Quantum Gravity: Photons, Electromagnetic Radiation & the Universe

[backenvelope]

We know that a photon has no mass, but has relativistic mass - which means in theory, a photon will have a gravitational field.

What is the total possible gravitational affect of all photons (and electromagnetic radiation) emitted from the stars in the universe?

Could this account for the gravity seen in the universe – which cannot be explained by ordinary matter?
And could this explain the accelerating expansion of the universe, instead of the dark matter and dark energy interaction that's otherwise predicted?

 

[PeterDonis]

We know that a photon has no mass, but has relativistic mass

A better term would be "energy" (still better would be "stress-energy").

Wwhich means in theory, a photon will have a gravitational field.

Yes.

What is the total possible gravitational affect of all photons (and electromagnetic radiation) emitted from the stars in the universe?

Very small; several orders of magnitude smaller than the effect of the ordinary matter. (Actually, AFAIK the largest component of the stress-energy of photons and EM radiation comes from the CMBR; the amount of stress-energy in starlight is considerably smaller than that.)

Could this account for the gravity seen in the universe – which cannot be explained by ordinary matter?

No. It's way too small.

And could this explain the accelerating expansion of the universe, instead of the dark matter and dark energy interaction that's otherwise predicted?

No. Stress-energy due to radiation can't cause an accelerating expansion, any more than stress-energy from ordinary matter can. (Dark matter can't either; only dark energy can.)

 

[Chalnoth]

No. Stress-energy due to radiation can't cause an accelerating expansion, any more than stress-energy from ordinary matter can. (Dark matter can't either; only dark energy can.)

In fact, radiation causes the expansion to slow even more than normal matter does.