Number density of air is ##2.5\cdot 10^{25}## molecules per cubic meter... It's larger by 16 orders of magnitude.
Now that makes me wonder how could light be "trapped" in such "low" density plasma before the recombination...
Right... So the formula for the total density of hydrogen is ##(1+z)^3\cdot 1.6\,m^{-3}##.
So for ##z=0## it's 1.6 hydrogen atoms per cubic meter... Right after the decoupling or today? More like today I guess.
So for ##z=1100## it's ##1101^3 \cdot 1.6## hydrogen atoms per cubic meter... That...
https://en.wikipedia.org/wiki/Recombination_(cosmology)
Was a matter density right after the decoupling low enough to consider the vacuum as the actual vacuum, and not the medium through which the light propagates with the speed lower than ##({\epsilon_0\mu_0})^{-1/2}##? I'm asking this in...