At recombination, Universe became transparent to electromagnetic radiation after universe expanded enough to cool down to form neutral atoms. Before that, the matter plasma was effectively opaque to electromagnetic radiation due to Thomson scattering by free electrons, as the mean free path each photon could travel before encountering an electron was very short.
This Thomson scattering is good to stop photons, but not Neutrinos. There are known Neutrino scatterings in theory, but it's probability is very low, AFAIK.
Why can't we detect neutrinos from before recombination era which can tell something more about initial phases of universe?
Answer
Detecting cosmic neutrino background (~1.95K) is extremely difficult (compared with cosmic microwave background) and never performed directly so far. That's because neutrinos interacts with matter very weakly, unlike photons. We have to build very large detectors. (But if they behave like photons we can't use them to observe early universe.)
I also found quite useful introductory slides about this problem at lbl.gov: http://www-physics.lbl.gov/seminars/old/Petr_Vogel.pdf
No comments:
Post a Comment