How many neutrinos are in the universe?

Answer 1

There should be in excess of #10^79# neutrinos in the Universe.

Estimates put the number of atoms in the Universe at #10^80# in the observable Universe. Actually estimates vary from #10^78# to #10^82#.

About 75% of the mass of matter is Hydrogen and most of the remaining 25% of the mass is Helium. Heavier elements are relatively rare. Given that Helium has two protons and two neutrons, we can estimate the number of neutrons to be about 10% of the number of protons or a number of #10^79#.

So, as creating a neutron also creates a neutrino, they should occur in similar numbers at about #10^79#.

If an neutrino and an antineutrino of the same flavour collide they will annihilate each other. As neutrinos don't interact much such collisions will be quite infrequent. Also, the neutrino-antineutrino annihilation is a weak force process where they become a #Z^0# boson. The Z decays rapidly. Depending on the neutrino energy the Z can decay into two photons, but it is more likely to decay back into a neutrino-antineutrino pair.

So, in summary the number of neutrinos is in the order of #10^79# to within two orders of magnitude.

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Answer 2

Anna Armstrong

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.