The matter-antimatter asymmetry of the Universe

► Is Leptogenesis the answer? 

Most particles in our Universe have an antimatter counterpart, which has the same mass but opposite charge. For example, the electron and positron are each other's antiparticles.


Therefore, one cannot but wonder why the Universe is not made of antimatter. The construction of an anti-Universe would have followed the same building rules as our matter-Universe. So why did it turn out to be made from particles and not from antiparticles? This mystery is one of the most intriguing questions in modern physics, known as the matter-antimatter asymmetry of the Universe.


Our current understanding suggests that there was an equal amount of matter and antimatter following the Big Bang. When matter and antimatter interact, they annihilate, producing a photon. If we assume that equal amounts of matter and antimatter were produced in the Big Bang, then it is not clear why there should be an asymmetry in the modern day. To put it another way, if the Universe was born of equal amounts matter and antimatter, it should exist in that state now, which we do not observe. What we observe instead, is a matter-dominated Universe, i.e. you and everything around you exists, it's not just made of radiation.


One theory that explains this asymmetry is called Leptogenesis. Leptogenesis proposes that the asymmetry is produced by a violation of lepton-number conservation. Leptons are a subset of fundamental particles that include the electron, muon, tau, and neutrinos. The Standard Model of particle physics stipulates that the number of leptons and anti-leptons participating in a physical process must be conserved. A discovery that this number is not conserved, would open the door for new physics beyond the Standard Model and offer an explanation for the imbalance of matter and antimatter in our Universe. If Leptons could be created without their counterpart anti-leptons, stuff could be created without having to also create anti-stuff; that would help explain why are we here and why our Universe is not only made of radiation.

Find out more about Leptogenesis below:

Matter-antimatter asymmetry in the Universe

We live in a matter-dominated Universe. It is a mystery why this asymmetry exists. Leptogenesis is a promising model that could explain this imbalance providing that lepton-number conservation can be violated in nature.

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