The Matter / Antimater Imbalance in the Universe

Physicists have long pondered the problem and may have an answer. It seems that just before the universe was one millisecond old, matter and antimatter annihilated each other in a sweeping extinction. But a tiny asymmetry was built into the universe so that matter dominated over antimatter by one part out of 100 billion. Why the built-in asymmetry? In the first 100 billion-billion-billionth of a second of the universe's history, particles called X particles and their antiparticles were created in equal numbers. These began to decay into other particles—quarks and leptons (which include electrons and positrons). But the decay rates of Xs and anti-Xs are not equal, for reasons that are still not fully understood. Equal numbers of Xs and anti-Xs decayed into unequal numbers of quarks and antrquarks, electrons and positrons. Quarks and antiquarks linked up to form protons and antiprotons. As the universe expanded and cooled, a tiny preponderance of matter over antimatter w^as frozen in. When the epoch of annihilation ended, only matter remained.

All of this happened before the universe w^as less than a thousandth of a second old!

In less than the first second of the Universe's existence, 1/100 billionth of the matter was left over from the annihilation with antimatter.