Hunt for the Small and Slow

With the recent discovery of gravitational waves, we now have a target for probing the very early universe, close to the big bang.  This is because gravitational waves can travel across the universe unimpeded, meaning those created after the big bang are still bouncing around today.  It’s like the big bang was the ringing of a giant bell, and the ringing can still be heard.  But all of our Easter eggs are not in one basket.  There is another way to probe the very early universe, one we haven’t found yet, because it involves particles that are very tiny and very slow-moving.  Big bang neutrinos.

Neutrinos are produced in the hearts of stars during the nuclear fusion reactions that power them.  They are also produced when charged particles interact with Earth’s atmosphere.  But there is a third source that may be the largest source of all – the Big Bang.

Big bang neutrinos have been flying around the universe since the beginning.  The expansion of the universe and the subsequent stretching of space-time has cooled them down to billions of times less energetic than the neutrinos from the Sun.  This will make them very difficult to find, since even solar neutrinos pass through matter as if it wasn’t there.

Only experiments like those at the Sudbury Neutrino Observatory can detect the ghostly particles. Their detectors lie under massive amounts of thick rock that shields them from other solar radiation, allowing the occasional neutrino interaction to be observed.

But this might not be enough for big bang neutrinos.  In order to detect these, scientist are planning to use a method exploiting the fact that neutrinos can be captured by tritium, a radioactive isotope of Hydrogen.  This interaction gives a slight boost to the energy of electrons released when tritium decays. By measuring the energies with unprecedented precision, researchers hope to identify the signature of a big bang neutrino.  Sounds simple right?

It’s not. It requires a detector that is kept colder than the vacuum of space, at a temperature only a fraction of a degree above absolute zero.  And that is not cheap.  But if it works, it could pave the way for future experiments that measure big bang neutrinos en masse, with the hope of probing the very early universe and testing if our current model of the big bang is accurate or needs refinement.

What deep unknown physics lie beyond the veil of the cosmic background radiation?


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