. How accurate are the energy levels for the Planck epoch ? The link stated on the order of 10^19 Gev is that a measured result? $\endgroup$
    – newguy
    Sep 4, 2015 at 6:35
  • $\begingroup$ It is a result convoluted from measurements. en.wikipedia.org/wiki/Planck_length It is connected to the Planck length, which is derived from three basic constants which have been measured. The time comes from "The Planck time is the time it would take a photon travelling at the speed of light to cross a distance equal to the Planck length."physlink.com/Education/AskExperts/ae281.cfm . So the Planck epoc, is the time from the Big bang to the size of the universe of a Planck length, as close as we can go setting limits from measurement extrapolations. $\endgroup$
    – anna v
    Sep 4, 2015 at 10:51
  • $\begingroup$ "effect similar to redshift, because the wavelength becomes larger". How is the effect not identical to redshift, meaning, how is it not actually redshift? Is redshift not merely an increase in wavelength? Or is redshift an increase in wavelength from a specific cause, which is not the same cause as the wavelength increase in photons from the time of decoupling? $\endgroup$ Sep 4, 2015 at 11:53
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    $\begingroup$ @ToddWilcox you are correct, the light is redshifted, the cause is different. $\endgroup$
    – anna v
    Sep 4, 2015 at 12:31
  • $\begingroup$ Anna v would it be possible to contact you directly I have a few more question but do not feel this forum would be the place to discuss them do you have a face book account $\endgroup$
    – newguy
    Sep 4, 2015 at 18:05

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