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4 months ago in Cosmology By Akash

Where did the cmb’s energy go?

The cosmic microwave background started as a hot, dense, high-energy soup. Today it's cold and faint. Where did all that energy go?

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By Shraddha Answered 2 months ago

It didn't vanish it redshifted away. As the universe expands, the wavelengths of CMB photons stretch. Longer wavelength means lower frequency, lower energy per photon (E = hf). The total energy density of the radiation field decreases because space itself has grown. But energy is not lost without a trace; in General Relativity, that energy does work against the gravitational pull of the universe. It's accounted for. It's just not where it started.

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