Science & Technology

Scientists Detected Neutrinos from One Second After the Big Bang

The Big Bang — that faster-than-light expansion that marked the beginning of the universe — seems like it would be a hard thing to prove. It would have happened a long, long time ago somewhere far, far away. We don't have time machines or intergalactic spacecraft, so it'd be understandable to assume that the Big Bang is just a theory we'll never prove. On the contrary: If the Big Bang happened, it would have left some telltale signs we can detect today, from our cozy confines here on Earth. It turns out that, over the last century, scientists have found almost all of those signs, one by one. And just last month, a research team announced that they'd found a piece of evidence that many thought would be impossible to detect: the first neutrinos from one second after the Big Bang.

Bang Bang, Into the Void

Here's how the beginning of the universe went down, according to the Big Bang theory: 13.8 billion years ago, everything you know started as a small, super-dense point known as a singularity. Then, in a trillionth of a second, the singularity expanded by double and double and double again at a rate faster than the speed of light. A (very) short time after that, the universe as we know it was a dense plasma soup of fundamental particles broiling hundreds of times hotter than the sun, rippling with density waves that traveled out into the expanding universe.

A visual history of the expanding universe includes the hot, dense state known as the Big Bang and the subsequent growth and formation of structure. The full suite of data, including the observations of the light elements and the cosmic microwave background, leaves only the Big Bang as a valid explanation for all we see. The prediction of a cosmic neutrino background was one of the last great unconfirmed Big Bang predictions.

Eventually, the plasma cooled enough that those fundamental particles were able to coalesce into atoms of hydrogen and helium, and the free electrons zipping around found homes around atomic nuclei. That left enough room for radiation to flow through the universe unhindered. But as the universe kept growing, that radiation would have cooled too, eventually turning into invisible microwaves.

Every one of the steps in the theory makes testable predictions. For example, it says that the universe was smaller in the past and will likely keep on getting bigger. In 1929, Edwin Hubble discovered that the universe is indeed expanding. Because we know how fundamental particles play together, the theory also says that the early plasma should have coalesced into a particular ratio of hydrogen, helium, and various other elements. Those are the ratios you find in our current universe. And that radiation — the stuff that cooled into microwaves? You can see it, too, in what's known as the cosmic microwave background.

Fly, My Pretties

There are other predictions from the theory, however, that have proven a bit trickier. Those involve a super-tiny, super-light particle known as a neutrino. See, if there's enough energy around — like there was at the beginning of the universe — the collision of two random particles can produce a new pair of particles: one particle, and one antiparticle (the antimatter version of that particle). Over time, most of those particle-antiparticle pairs canceled each other out or decayed into other particles. But neutrinos can't decay, so once they stopped interacting with antineutrinos — which happened about a second after the Big Bang — they just hung around, surfing the density waves of the expanding universe.

That brings us to another prediction: Neutrinos don't interact with anything, so they likely would have traveled a few steps ahead of the rest of the universe's matter as it expanded. That would have left nearly imperceptible adjustments in the pattern of the cosmic microwave background and in the overall structure of the universe. Of course, neutrinos are hard to detect even when they're new and excited. Neutrinos that have been traveling through space, gradually losing steam over billions of years? You could assume it's never going to happen.

Well, never say never. In 2015, researchers from the University of California, Davis detected evidence of those ancient neutrinos in the cosmic microwave background. And in February of this year, an international team detected their evidence in the universe's baryon acoustic oscillations — a fancy term for the quirks in the universe's structure caused by the density waves that traveled through that primordial plasma soup after the Big Bang. The effects of neutrinos at the birth of our universe were detected in the cosmic microwave background, and now they've been detected in the universe's overall structure.

While there are certainly refinements to be made on this discovery, it's worth pausing to celebrate. The Big Bang is our best theory of how the universe began, and it just keeps getting better.

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Learn more about our universe's birth in "Big Bang: The Origin of the Universe" by New York Times bestselling author Simon Singh. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer March 26, 2019

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