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In the early universe, neutrinos were king. These tiny particles interact with other matter so weakly that about 100 trillion of them pass through your body unnoticed every second, but they may have had enormous effects on the structure of matter just after the big bang.

In the first few thousand years of the universe, before the formation of galaxies or stars, very small ripples of matter started to form, says Francis-Yan Cyr-Racine at the University of New Mexico. 鈥淭he way those ripples form depends dramatically on how neutrinos behave.鈥�

Yu Liu at Shanghai Jiao Tong University in China and his team developed a new way to analyse the large-scale structure of the universe for the effects of neutrinos by scanning its density. They found that in the early universe, areas without much matter still contained neutrinos, and that large clumps of matter were a little blurry around the edges.

Liu suggests this is because neutrinos, even with their slight mass, pulled matter away from denser regions, slowing down the accumulation of matter and making the edges of those clumps less defined than they would otherwise have been.

While we don鈥檛 know the exact masses of neutrinos, we know they are extremely light and so can move incredibly fast 鈥� nearly the speed of light. In the early universe, as other particles were starting to clump together, neutrinos鈥� speed and low interaction allowed them to continue zipping around without getting caught in the clumps.

But their mass still attracted other matter, pulling it away from the clumps. The locations where neutrinos dragged matter nearly 14 billion years ago guided where stars and galaxies would eventually form.

鈥淎t the time that structures begin to clump together and create density contrast, neutrinos tend to damp that clumping,鈥� says George Fuller at the University of California, San Diego. Not only does this effect elucidate how crucial neutrinos were, studying it further could help solve the mystery of neutrinos鈥� mass, he says.

Neutrinos themselves could help us solve another major mystery: the precise amount of lithium in the universe. Based on how much lithium was produced in the big bang, we expect old stars to contain far more of this element than they do, and it isn鈥檛 clear why it is missing.

Alexander Heger at Monash University in Australia and Stan Woosley at the University of California, Santa Cruz, attempted to solve this discrepancy by looking into the differences in how the earliest stars exploded and how stars explode now.

When a star explodes in a supernova, it releases high-energy neutrinos that can kick-start cascading nuclear reactions. The first stars would have been relatively compact, which means the neutrinos would hit the stellar material much harder because it was closer to the exploding core.

The researchers found that this high density and extra energy meant the neutrinos would kick off extremely efficient reactions, which should have produced more lithium in old stars than we see. 鈥淚t does not solve the problem, it makes it worse,鈥� says Heger.

A significant amount of the lithium 鈥� as well as some other elements 鈥� in the universe now may have been made by neutrinos in these sorts of supernovae, he says.

鈥淣eutrinos are very important in the history of the early universe,鈥� says Fuller. 鈥淏ut they are hard to measure in the laboratory. They鈥檙e very slippery.鈥� So, we may have to rely on their effects across the cosmos to understand them.

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