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The Cosmic Microwave Background shows us the oldest light in the universe, but to really understand the early universe we need something even older: The Cosmic Neutrino Background.

Host: Reid Reimers

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[ ♪ Intro ].

If you like cosmology, you’ve probably seen this picture before. It’s called the Cosmic Microwave Background, or the CMB, and it’s a false color image of the oldest light in the universe that physics allows us to see.

It’s a baby photo from when space was around 400,000 years old. But it not the oldest image we might one day capture. There’s another, elusive cosmic background created by some of the most mysterious particles physics has described: neutrinos.

Appropriately, it’s called the Cosmic Neutrino Background. And if astronomers are able to snap a photo of it, well, it’ll open up a treasure trove of knowledge about the universe when it was only a second old. Both the CMB and its neutrino counterpart have to do with a phenomenon called decoupling.

These were moments when certain particles stopped interacting with the rest of the matter in the universe, and could stream through space without, for the most part, hitting anything. See, at the moment of the universe’s birth, it was so hot that everything was just a soup of fundamental subatomic particles and light. Then, as space expanded, temperatures started dropping, and particles started slowing down.

Eventually, that allowed the formation of protons and neutrons, then atomic nuclei, and then atoms as a whole. And so on and so forth. The Cosmic Microwave Background formed when photons separated from this soup.

For the first several hundred thousand years, there were so many lone electrons zipping around that the universe was opaque, because photons couldn’t travel very far before getting scattered. Then, as the universe grew, the density of these free electrons decreased. Many also started getting locked up into newly-formed atoms, so the average time between photon scattering increased.

And 380,000 years after the Big Bang, light was able to stream unimpeded through the universe. Scientists say that this is when photons decoupled from matter. And images of the CMB show us that moment.

But photons weren’t the first particles to separate from the primordial soup. When the universe was only a second old, neutrinos high-tailed it outta there, freely flying through space. They produce their own background radiation distinct from the CMB, called the C𝝼B, or CNB.

The Cosmic Neutrino Background. Neutrinos are in the same family of particles as electrons. But unlike electrons, they’re really hard to detect because they almost never interact with anything.

Like, you literally have trillions of them streaming through your body right now. To neutrinos, even entire planets mean nothing. This is why they were able to decouple from matter much faster than photons did.

They only had to wait for the universe to cool to 35 billion Kelvin, as opposed to a few thousand. At that point, things were moving slowly enough, relatively speaking, that neutrinos stopped crashing into other particles all the time. Now, it’s worth noting that not all neutrinos were made in the Big Bang.

They’re also produced by stars as they undergo nuclear fusion, and by your own body as certain radioactive atoms decay. But cosmic neutrinos are a lot sneakier. And right now, we don’t have technology sensitive enough to find direct evidence of them.

Our current detectors can isolate neutrinos with energies on the order of 0.1 Megaelectron volts, but that’s over a billion times more energetic than cosmic neutrinos. So we’re working on indirect detection. And there are a couple ways we can do that.

First, there’s studying the CMB for any subtle imprints the CnuB may have made. Basically, after neutrinos decoupled but before photons did, the neutrinos would have created tiny sonic booms in the primordial soup. They would’ve produced regions that were slightly hotter or colder than others nearby.

So far, some papers have reported detecting cosmic neutrinos’ influence on the CMB. A 2005 report in Physical Review Letters used data from the WMAP satellite and the Sloan Digital Sky Survey. And Planck telescope data provided less ambiguous results a decade later.

It doesn’t confirm anything for sure yet, but it is a promising start. The other indirect detection method requires monitoring the radioactive decay of tritium. That’s a hydrogen atom with two extra neutrons in its nucleus.

Tritium naturally decays by emitting an electron, but it can be forced to decay faster than usual if it absorbs a neutrino. In that case, the electron it emits has a measurably different energy. That energy actually depends on the energy of the neutrino that was absorbed.

So by tracking it, physicists would be able to tell the difference between the tritium absorbing a cosmic neutrino, or one from another source. The problem with this method, though, is scale. Because the energy of cosmic neutrinos is so low, and our detectors aren’t very large or sensitive, we can only hope for a single detection a month.

If that. The KATRIN experiment in Germany, for example, uses 20 micrograms of tritium. And under the most ideal of conditions they estimate they’ll get 1.7 hits a year.

The PTOLEMY experiment at Princeton, on the other hand, is currently operating a prototype device to track even more neutrinos. It involves a detector the size of a postage stamp made of a single, atom-thick layer of tritium on top of an atom-thick layer of carbon. Ultimately, they hope to expand their amount of tritium up to 100 grams, where they might capture 10 cosmic neutrinos a year.

So we’ll see. There’s a ton of effort and money going into these techniques, but scientists aren’t doing it just for the thrill of the hunt. Finding cosmic neutrinos would push back how far into the universe’s history we can actually observe.

Right now, math can take us back further than the CMB, but we don’t have the experimental data to confirm it. It’s all hypothetical. The CnuB would push us back to a time where matter and light readily interacted.

And knowing more about these cosmic neutrinos would inform astronomers how what we normally think of as anti-social particles actually affected the structure of the universe. So thanks, cosmic neutrinos currently flying straight through my… yep. There they go.

Through the entire planet. What are you going to do? Thanks for watching this episode of SciShow Space!

If you’d like to learn more about the Big Bang, you can watch our episode about the first few moments of the universe that physics can’t quite explain. [ ♪ Outro ].