There are other questions on this site about the CMB, but none of them answer my question specifically. I am not asking about fluctuations in the CMB.
Now as far as I understand, photons do have stress-energy, and do exert gravitational pull on each other, especially in the early universe, when the CMB was very dense.
If you use General Relativity instead you'll find that photons make a contribution to the stress energy tensor, and therefore to the curvature of space.
How did space expansion overcome uniformly this gravitational pull that the CMB photons' had on each other?
Now just to clarify, imagine the early universe, very energetic photons, very dense CMB, so the photons' gravitational pull on each other must be significant. How can space expansion overcome this uniformly? Analogously space was filled with matter particles, those had gravitational pull on each other too. These matter particles clumped up into denser parts, which became galaxies, and voids inbetween.
Why didn't this happen with photons the same way? Those energetic photons in the early universe had significant gravitational pull on each other, like matter particles. But, CMB is uniform everywhere, no denser parts.
Another interesting thing is, there is even something called the geon.
In theoretical general relativity, a geon is a nonsingular electromagnetic or gravitational wave which is held together in a confined region by the gravitational attraction of its own field energy.
- Why didn't the CMB clump into denser parts like the matter in galaxies with voids inbetween since the photons exert gravitational pull on each other?