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- Thread starter Ascending One
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The mechanism is not fully understood or agreed upon according to any theory - many aren't convinced that it is fore-sure expanding (although it really does seem to be).

The most popular theory is dark energy - to my knowledge a magic aether that permeates the universe magically causing accelerated expansion... surely the details are better-understood - but certainly not by me.

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This is in spite of the fact that most scientists cling to a belief in GR which asserts that "gravitational redshift" must occur.

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cristo

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The most popular theory is dark energy

Note that the universe "expanding" has nothing to do with dark energy; it is the present day acceleration of this expansion which dark energy is conjectured to explain.

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cristo

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the uuniverse was not always expanding so i'd say No.

What makes you say this? The expansion of the universe is an initial condition in the standard model of cosmology.

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Its a consequence of Einsteins Equations. Matter an energy couple to spacetime generating expansion...its as as simple as that.

Formally the metric for an FRW universe reads (in cartesian coordinates)

[tex]ds^2=dt^2-a^2(t)(dx^2+dy^2+dz^2)[/tex]

The expansion of the universe is 'controlled' by the scale factor a(t). To find a(t) you need to solve Einsteins equation:

[tex]G_{\mu\nu}=\frac{8\pi G}{c^4}T_{\mu\nu}[/tex]

This is pretty easy actually. Anyhow, you discover that a(t) is related to the energy density and pressure of the universe if you model it as a perfect fluid. Some fiddling with conservation of energy equations demonstrate that the pressure is negative which drives the expanding (accelerated) universe.

Summary - Forget what you know about forces and basic classical mechanics, matter and energy couple to spacetime and generate expansion. Thats it. I'm sorry of you find this hard to visualize or understand mechanically, but sadly general relativity is not really like that. That was Einsteins genius.

- #9

Fredrik

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Robousy's answer is definitely the best so far (to the OP's question). *All* the solutions of Einstein's equation that describe homogeneous and isotropic universes describe expanding universes. If we modify the equation by adding the cosmological constant (which is equivalent to a non-zero density of vacuum), there are some solutions that describe a more complicated behavior (like an accelerating expansion).

How can the "force" be gravity? General relativity describes gravity as a relationship (expressed by Einstein's equation) between the geometry of space-time and its matter content. So in GR, gravity is not really a force. It's just geometry. Note that it's the geometry of space-time, not just space. The fact that time is involved is what makes things like expansion possible even though geometry is usually just about shapes.

I also agree with everything that Cristo said.

How can the "force" be gravity? General relativity describes gravity as a relationship (expressed by Einstein's equation) between the geometry of space-time and its matter content. So in GR, gravity is not really a force. It's just geometry. Note that it's the geometry of space-time, not just space. The fact that time is involved is what makes things like expansion possible even though geometry is usually just about shapes.

I also agree with everything that Cristo said.

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