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Astrophysics/Redshift of cosmic background radiation


I read that the redshift of the cosmic background radiation has a value
z = 1100
Could you please explain:
1) As it turns out this number (1100)?
2) What is the difference between the redshift of the cosmic background radiation and the redshift of a distant galaxy? How do you justify this -large difference- redshift that the cosmic radiation as has been said applies z = 1100 whereas for the most distant galaxies that z is more or less 7 or 8?

Hi Dario,

What an excellent question! And most of the online sources do not give a very good explanation of how z = 1100 is determined. First, we don't directly measure that z value from Doppler shifts. All we measure is the cosmic background radiation, which has a peak energy at about a wavelength of 1 mm. Which is why we say the temperature is at about 2.7 degrees K - a blackbody at 2.7 degrees K would have a peak spectrum at about 1 mm.

So now we add a little physics. The photons we now see (peaking at a wavelength of 1 mm) were actually emitted by photons at a temperature = 3000 K. The epoch at which atoms form, when the universe was at an age of 300,000 years and a temperature of around 3000 K, is referred to as "recombination". At a temperature of greater than about 3000 K, the universe consisted of an ionized plasma of mostly protons, electrons, and photons, with a few helium nuclei and a tiny trace of Lithium. The important characteristic of this plasma is that it was fairly opaque. As the universe cooled and expanded, the plasma "recombined" into neutral atoms and the plasma became more transparent.

So since photons could now escape (the universe became transparent during the recombination phase), we assume the CMB photons we see now were actually emitted when the gas was at a temperature of 3000 K. Those photons must have had a blackbody wavelength of 10^(-6) m. The photons now peak at 10^(-3) m ( or 1 mm). So they're red-shifted! We can calculate the redshift by Z = (10^-3 - 10^-6)/10^-6 = 999. That's close to Z = 1100 (if you do the calculation more carefully, you'll get z = 1100). Can we interpret that as a velocity = 0.999 c? I don't know!

The cosmic redshift of distant galaxies is actually measured. We know what the spectrum of nearby galaxies looks like, and (assuming the spectrum of distant galaxies is the same), we can calculate the red shift. As you say, redshifts of greater than 7 or 8 are very difficult to measure - the light simply gets too red / infra-red, and we can't (yet) measure very faint sources at those wavelengths.

All this is still speculative, in my opinion. It is 'generally' assumed that the Big Bang occurred some 13.7 billion years ago. Although this idea is believed to be valid by the majority of cosmologists, it is by no means universally accepted. The "Big Bang", based on Hubble's velocity-distance relation and the microwave background radiation, is very much still a theory, and there are alternative theories. Some observations do not support an expanding universe or the Big Bang. There are several references concerning this, but perhaps the most authoritative person is Halton Arp, a leading astronomer and researcher on galaxies, who wrote "Seeing Red". That book is highly recommended to get an alternative view.  Or read "A Different Approach to Cosmology" by Hoyle, Burbidge, and Narlikar.  That's another great book which gives a scientific view on how the universe has always been in a steady state. See Halton Arp is a leading researcher at the Max Planck Institute in Germany.

Hope this helps.

Prof. James Gort  


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James Gort


Questions on observational astronomy, optics, and astrophysics. Specializing in the evolution of stars, variable stars, supernovae, neuton stars/pulsars, black holes, quasars, and cosmology.


I was a professional astronomer (University of Texas, McDonald Observatory), lecturer at the Adler Planetarium, professor of astrophysics, and amateur astronomer for 42 years. I have made numerous telescopes, and I am currently building one of the largest private observatories in Canada.

StarDate, University of Texas, numerous Journal Publications

B.A. Physics and Astronomy M.Sc. Physics Ph.D. Astrophysics

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