- #1

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- TL;DR Summary
- It seems there are two ways to think about the energy conservation in the universe.

First, "Energy is not conserved" as e.g. explained by Sean Carroll in https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ .

Second, the Friedmann Equations are expressed in energy conservation, e.g. https://core.ac.uk/download/pdf/25318877.pdf equation (16).

Do we talk about two different "kinds" of energy, one which is conserved and another one which isn't?

In fact, thinking of the universe as a closed adiabatic system then according to the first law of thermodynamics the energy is conserved:

dU + PdV = 0, whereby U is the internal energy of the universe. But this assumes that pressure is doing work which rises the question how this is possible as the pressure is the same everywhere.

Final question. It it correct that in an expanding universe time-translation invariance does not hold? Which would mean that energy is not conserved.

Second, the Friedmann Equations are expressed in energy conservation, e.g. https://core.ac.uk/download/pdf/25318877.pdf equation (16).

Do we talk about two different "kinds" of energy, one which is conserved and another one which isn't?

In fact, thinking of the universe as a closed adiabatic system then according to the first law of thermodynamics the energy is conserved:

dU + PdV = 0, whereby U is the internal energy of the universe. But this assumes that pressure is doing work which rises the question how this is possible as the pressure is the same everywhere.

Final question. It it correct that in an expanding universe time-translation invariance does not hold? Which would mean that energy is not conserved.