Neutrinos may have an interesting role to play in the greater scheme of things. But, again to answer that question for the kid who’s curious, you have to say, “Why? What has this got to do with the universe?” And, in fact, it has a lot to do with the universe. It turns out, if we want to understand the universe, which means the stars and the galaxies, and all the great big things, we have to know the small things. That’s what we call “the inner space-outer space connection.” We have to know about the small things in order to understand how the big things work.
That particular example is a very fascinating one. We know that the universe is expanding as if there were an explosion roughly 15 billion years ago. It’s called, the Big Bang. In that explosion, all the matter in the universe spewed out and space and time, radiated. Got bigger and bigger and bigger. Now the question is, will this continue forever? Or will it slow down because gravity is an attractive force? Gravity says “Come back, come back!” How strong is gravity compared to the initial explosive force? That’s an issue. That depends. It’s an experimental issue. We can find this out. It depends on how much mass there is. The more mass, the more gravity. If we add up all the mass, count all the stars we can see with the most powerful telescopes, we estimate how much mass they have, we can calculate whether we have enough mass to slow up the expansion. When we do, we find we don’t have nearly enough mass.
But, on the other hand, there are the neutrinos. We know they’re there because we know enough about the big explosion to know that it spewed enormous number of neutrinos around. The neutrinos themselves would have no impact on the expansion if they have zero mass. On the other hand, if it’s not zero, but teensy-weensy, but not zero, just a small amount, then the neutrinos might act to break the expansion, slow it down. So, the issue then after we discovered the neutrino, was to see whether in fact, since the 1960s, we now know that in the universe, in nature, there are three neutrinos. Part of the zeros and ones is there are three different kinds of neutrinos. We don’t know very much about the masses. If they have a small mass, they can act to change the future of the universe. Those are experiments one can do.