Re: Energy conservation in an expanding universe

Thus spake ebunn@xxxxxxxxxxxxxxxxxxxxxx
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.

I don't know about that. I find it philosophically important. But the
place to take about philosophical issues in physics is
sci.physics.foundations, not s.p.r.

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.)

I would have said that WMAP which gives our best measure of spacial
flatness was actually quite important, and that its fairly relevant in
the study of supernova redshifts also. But apart from those two, I
agree, I can't think of any other measurements we can make where we can
look far back enough, and clearly enough, that global properties come in
to play to any great extent. That may change with the next generation of
very large telescopes. We are already seeing galaxies so far back in
time that we don't know how to explain how they evolved so quickly after
the big bang. If that trend continues it would throw the consistency of
the standard model into doubt.

Regards

--
Charles Francis
moderator sci.physics.foundations.
substitute charles for NotI to email

.

Relevant Pages

  • Re: Energy conservation in an expanding universe
    ... Friedmann equation, it's rarely necessary or fruitful to think much ... about truly global properties. ... meaning the net curvature in a global ... Charles Francis ...
    (sci.physics.research)
  • Re: Energy conservation in an expanding universe
    ... e.g. solutions of the Friedmann equation. ... Well, I admit it's an oversimplification, like most morals. ... it's rarely necessary or fruitful to think much ... about truly global properties. ...
    (sci.physics.research)
  • Re: Energy conservation in an expanding universe
    ... Friedmann equation, it's rarely necessary or fruitful to think much ... about truly global properties. ... property of spacetime, not a global one. ...
    (sci.physics.research)