So from where we are today, we can look out in any direction we like and see the same cosmic story unfolding. Today, 13.8 billion years after the Big Bang, we have the stars and galaxies we know today. Earlier, galaxies were smaller, bluer, younger and less evolved. Before that, there were the first stars, and prior to that, just neutral atoms. Before neutral atoms, there was an ionized plasma, then even earlier there were free protons and neutrons, spontaneous creation of matter-and-antimatter, free quarks and gluons, all the unstable particles in the Standard Model, and finally the moment of the Big Bang itself. Looking to greater and greater distances is equivalent to looking all the way back in time.

Although this defines our observable Universe — with the theoretical boundary of the Big Bang located 46.1 billion light years from our current position — this is not a real boundary in space. Instead, it’s simply a boundary in time; there’s a limit to what we can see because the speed of light allows information to only travel so far over the 13.8 billion years since the hot Big Bang. That distance is farther than 13.8 billion light years because the fabric of the Universe has expanded (and continues to expand), but it’s still limited. But what about prior to the Big Bang? What would you see if you somehow went to the time just a tiny fraction of a second earlier than when the Universe was at its highest energies, hot and dense, and full of matter, antimatter and radiation?