“Prediction is very difficult, especially if it’s about the future.” -Neils Bohr
As many of you know, the Big Bang is one of the most powerful scientific theories we have.
It’s also one of the most unsettling. The idea that our entire Universe — filled with practically a trillion galaxies — is only three times as old as our Sun and was once compressed into the size of your thumb is pretty unsettling.
As an astute commenter noted earlier this week,
I’m asking what persuasive predictions BB has to its credit. Most usually the best I see is new observations favoring one BB variant over another, with some variant or other predicting every possible outcome.
I often speak about the cosmic microwave background, nucleosynthesis of the light elements, and the formation of large-scale structure as strong pieces of evidence for the big bang, and this is completely true.
Well, there is one last prediction that the Big Bang Theory makes that we have not yet been able to successfully test. Just as there is a cosmic background of photons predicted by the big bang,
there should also be a cosmic background of neutrinos!
A neutrino is a tiny particle produced either by radioactive decays or by nuclear reactions in stars or power plants. Or, if you ask the big bang what it predicts, during the first two or three seconds of the Universe.
Although neutrinos are hard to detect, we’ve been able to do it by building giant tanks of water with something like 1035 protons in it, capable of registering and measuring the energy a single, solitary hit from a neutrino.
Neutrinos, which come in three types, are fun little things. Although they have an incredibly low mass — the heaviest may be 10 million times lighter than an electron, the next lightest particle — they do weigh something! (As opposed to photons, which weigh nothing!)
Early on in the big bang model, neutrinos acted just like radiation, since the amount of energy they had was much larger than their mass. But today, the Universe has cooled an incredible amount. While the photons have a temperature of 2.725 Kelvins and a density of about 400 photons per cubic centimeter, the neutrinos are even cooler (at only 1.95 Kelvin) and less dense (there are only about 100 of each type per cubic centimeter).
And while the big bang predicts this neutrino background exists, we have not yet detected it directly.
Why not? After all, we do detect neutrinos, as shown above for an event at Super-Kamiokande. Why can’t we detect these neutrinos?
They’re simply not energetic enough. Neutrinos with more energy are easier to see, and these neutrinos have less kinetic energy than a single atom moving at the speed of a slug.
Only, the slug (or even the atom) makes its presence felt. We don’t know how to see neutrinos of such low energy, but according to the big bang, millions of these leftover neutrinos from the big bang are passing through your body every second!
These neutrinos can’t be the dark matter (they’re neither heavy enough nor cold enough to form the proper structure), but they are expected to fall into each galaxy in a giant halo-shape.
So that’s what the big bang predicts, and as far as I know, this is the last unverified prediction of the big bang. It’s a great one, and I think if this thermal, low-energy neutrino background is discovered, many of the doubters will finally be silenced.
Of course, nobody knows how to detect this neutrino background, yet, myself included! But every once in a while I come back to thinking about this problem, because there’s got to be a way to find out for sure. Any ideas among you?