# Does the redshifting of photons from the Universe's expansion violate conservation of momentum?

The energy-momentum relation,

$$E^2 = m^2c^4 +p^2c^2,$$

lets us derive the momentum of a massless particle:

$$p = \frac{E}{c} = \frac{h\nu}{c}$$

However, the expansion of the Universe redshifts light. This should decrease the momentum of photons. Where would the momentum go, in order for conservation of momentum to hold?

• Is this a different question to astronomy.stackexchange.com/questions/18613/… ? – Rob Jeffries Nov 24 '16 at 16:33
• @RobJeffries Yes, because as far as I know, conservation of energy does not hold in GR. I'm asking about momentum. – Sir Cumference Nov 24 '16 at 16:35
• Light blue-shifts as it falls into super-clusters, then red-shifts a little less as it climbs out of the ever-expanding cluster's gravity well. Basically expansion causes universe to not act like a closed system energy or momentum-wise; but this is only on extremely large scales. – Wayfaring Stranger Nov 26 '16 at 16:00