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Originally Posted by Nereid
In his post in the thread "Mass Exodus From Big Bang Begins" (in the ATM section of BAUT - here), ngeo writes " does GR contain a mathematical description for ‘empty’ space, like a kind of ‘inert field’, or is it that space-like characteristics arise out of the mathematics, or do these characteristics exist only as spatial measurements? |
Not quite sure what he's looking for here. GR treats spacetime as a Reimann manifold, so I guess you could say it's in the math. Although most relativists will say spacetime is the actual physical component of the mathematical manifold.
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Originally Posted by Nereid
If the addition of matter/mass-energy to ‘empty’ space leads to expansion, what is it that expands?
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In terms of the entire universe, the addition of stress-energy to the manifold causes
either an expansion or contraction. The static solution resembles a pencil balanced on it's point. As soon as something changes, the manifold either contracts or expands. Within the math, it's the manifold that expands (as currently viewed). So, since spacetime is the physical componet of the manifold, it is spacetime that is expanding.
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Originally Posted by Nereid
I've seen good questions about GR, and what sort of universe follows from applying the equations, several times.
While there are good resources on the internet, at many different levels, which explain these sorts of things (and more), I don't think there's a single place in BAUT where our members can go to ask questions about this, at the level they're comfortable with, and get helpful answers.
This, then, is the purpose of this thread.
In particular, let's take a look at Einstein's GR equation (G = T), and apply it to the universe.
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Not sure what you are looking for here Nereid, but
here's an application of the Friedman equation. It doesn't start with the EFE, but the discussion starts with if EFE is correct, the first equation is correct. I'll see if I can scare up a page that shows how to get to the Friedman equation relates to the EFE.
EDIT Ok, I found
this rather quickly.
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Originally Posted by Nereid
Let's also consider what GR does NOT get into (e.g. hadrons and leptons; galaxies and superclusters) ... i.e. what extra 'bits' (from other parts of physics) you need to make a 'real universe' (the kind we can see around us, with our eyes).
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Well, I don't think I have the math for this part of the question as I haven't studied QM, much less superstring theory, in mathematical depth. I think I could get concepts across. Although, since there are practicing particle phycisists on the board, they would be more qualified than I to describe these things.