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How much of the universe is in the form of electromagnetic radiation?

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Mar8-14, 11:49 AM
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Can you make an educated guess on the amount of energy in the universe in the form of electromagnetic radiation (photons), considering the vast amount of photons moving in every direction throughout the vast universe, there is literally no point in the universe that you can be in and not observe photons coming from every direction around you, meaning most of the vacuum of space has EMR moving through it. I'm guessing it should add up to a really significant portion of all the available energy in the universe? Also how much energy do you think a sun like ours looses in EMR throughout it's life time?
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Mar8-14, 12:40 PM
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No, it is an insignificant amount in comparison to the energy content of the total mass content.

You can make a rough calculation from the peak energy of the cosmic background radiation (CMB), which represents the peak temperature (2.7 K) for a blackbody spectrum. For more on the CMB see:

From the temperature you can calculate the energy of each photon, and then calculate the energy content per cubic meter of outer space from the density figures (~10^9 CMB photons per cubic meter).

Then compare it with the energy content of the particles in that same cubic meter - perhaps one hydrogen atom.

A good exercise in conversion of units of energy, and making physical estimates.
Vanadium 50
Mar8-14, 04:15 PM
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It's been a very long time since the universe was radiation dominated - over 13 billion years.

Mar8-14, 07:47 PM
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How much of the universe is in the form of electromagnetic radiation?

Photons contain about 0.01% of the energy of the universe according to the most popular modern cosmological model - the Lambda-CDM. For comparison, regular matter (baryonic matter) contains about 5%, dark matter contains about 25%, and dark energy about 70%. Other models are possible but that one seems to be the front runner at least for now. At any rate, the fraction of the universe's energy in the photons is minimal.

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