The neutrino decoupling and the photon decoupling happened after the big bang, though at different times. The CMB is very detectable, and although there is no universal reference frame, the CMB is quite often used as some kind of a reference frame.

Understanding the CMB background as a reference frame

Now theoretically it is possible that in the future we will be able to detect the CNB, though these neutrinos are much less energetic then the ones currently detected.

Big Bang cosmology makes many predictions about the CνB, and there is very strong indirect evidence that the cosmic neutrino background exists, both from Big Bang nucleosynthesis predictions of the helium abundance, and from anisotropies in the cosmic microwave background.


My question is whether the CNB should be comoving with the CMB or, different from the CMB?


  1. Should the CNB be comoving with the CMB?
  • $\begingroup$ @RobJeffries thank you I have seen that answer, as I understand it bases the answer on the neutrino masses and if they are small (<0.1eV) then the CNB should be comoving with the CMB. What I do not understand is, since neutrinos interact only through the weak interaction, and gravity, but photons through the EM force too, we usually say that neutrinos fly through matter almost unimpeded. At the time of decoupling, the universe was filled with dense matter and that affected photons and neutrinos differently. Neutrinos flew through dense matter, but photons did not. $\endgroup$ Apr 26, 2020 at 19:22
  • $\begingroup$ @RobJeffries That is why I do not understand how they can be comoving. For neutrinos, the fact that the universe was filled with dense matter does not really count, since they just flew free through it. For photons, it is completely different, since photons get scattered all the time, even when the universe becomes more transparent to photons. Scattering changes the direction of the photons all the time. $\endgroup$ Apr 26, 2020 at 19:24
  • $\begingroup$ TheCMB is (almost) isotropic, except the dipole due to the Galaxy's motion wrt to the comoving cosmological frame. The neutrino background would also be (almost) isotropic in exactly the same way if neutrinos were massless, for exactly the same reason - they formed an isotropic "radiation" field shortly before the decoupled. $\endgroup$
    – ProfRob
    Apr 26, 2020 at 19:27
  • $\begingroup$ @RobJeffries thank you can you please tell me where I can read about this " they formed an isotropic "radiation" field shortly before the decoupled." $\endgroup$ Apr 26, 2020 at 19:34
  • $\begingroup$ Why would it be anisotropic? Other than the dipole caused by the Galaxy's motion and the even smaller anisotropies caused by primordial fluctuations? $\endgroup$
    – ProfRob
    Apr 27, 2020 at 0:04