baryon asymmetry of the universe
/ BAR-ee-on /
Here is one of the biggest unanswered questions in science, hiding in plain sight: why is there anything at all? The universe is made of matter — stars, planets, you — with essentially no antimatter around. But our best physics says the hot early universe should have produced matter and antimatter in equal amounts, and equal amounts would have annihilated each other completely, leaving a cosmos of nothing but light. The fact that matter survived means there was a slight, mysterious imbalance. This leftover excess of matter over antimatter is called the baryon asymmetry of the universe.
Baryons are the heavy particles built from three quarks, with the proton and neutron as the familiar examples; 'baryon asymmetry' refers to the excess of these matter particles over their antimatter counterparts. We can actually measure how lopsided things are. By studying the cosmic microwave background and the abundances of light elements made just after the Big Bang, cosmologists infer that for roughly every billion antimatter particles in the early universe, there were about a billion-and-one matter particles. Those billion pairs annihilated into the sea of photons we still see as the cosmic microwave background, and the tiny surplus of one-in-a-billion is everything material in the universe today, including ourselves.
Explaining that one-part-in-a-billion imbalance is a central open problem in physics. It cannot have been there from the start in standard cosmology — inflation would have diluted any initial excess away — so it must have been generated dynamically by physical processes in the early universe, a scenario called baryogenesis. We know the broad conditions any such process must satisfy (the Sakharov conditions), and we know the Standard Model contains some CP violation, a key ingredient. The honest and important caveat is that the CP violation we have measured is far too weak, and the Standard Model's other ingredients too feeble, to produce the observed asymmetry — so the reason anything exists remains genuinely unexplained and is one of the strongest hints that there is physics beyond the Standard Model.
From the cosmic microwave background, the early universe contained only about one extra matter particle for every billion matter-antimatter pairs; those pairs annihilated into the photons that still bathe the sky, and the leftover one-in-a-billion is all the matter we see today.
We are the one-in-a-billion survivors of a near-total annihilation.
We have no evidence for large regions of antimatter elsewhere in the universe; if matter and antimatter galaxies coexisted, the boundaries between them would glow with annihilation radiation we do not observe, so the asymmetry appears to be genuine, not just local.