Why does the universe exist?

Is there a scientific reason why the universe exists? In other words, what is the science of why there is anything at all, instead of only nothing?

The answer has to do with opposites. Scientists have found that the universe exists because it began with a slight imbalance between matter and antimatter. Particles of matter — that is, all of the electrons, protons and neutrons in the atoms and molecules of regular stuff — differ from particles of antimatter, which carry the opposite electric charge but are similar in many ways.

Matter and antimatter do not get along. When their particles collide, they annihilate each other in an intense burst of gamma-rays. Fortunately, antimatter is now extremely rare. Although antimatter had a foundational role in the formation of the universe, the fact that there is now so little of it is one of cosmology's great mysteries.

Antimatter was predicted by English physicist Paul Dirac almost 100 years ago as part of his pioneering work on quantum mechanics, and it has been confirmed experimentally since the 1930s. Nowadays, scientists can create antimatter in particle colliders like the Large Hadron Collider.

But Dirac predicted there should be equal amounts of matter and antimatter, according to Pasquale Di Bari, a professor of physics and astronomy at the University of Southampton in the U.K. So the fact there is now so little antimatter and so much matter — including all the stars in all the galaxies in the universe, although some scientists once suggested there might be "anti-galaxies" of "anti-stars" — is a big scientific problem.

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"We think the universe started as 50-50 matter-antimatter in the Big Bang but very quickly afterwards became dominated by matter," Tara Shears, a particle physicist at the University of Liverpool, told Live Science in an email. "For this to occur there needs to be a very slight difference, or asymmetry, in the behaviour of matter and antimatter to allow one to ultimately dominate over the other."

But "this difference is not predicted, it is not understood and it is certainly not explained," Shears continued. "Understanding this difference is the problem we want to solve; this is the matter-antimatter asymmetry problem."

According to Dirac, the terms "matter" and "antimatter" are almost arbitrary. "Matter" refers to regular particles, and "antimatter" refers to antiparticles — but it could have been the other way around. If they weren't mostly annihilated, antimatter particles might have formed a universe of anti-atoms and anti-molecules. In the end, whatever predominated was named matter, and its opposite was named antimatter.

Cosmic leftovers

Using observations from particle colliders, traces of the decay of antimatter in astronomical spectra, and gravitational waves, physicists are trying to better understand why there is this large unexplained discrepancy in the universe that has given rise to everything it contains.

Di Bari estimates there might originally have been many billions of times more matter and antimatter particles than there are now, before they mostly annihilated each other in the first fractions of a second after the Big Bang. "What we are made of is the leftovers," he told Live Science.

Raymond Volkas, a theoretical particle physicist at the University of Melbourne, added that a reason for the asymmetry was outlined in 1967 by Soviet physicist Andrei Sakharov. (Sakharov, a critic of the Soviet system, was sentenced to "internal exile" for political dissent in 1980; he was freed in 1986, and died in 1989.)

Sakharov proposed that the asymmetry existed because matter and antimatter particles were not exact opposites but instead reacted differently to some fundamental forces in certain circumstances — a phenomenon known as "C and CP violation."

The general principles of "C and CP violation" are known, but the specifics are not, Volkas told Live Science in an email. "There are many possibilities on the table!" he said. "The challenge is to experimentally distinguish between them."

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