Topic

Antimatter

The matter-antimatter asymmetry problem

A composite view of the Crab nebula, an iconic supernova remnant in our Milky Way galaxy, viewed by the Herschel Space Observatory and the Hubble Space Telescope (Image: NASA)

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 an asymmetry between matter and antimatter.

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 antiparticle 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 past few decades, particle-physics experiments have shown that the laws of nature do not apply equally to matter and antimatter. Physicists are keen to discover the reasons why. Researchers have observed spontaneous transformations between particles and their antiparticles, occurring millions of times per second before they decay. Some unknown entity intervening in this process in the early universe could have caused these "oscillating" particles to decay as matter more often than they decayed as antimatter.

Consider a coin spinning on a table. It can land on its heads or its tails, but it cannot be defined as "heads" or "tails" until it stops spinning and falls to one side. A coin has a 50-50 chance of landing on its head or its tail, 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 a table of spinning coins and caused every coin it hit to land on its head, it would disrupt the whole system. There would be more heads than tails. In the same way, some unknown mechanism could have interfered with the oscillating particles to cause a slight majority of them to decay as matter. Physicists may 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.