Comment by neildymium on 15/07/2022 at 21:28 UTC

13 upvotes, 1 direct replies (showing 1)

View submission: AskScience AMA Series: We are Cosmologists, Experts on the Cosmic Microwave Background, The Cosmic Web, Dark Matter, Dark Energy and much more! Ask Us Anything!

View parent comment

This is a great question, or rather lots of great questions! I'll answer them in order:

• Light does contribute to the total energy density of the universe, but it's a very small contribution. Light and relativistic matter such as neutrinos are often clumped together and called "radiation" in astronomy. Radiation actually used to dominate the energy content of our universe at very early times, but because of redshifting and the energy of light being related to its wavelength, the energy of radiation falls off faster than matter. So at some point, non-relativistic matter and radiation reached equal energy density (when the universe was about 50,000 years old). Past this point, radiation contributes less and less with time.

• Light does have gravitational influence, but it's completely negligible in astronomical systems.

• Radiation does indeed lose energy to the expansion of the universe. A way to see this without relying on the wavelength is that the expansion of the universe is gravitationally sourced, and gravity can be thought of as sourced by particles called gravitons. So as light propagates through curved spacetime, you can think of it as emitting gravitons steadily, and this emission results in energy loss which we see as redshifting.

• As for the CMB passing through matter, this is something cosmologists absolutely account for! It does have an effect. However this effect isn't quite sensitive enough that we can use it to probe astronomical structure. There are other ways however of getting around dust, such as taking images in the infrared rather than optical, which is exactly what JWST is doing.

Replies

Comment by TeeDeeArt at 16/07/2022 at 06:52 UTC*

2 upvotes, 1 direct replies

Thanks for all the answers :D

So the loss of energy of light to the expansion, does that weakening then itself cause more expansion/inflation? (or rather, did it, back when radiation was actually a significant contributor to the total energy density?)