Question

Neutrinos are very abundant in the Universe (our Sun for example produces them in huge amounts as as by-product of nuclear fusion). A recent discovery has given them a (tiny) amount of mass. Discuss how this would/could change our view of whether the expansion of the Universe is slowing or not. (Hint: Think about gravity and potential energy due to gravity)

Answer 1

The simple answer would be yes, since gravitational potential increase the positive acceleration of the universe will be slowed down due to the mass of the neutrons. To have a better idea of how gravity slows down acceleration, we shall take a look at the friedmann equations:

Making a few assumptions like, uniformity and flatness of the universe we have :

where is 'a' is a scale factor which tells us how expanded the Universe is. And it's first derivative gives the rate of expansion and it's second derivative gives the acceleration of the universe.

So if G (gravitational potential) increases, due to the mass of neutrons. Then the acceleration would have a negative value which would mean slowing down.

All the above however does not consider dark energy , which would modify the equations. We still have to understand the nature of dark matter to be able to predict how much in the long run would gravity affect the acceleration.