What the ‘ghost particle’ reveals about the universe’s evolution

Every second of your life, trillions of particles called neutrinos pass unnoticed through your body. Don’t worry, these so-called ‘ghost particles’ very rarely interact with other particles, including the ones that make you. But these ghostly particles are far from spooky – they're key to helping cosmologists understand the formation and evolution of our universe.

Neutrinos are one of the elementary particles found in the Standard Model of particle physics.  While researchers predicted the existence of neutrinos in the 1930s, it wasn’t until 1956 that two physicists named Clyde L. Cowan and Frederick Reines detected the elusive particle at an underground reactor in South Carolina, USA. Their name for the experiment? “Project Poltergeist.” Reines would accept the Noble Prize in Physics in 1995 for their discovery, as Cowan passed away in 1974.

Neutrinos: the particle that’s full of spooky surprises 

Neutrinos have since developed a habit of defying scientists’ expectations. The Standard Model in the 1970s predicted that neutrinos were massless, similar to the photon. But our ghost particles would be the catalyst for a Nobel Prize again in 2015, when researchers Takaaki Kajita and Arthur B. McDonald discovered that these supposedly massless particles do, in fact, have mass.

“We know that neutrinos have mass because we observe that they mix into each other,” explains Zach Weiner, a postdoctoral researcher in cosmology at Perimeter Institute.

Neutrinos come in three different types, called ‘flavours.’ As they travel, they can change from one flavour to another. “That would only be possible if they have some mixing, and that would only happen if they have mass,” says Weiner.

Neutrinos having mass is another proof point that the Standard Model  – while incredibly successful in predicting characteristics of particle physics - is incomplete. But neutrinos are "ghoul-ing against the grain” in cosmology as well.

“Neutrinos are one of the dominant players in cosmology for a large part of the evolution of the universe,” says Weiner.

A universe of ghostly particles 

There are two important predictions about neutrinos in cosmology that have been shown to be true. The first, explains Weiner, relates to how in the early universe all the particles we know about were in a dense plasma state at very high temperatures. Neutrinos were predicted to contain about 40% of the total energy budget of the universe, a fact since confirmed through investigations into the Cosmic Microwave Background, a snapshot relic of the universe as it was at 300,000 years old.

The second prediction which has been shown to be correct, is that neutrinos became collisionless a few seconds into the formation of the universe. “This characteristic gives neutrinos a key dynamic signature that they affect the evolution of photons and baryons through gravity in a distinct way.”

These two predictions are extremely important components of the current cosmological model of the early universe, but the neutrino defied a third prediction that’s proving tricky for cosmologists.

Cosmology has revealed that neutrinos, like photons, were very relativistic at early times, meaning they moved near the speed of light, says Weiner. That didn't last. Neutrinos eventually became non-relativistic and their velocity became much lower than the speed of light. This change happened well before the present day, meaning neutrinos have behaved as non-relativistic matter components for much of the recent universe.

Normally, this matter component would be hard to distinguish, says Weiner. But neutrinos – while not moving as fast as they once did – were still moving very fast compared to other matter.

“The fact that they’re moving fast means that while dark matter and Standard Model matter are growing and clumping together in the late universe, neutrinos are whizzing past these structures,” says Weiner.

Cosmologists predict that the more dilute density of neutrinos resists the growing clusters, causing other forms of matter to grow more slowly. And, because neutrinos have mass, they are also predicted to have more energy density in late stages of the universe than if they were massless.

But cosmology has been unable to find signatures for either of these effects. “The odd thing about measurements right now is that they suggest that there’s more structure than we expect, and in addition, instead of there being more matter than we would expect from the mass of neutrinos, there’s less,” says Weiner.

In a paper that Weiner published with Marilena Loverde from the University of Washington, the two researchers explored this contradiction. “There’s two physical effects, they’re both opposing expectations, and they're both comparably important.”

“The reason why this interests me is that cosmology is failing to find both of these signatures,” explains Weiner. “This tells us there’s something wrong with either our measurements or with the way we’re modelling the universe. This is exciting – it's an opportunity to search for new physics beyond what we’ve already discovered.”

So, the ‘ghost particle’ continues to both inspire and surprise physicists. There’s no supernatural spook here – just one of the fundamental particles challenging researchers to dig further into the nature of the universe.