Matter / antimatter asymmetry

The big bang should have created equal amounts of matter and antimatter in the early universe. But today, everything we see from the smallest life forms on Earth to the largest stellar objects is made almost entirely of matter. Comparatively, there is not much antimatter to be found. Something must have happened to tip the balance. One of the greatest challenges in physics is to figure out what happened to the antimatter, or why we see matter/antimatter asymmetry.

Antimatter particles share the same mass as their matter counterparts, but qualities such as electric charge are opposite. The positively charged positron, for example, is the anti-particle to the negatively charged electron. Matter and antimatter particles are always produced as a pair and, if they come in contact, annihilate one another, leaving behind pure energy. During the first fractions of a second of the big bang, the hot and dense universe was buzzing with particle-antiparticle pairs popping in and out of existence.

If matter and antimatter are created and destroyed together, it seems the universe should contain nothing but leftover energy. Nevertheless, a tiny portion of matter – about one particle per billion – managed to survive. This is what we see today. In the preceding few decades, scientists have learned from particle physics experiments that the laws of nature do not apply equally to matter and antimatter. They are keen to discover the reasons why.

Researchers have observed particles spontaneously transforming, or oscillating, into their antiparticles at a rate of millions of times per second before decaying. Some unknown entity intervening in this process in the early universe could have caused oscillating particles to decay as matter more often than they decayed as antimatter.

Consider a coin spinning on a table. A spinning coin has the potential to land on heads or tails, but it cannot be defined as one or the other until it stops spinning and falls definitively to one side. A coin has a 50-50 chance of landing on heads or tails, so if enough coins are spun in exactly the same way, half should land on heads and the other half on tails. In the same way, half of the oscillating particles in the early universe should have decayed as matter and the other half as antimatter.

However, if a special kind of marble rolled across the table of spinning coins and caused every coin it hit to land on heads, it would disrupt the whole system. There would be more heads than tails. In the same way, scientists think some unknown mechanism interfered with the oscillating particles to cause a slight majority of them to decay as matter.

Scientists could find hints as to what this process might be by studying the subtle differences in the behaviour of matter and antimatter particles created in high-energy proton collisions at the Large Hadron Collider. Studying this imbalance could help scientists paint a clearer picture of why our universe is matter-filled.