eli5: does redshift disobey the first law of thermodynamics
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I’m going to assume you mean *cosmological* redshift.
The answer is “no”, at least not as conservation of energy is usually framed. Conservation of energy isn’t even necessarily well-defined across the universe as a whole: energy is observer-dependent (even though it’s conserved for each observer) and conservation of energy is only true locally. There’s no way to “patch” local measurements of energy together into some universal “energy of the Universe”.
Put another way, since conservation of energy is equivalent to time-symmetry of physical laws and redshift emerges from the fact that the Universe is expanding (and that, therefore, the physical laws *do* depend on time and are *not* time-symmetric), it’s not so surprising that it violates conservation of energy.
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