Horizon and hydrogen recombination

Before the (re)combination of hydrogen, the Universe was opaque and light could not travel. Does this correspond to what one calls horizon?
The answer is no.

The black wall that one reaches when one watches far enough to observe the epoch of hydrogen recombination is opaque because, beyond it, light emitted is immediately trapped by charged particles. This means that light emitted before this epoch cannot be observed, but it does not mean that information cannot be obtained from earlier epochs. This information could be transported by neutrinos which interact very weakly very matter, or by gravitational waves. In other words, the black wall is opaque to light but not to other “messengers”.

On the other hand, an horizon corresponds to a surface which limits the region from which, at a given time, we cannot receive any information (whether in the form of light, neutrinos or gravitational waves).

One comment


    That has been very helpful, I like the potential to use Neutrinos for further penetration. Are there any possible messengers from beyond the Big bang??

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