When the Universe as we know it began with the hot Big Bang, it was filled with all sorts of energetic particles, antiparticles, and quanta of radiation: a primordial soup of the cosmos. Over time, it expanded and cooled, finally becoming cool enough to produce stable, neutral atoms after a few hundred thousand years had passed. Although the earliest stars and galaxies likely formed within the first ~150 million years of this cosmic history, the Universe remained largely dark and opaque to light until an impressive ~550 million years would pass, as the neutral atoms formed much earlier are remarkably effective at blocking optical wavelengths of light. It’s only through the gradual, slow process of cosmic reionization that the Universe became transparent to light at all.
Ask Ethan: Can we see the cosmic neutrino background?
One of the hardest concepts to wrap our head around is that of the hot Big Bang: the notion that our Universe began 13.8…