Where did all matter come from?

Question

I’ve hear before that the big bang wasn’t really an explosion, more of the point in time where the universe started rapidly expanding. Can someone explain where the all the matter that makes up planets, stars, etc. came from?

In: Physics

Answer 1

The very early Universe was extremely hot, hotter than even the cores of the hottest supernovas today. It was filled with a soup of particles that, under the conditions we see today, would be incredibly unstable.

As the Universe expanded, cooled, and aged, those particles decayed into other particles (in the same way that a radioactive element can decay today, but much more quickly). Eventually (and by “eventually” I mean “roughly 1 second after the Big Bang”), they decayed into the only particles that are stable for long periods of time (where “long” here means “minutes”): photons, protons, electrons (stable) and neutrons (unstable by themselves but relatively long lived) and the antimatter equivalents of each.

For reasons that are not completely understood, more protons, electrons, and neutrons were made than antiprotons, antielectrons, and antineutrons: for every billion or so of each of them, there was one extra proton/electron/neutron. Each particle and antiparticle paired off and destroyed each other, which left just those few leftover protons, electrons, and neutrons.

Over the next few minutes, some of the neutrons decayed, but most of them got stuck to protons. And finally, the Universe cooled down that electrons could attach to protons to form the first atoms. That left the following types of particles in the Universe:

* A huge number of Photons (light)
* A moderate number of Hydrogen atoms (electrons ‘orbiting’ [not really] protons)
* A few Helium atoms (electrons ‘orbiting’ [not really] protons and neutrons stuck together)
* A very small number of lithium atoms, which had even more protons and neutrons stuck together.

The history of the Universe pauses here for a long time – many hundreds of thousands of years – in which pretty much nothing happens. But eventually stars formed, and the reactions within stars would convert hydrogen into the other elements we see today (in particular, all the elements other than Hydrogen that you see on a rocky planet like Earth).