Comment by TeeDeeArt on 15/07/2022 at 12:30 UTC*

17 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!

I always hear how only 5% of the total energy in the universe is in ordinary baryonic matter, 25% is dark matter, 70% dark matter (or thereabouts)

What of light? Does it not contribute to the total budget, and does it not exert a gravitational force along with the ordinary and dark matter, can enough of it all together not create blackhole? What then is light's contribution? Or does it not have a gravitational effect unless all together like a kugelblitz blackhole and so each star is gradually lowering the amount of gravitational force of its galaxy and in the universe as a whole with each photon it produces?

Which then brings me to the next part of the question. Does the expansion of the universe, as it stretches out those photons and gravitational waves, does it actually mean they have less energy? Or is it the same amount of energy just stretched out over a longer wavelength? And if so, is light's contribution to the 'energy budget' of the universe (and it's gravity) being continually weakened as the universe underwent inflation and expansion.

Anotherthing I've wondered, about the CMB. Given how its light has had to pass through 13B years worth of vast clusters, filaments and voids, getting stretched and distorted and lensed before reaching us, how is it worked out that the CMB has true 'hot' and cold spots, and not just distortions given the intervening matter (or lackthereof) which isn't yet fully mapped?

Obligatory james web 3rd question: It can see through dust well I've heard? What then are the chances of then of seeing what the 'great attractor' is all about then? Can it see through all that damn dust through the milkyway and out to the other side so we can see what's going on?

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

15 upvotes, 1 direct replies

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.