Re: Energy conservation in an expanding universe

In article <JAsMgjExlr3FFwGr@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Oh No <NotI@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Thus spake ebunn@xxxxxxxxxxxxxxxxxxxxxx
And one of the key morals of general relativity is that you're better
off thinking of physics locally rather than globally anyway.

This is at once true and false. Ted doesn't mean this to apply why
thinking of global structure, e.g. solutions of the Friedmann equation.

Well, I admit it's an oversimplification, like most morals. But not
all that much of one. Even when talking about solutions to the
Friedmann equation, it's rarely necessary or fruitful to think much
about truly global properties.

For instance, is the Universe spatially finite or infinite? That's a
global question, and it's one that lots of people find interesting to
think about. But it has remarkably little relevance to anything that
we can test observationally. It's much more important to think about
observables that live in small to medium-sized volumes rather than
global properties.

The biggest exception in cosmology is searches for nontrivial global
topology, but frankly that's a bit of a sideshow. (I feel like I've
earned the right to say this, since I've written papers on the
subject.)

-Ted

--
[E-mail me at name@xxxxxxxxxx, as opposed to name@xxxxxxxxxxxxxxxxxxx]

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