Adam Brown

A static universe.

A universe that is expanding, energy is not conserved.

It can just sort of appear.

And general relativity is quite clear on that.

General relativity, Einstein's theory of space and time, one of our most beautiful and best tested theories, is quite clear on that point.

Energy is not conserved.

To ask what happened to the energy, you can ask at a local level what happened to the energy density.

But at a global level, energy is simply not conserved.

Yeah, this is a matter of extreme interest, I would say, to us.

It won't be relevant for tens of billions of years, probably, because that's the timescale on which the cosmological constant operates.

But if the cosmological constant is truly constant,

And we've only known about it for 25 years, and there are, you know, astronomical observations that seem to be in tension with that.

But, like, if it is truly constant, then there is a finite amount of free energy in our universe.

If it's not constant, if we can manipulate it, or even if it naturally decays on its own, then there is the possibility of an unbounded amount of free energy in our future, and we would avoid a heat death scenario.

Yes.

In any of these scenarios in which our universe is a bubble formed in a sort of bigger, what's called a multiverse or a universe,

That's a loaded term, but a sort of larger universe in which our universe is just one bubble.

The higher, the meta universe also has a cosmological constant and it is higher than the value in our universe.

That is the one sense in which there's some version of energy conservation is that you can go down from high to low.

It is considerably harder to go from low to high.