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Universal expansion

by shelanachium
Tags: expansion, universal
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Apr28-08, 09:19 PM
P: 131
Amen, Shelanachium !!

Either Hubble expansion in bound systems is zero or it is not. Very curious as to whether this point can be conclusively resolved within current theory.
Apr28-08, 09:21 PM
jonmtkisco's Avatar
P: 531
Quote Quote by shelanachium View Post
Is there someone out there of Asimovian lucidity can explain this stuff to the likes of me?
Hi Shelanchium, yes he had an understudy in Sri Lanka, we're tracking him down as we speak...

I don't think your expression of distress is justified. Nothing is rotten in the state of cosmology. As was explained earlier, the underlying facts of the expansion are fairly well accepted. It's the analogies we use to simplify the explanation that tend cause debate.

Here's what I suggest as your take-away package from this discussion (doggie bag):

1. Go with Wallace's description of what is probably the majority view, that expansion of the universe is caused simply by matter objects moving apart from each other because they were previously moving apart from each other. The increase in empty space being the result, not the cause of expansion. This description is straightforward because it analogizes to the concept of momentum.

2. Go with MeJennifer's admonition that there currently is no exact GR solution for local energy inhomogeneities, such as gravitationally bound collections of matter (e.g. galaxies) and underdense regions (e.g. voids). However there is a solution for spherical regions of cosmic-average density, where the matter is collected in spherically symmetrical clumps (such as a single central mass). This solution is called the "Swiss Cheese" solution, and it is based on Birkhoff's theorem.

3. I don't think you will find it intuitive to think of objects which are gravitationally bound together as having "lost their expansionary momentum only with respect to each other." It is too abstract to say that the Schwartzschild static-space metric applies as between two bound objects, while the FLRW expanding-space metric applies as between those same two objects and all other objects in the universe. It is more intuitive to say that so long as two objects are gravitationally bound, their local expansion away from each other is more or less suppressed, i.e. the "pause" button is pushed.

4. As Wallace said, your question, "at what distance the Hubble expansion exceeds the force of gravity" requires clarification. You must be clear whether you are referring to accelerated expansion resulting from dark energy (cosmological constant). If you are, then the "zero-gravity sphere" can be calculated as I set out in a prior post.

If you are referring to the "original" Einstein-de Sitter expansion, with the cosmological constant (Lambda) = 0 (not the current concordance model), then the Hubble expansion is merely a decelerating (due to gravity) velocity, not an accelerational force. A nonaccelerating expansion velocity can temporarily offset gravitational collapse, but cannot prevent it from occurring eventually.

If the terminology is not clear, please ask specific questions. But it won't be helpful to assert that cosmology is lost in some tractless desert.

Apr28-08, 09:32 PM
jonmtkisco's Avatar
P: 531
Quote Quote by sysreset View Post
Either Hubble expansion in bound systems is zero or it is not. Very curious as to whether this point can be conclusively resolved within current theory.
I would like an answer to the same question! My understanding is that any expansionary "offset" to gravity is insignificant within our solar system and even within the MW galaxy. However, cosmologists have recently observed what they interpret to be Hubble expansionary effects within our Local Group, and within our local supercluster, both of which appear to be gravitationally bound structures. See this thread for more on this topic.

Apr29-08, 02:53 PM
P: 41
Just I think Cosmology is getting a bit like Ptolemy's epicycles. We got inflation, then dark matter, then a resurrected cosmological constant - and what next?

I know it is only an act of faith to believe like Dirac that the fundamental rules of the Universe are simple. (I prefer RULES to LAWS, for the universe works more like a game than a courtroom).

When simplicity fails to appear, it is usually because we have not gone deep enough. Ptolemy sought simplicity at the level of ORBITS, which therefore had to be compounded of combinations of perfect circles, to him the most elegantly simple paths. The true simplicity was unveiled by Newton as lying much deeper, in the forces controlling the orbits - and even a ten-year-old can grasp Newton's rules, even if their consequences have kept great mathematicians busy for centuries.

For the same reason I have no fear that if a simple Theory of Everything is discovered, science will cease to be fun. Chemists still have great fun, though the basic physical rules underlying chemistry have been known now for decades. When we get the TOE we shall be like bright kids who've just learnt the rules of Chess. Care to put up a bright 6-year-old against Kasparov?

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