The overall energy density of light today is about one ten thousandth (10^(-4)) that of matter or dark energy. Thus, its contribution to the overall spacetime curvature is negligible.

That was not always the case, though. Despite all of the light emitted in galaxies, the cosmic microwave background (CMB) still dominates the energy density in radiation (see for example the first figure in this article). This is not light that was "emitted" per se; rather it is left over from a time when the universe was much hotter and denser. Between a few minutes and roughly 50000 years, this light dominated the energy density of the universe. (In particular, the energy density was about 60% photons and 40% neutrinos.)

The gravitational influence of this radiation (that is, its influence on spacetime curvature) led to a different cosmic expansion history. When radiation dominates, cosmic expansion decelerates more efficiently than when matter dominates (the size scales as time^(1/2) rather than time^(2/3)).

Another major impact of radiation domination is that structure growth is suppressed. Matter can cluster, so for example, any small density excess tends to pull in surrounding matter, becoming even denser. That's what is meant by the growth of structure. However, radiation cannot cluster, so when radiation dominates, structures grow much less efficiently.

[removed]

Geez you should be an astrophysicist ;) this is an excellent breakdown, do you have any articles or papers you recommend (or published) on the early universe that focus on expansion history and the emergence of cosmic structures?

In the cosmological context the current contribution of photons to the energy density of free space is negligible.

However there are entirely theoretical phenomena that occur under extreme radiation-dominated energy densities. Most notable is the Kugelblitz: a black hole formed by concentrating light (or heat) sufficiently dense as to curve spacetime and create an event horizon. This entirely theoretical black hole is one of a class of phenomena proposed by Wheeler in 1955 called "Geons" in his paper of the same title.

A model of these (again theoretical) geons has been extended to a candidate for dark matter called Graviballs. Which is a neat idea, but has no supporting observational evidence that I'm aware of.

I used Dodelson's Modern Cosmology originally. However:

• There's a recent, very concise review article on the growth of structure.
• For the expansion history, the Wikipedia article on Friedmann equations has most of the relevant mathematics.
• The thermal history is more complicated. I hinted at some of its features when I mentioned 40% neutrinos and that photon domination only begins at a time of a few minutes (that's about when the electron-positron phase transition ends). I frequently refer to this article for details about the thermal history.

[removed]

[removed]

And here I sit thinking the kugelblitz was just a lot complication from Umbrella Academy.

[removed]

The CMB is light that was emitted, but not until about 380k years after the big bang. Before that matter was so dense in the universe that any light that was emitted was reabsorbed by other matter. There were also other elementary particles in the early universe ~1 second after the big bang like quarks and electrons. By 3 minutes after the big bang, quarks we're forming neutrons and protons. Around 24k years after the big bang, there was more matter than energy in the universe. At 380k years after the big bang, things had cooled Enough for electrons to get captured by hydrogen and helium nuclei, throwing off photons that we still see some of today, the CMB. The CMB doesn't dominate the energy in the universe. It's about 10 orders of magnitude lower than the average matter/energy density of the universe.

> The CMB is light that was emitted, but not until about 380k years after the big bang. Before that matter was so dense in the universe that any light that was emitted was reabsorbed by other matter.

Since all photons are identical, there is no way to objectively say whether photons were absorbed/reemitted or scattered. Regardless, last scattering at ~370000 years did not cause any change in the energy density in photons (which continued to drop as a^-4 as usual), which is why for the purpose of this discussion, it is reasonable to ignore it.

> Around 24k years after the big bang, there was more matter than energy in the universe.

Matter is energy, but it's closer to 52k years that the energy density of matter began to exceed the energy density of radiation.

> The CMB doesn't dominate the energy in the universe. It's about 10 orders of magnitude lower than the average matter/energy density of the universe.

No, just 4 orders of magnitude today, as I mentioned.

[removed]

[removed]

Thanks for the answer, very educational. What's the current theory regarding the future of the CMB? How will redshifting change it further?

It will become continuously dimmer and redder, eventually moving into radio frequencies. If dark energy persists, the CMB frequency will halve (wavelength will double) every ~12 billion years.