How would a galaxy "bathing in radiation" behave?

Question

For the sake of understanding mass and radiation interaction, I hope somebody could explain how this hypothetical situation would look. I want to exaggerate the amount of photons, assuming that there is some correlation between amount of photons in the void and how large objects interact.

Hypothetical sitation: A spiral galaxy is subjected to such high levels of cosmic background radiation, that the gravity from radiation throughout the volume of the galaxy was equal to gravity originating from the mass of that galaxy. I.e. here is a somewhat isotropic, dense distribution of photons in the galaxy.

How would that galaxy behave, when seen from earth? Would it spin faster, or slower? Would gravitation between planets and their star appear weaker/stronger?

Would it be stretched, compressed, redshifted, blueshifted if we were observing it from a region of space where background radiation were much lower?

PS: This is question is not about the time immediately after the big bang, although one can of course agree that this is the only time in history that cbr was near the levels I am talking about.

Answer 1

Have a look at this history of the universe, according to the current model:

The Cosmic Microwave Background radiation decoupled 380,000 years from the Big Bang, before galaxies formed. It starts at temperatures of 3000 K to be compared with the sun which has a black body temperature of ~5000K. So already, even before the evolution into galaxies, during the time neutral hydrogen forms, the temperatures are not equal to stellar temperatures.

The energy of the CMB falls as time progresses to the right. By the time galaxies form the CMB has degraded a lot and the scenario you envisage is not possible. At present the black body temperature of the CMB is the cold 2.725 K.

To see it differently, when the photon energies are equal to the energies of nucleons, electrons etc before 380,000 years, the energy of the photons is large enough to contribute to gravitational effects, but there are no galaxies. By the time galaxies form the photon energy has deteriorated by the expansion and its gravitational contribution is very small.