(The third installment in the series “Alphabet of Cosmology“, in which I introduce a concept or project in cosmology that’s a little out of the ordinary. This one may be a little more familiar than the first two in the series!)
The primordial cosmic soup
Expansion of the Universe in diagrammatic form, from the Big Bang until today. This post is most concerned with the second and third boxes from the top, before any galaxies or stars.
Since the early 20th century, we’ve known the Universe is expanding: galaxies are getting farther apart and the density of stuff is getting smaller. That means in the past, the part of the Universe we observe today must have been much smaller and denser. As I discussed in the previous Alphabet of Cosmology entry, the environment in early times was very dense, with a high temperature maintained in part by collisions between photons and ordinary (baryonic) matter. Back when the Universe was very young, then, it couldn’t have looked much like it does today: no galaxies and no stars, since there wouldn’t be the right conditions for them to form.
In this era, the Universe consisted of a hot soup of matter and ...