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As per the title. I'm probably thinking too naively in a classical way, but intuitively it seems to me that:

  1. the presence of vacuum energy at fixed density throughout all space and,
  2. an expanding universe

would imply that more and more energy is constantly added to the universe.

I am assuming we have not given up on energy conservation quite yet, so where is the catch?

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Energy conservation is equivalent to the Lagrangian's partial time derivative being $0$. In an FLRW spacetime, this fails. (In fact, total energy isn't always even well-defined in general relativity.)

While ordinary matter has a density proportional to $a^{-3}$ in a universe of scale factor $a$, dark energy is far from the only component of the universe that doesn't work like that, resulting in a volume-dependent contribution to total mass-energy. Radiation has an $a^{-4}$ density, so actually diminishes over time. This is why a radiation-dominated era preceded the matter-dominated era.

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