Forgotten neutrino may have changed how matter won out over antimatter

Everything around us, from stars to people, is made of matter. That is actually strange. According to physics, the Big Bang produced almost exactly as much antimatter: a kind of mirror image of ordinary matter. When matter and antimatter meet, both disappear in a flash of energy. Had the amounts been exactly equal, nothing would have been left to make stars, planets and people from.

So shortly after the Big Bang, a small surplus of matter must have arisen. How exactly that happened is one of the great open questions of physics. Rishav Roshan of the University of Southampton in England and Sudipta Show of the Indian Institute of Technology Kanpur in India now point to a particle that was almost always skipped in this puzzle. Their calculations appeared in 2026 in the journal Physical Review Letters.

Neutrinos and their heavy partners

A popular explanation for the surplus revolves around neutrinos.

According to that idea, the heavy partners left a small surplus behind when they fell apart. That surplus eventually carried through to all matter. In the simplest version there are three heavy partners. Two of them are enough to explain what we now measure in ordinary neutrinos. The third has almost no contact with the rest of matter. That is why physicists usually leave it out of their calculations: it would not do anything anyway. According to Roshan, this particle has been in the scientific literature for a very long time, but almost nobody talks about it.

A particle that changed the pace

Roshan and Show calculated what that third particle can do after all. Precisely because it has so little contact, it falls apart very slowly. So it exists for a long time. As the universe expanded and cooled, it could therefore supply most of the energy in the universe for a while. During that period, the universe expanded differently than in the usual picture.

That pace matters. The surplus of matter did not arise in one go, but in a series of steps that each began at a certain temperature. Roshan compares such a moment with a switch. The forgotten particle flips that switch later. That changes how large the surplus of matter eventually becomes. And when the particle does fall apart, a lot of energy is released. That dilutes the surplus that was already there a little, like water in a glass of squash.

So the researchers are not saying that this particle made matter on its own. They show that it changed the conditions in which that happened. Anyone calculating how the surplus arose should from now on include this particle, they say.

Testing with vibrations in space

Can this ever be measured? Possibly, with gravitational waves: tiny vibrations of space itself. Detectors such as LIGO in the United States already pick up such waves from colliding black holes. A new generation of detectors should also be able to pick up the much weaker waves that arose just after the Big Bang. One of them is the Einstein Telescope. The border region around South Limburg, where the Netherlands, Belgium and Germany meet, is a candidate to build it; the choice of site is expected in 2027.

If the universe expanded differently for a while, that leaves a recognisable kink in those waves. In a follow-up study that fellow scientists are still reviewing, Roshan, Show and their colleague Stephen King calculated which variants of the idea can be tested. In a related variant, in which one of the two other partners is the particle with almost no contact, the kink falls within reach of the planned detectors. For the particle from this research, it is usually still out of their reach. Precise measurements of ordinary neutrinos also help determine which version of the idea holds up.

According to Roshan, an answer to this question could change our picture of the history of the universe considerably, and also teach us something about the laws of nature themselves.

What does this mean for the road to the singularity?

A particle that was left out of the calculations turns out to be able to change the first fraction of a second after the Big Bang. This research shows how closely different fields are connected: the particles everything is made of, the history of the universe, and measuring instruments that will be built in the coming years. For a single specialist that is hard to oversee. An AI model that knows all those fields at once can help track down such forgotten corners and work through the many variants of an idea, to see which ones can really be tested.

AI also helps with the measuring itself. Google DeepMind, together with LIGO, developed an AI method that in a first test dampens the vibrations of the mirrors in the detector 30 to 100 times better than the usual technique. The quieter a detector, the weaker the signals it can pick up. In this way theory and measurement come closer together, and one of the oldest questions in physics becomes step by step easier to investigate: why there is something rather than nothing.

What is the technological singularity? Read the explanation →