the matter-antimatter asymmetry

Here is a fact so basic that it is easy to overlook: the world is made of matter and almost no antimatter. Every star, every planet, your own body — all of it is matter, with antimatter showing up only fleetingly in cosmic rays, lab experiments, and certain radioactive decays. Yet the deepest principles of physics are nearly even-handed between the two. So why did the universe end up so lopsided? Why is there something made of matter at all, rather than a cosmos that wiped itself out?

The puzzle becomes sharp when you wind the clock back. In the hot early universe, matter and antimatter should have been produced in nearly equal amounts, and when a particle meets its antiparticle they annihilate into pure light. If the books had balanced exactly, every particle of matter would have found a partner of antimatter, both would have vanished, and the universe today would be a cold bath of radiation with no atoms — no galaxies, no planets, no us. The fact that we are here means there was a tiny surplus: for roughly every billion antimatter particles, there were about a billion-and-one matter particles. After the great annihilation, that one-in-a-billion leftover became everything we see.

Generating such a surplus requires three specific ingredients, spelled out by Andrei Sakharov: processes that change the net amount of matter, a difference in how matter and antimatter behave (called CP violation), and a period when the universe was out of balance. The Standard Model contains a small amount of CP violation, observed in the decays of certain mesons, but it is far too feeble to account for the surplus we see — by many orders of magnitude. That shortfall is a clear hint of physics beyond the Standard Model, and it is one reason experiments hunt so hard for new sources of CP violation, especially among neutrinos.