# How many gravitons are there in the observable universe?

1. May 4, 2014

### jimgraber

It is thought that there are approximately 10^80 protons in the observable universe, but there are approximately 10^90 photons in the observable universe. If my googling is correct, there are also approximately 10^90 neutrinos in the observable universe, but their temperature is only 1.9 degrees Kelvin, compared to 2.7 degrees Kelvin for the photons. How many gravitons are there in the observable universe? (And what is their temperature?)

Is this number also approximately equal to 10^90? Why or why not?

The photons and neutrinos are supposedly relics of the big bang.
But we have just (Bicep 2) (indirectly) detected gravitons which are relics of inflation.
Does this make a difference?

Are there photons and neutrinos which are relics of inflation also?

2. May 4, 2014

### iDimension

Gravitons are only hyperthetical. So 0, until we can prove otherwise.

3. Jul 15, 2014

### Staff: Mentor

No, at the end of inflation the universe was so hot that both photons and neutrinos didn't live long enough to "remain intact". The relic neutrinos come from 2 seconds after the big bang, and the photons from ~380 000 years after it.

There are good estimates about the amount of energy in gravitational waves, but I don't know their frequency distribution, so I cannot calculate back to hypothetical gravitons.

4. Jul 21, 2014

### jimgraber

So, if the relic photons and relic neutrinos come from different eras, is it still true their numbers are approximately equal?
(I had thought they were in quasi -equilibrium and the numbers were related similar to the temperatures.)
If not, how many neutrinos are there?
TIA. Jim Graber

5. Jul 22, 2014

### Staff: Mentor

This number depends on the unknown neutrino mass, but the current constraints allow a good approximation: take the neutrino energy density, approximate ~10-100 meV of energy per neutrino, and you get the neutrino density. Multiply with whatever volume you want to consider to get the number of neutrinos.

6. Jul 23, 2014

### yogi

approximate ~10-100 meV of energy per neutrino, and you get the neutrino density

Does the standard theory presume that neutrinos all travel within a range of velocities? It would seem that each neutrino would have a velocity determined by the type of event from which the neutrino issued.... in which case is it not possible to have extremely large neutrino energies and very small neutrino energies ...just curious where the range estimate came from.

7. Jul 25, 2014

### Staff: Mentor

The relic neutrinos where in thermal equilibrium at the time they formed, afterwards it just got cooler from the expansion of the universe, so we still expect their thermal distribution now (~2 K, a bit colder than the photon background). With less than 1 meV as corresponding energy, their kinetic energy has to be below the mass of the most massive neutrino type, so at least one of the three types has to be significantly slower than the speed of light now - those 10 to 100 meV are just an estimate for this mass.

8. Jul 31, 2014

### TheDemx27

May someone post a more complete explanation of this with some maths?