# Expansion of the Universe

1. Jun 2, 2008

### Ascending One

Is the expansion of the universe phenomenon considered a force or what? Basically I'm wondering how the mechanism of the expansion of the universe is represented in the standard model or any models of physics.

2. Jun 2, 2008

### lzkelley

"the expansion" is not a force, its a response to a force (presumably).
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.

3. Jun 3, 2008

### Ascending One

It's a response to a force? Which force? Is it one of the 4 fundamental forces? I read on Wikipedia that it is gravity, but how can that be?

4. Jun 14, 2008

### lonestone

The inadequacy of all current theories for explaining the expansion of the universe is the driving force behind HEP (high energy Particle physics) lzkelley is right when he suggests that the expansion theory is not universally accepted. The expansionist theory depends upon the notion (often included in the definition of Doppler) that relative radial velocity (rrv) is the only basis for explaining the excess of redshift observable.
This is in spite of the fact that most scientists cling to a belief in GR which asserts that "gravitational redshift" must occur.

5. Jun 14, 2008

### cristo

Staff Emeritus
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.

6. Jun 16, 2008

### rubecuber

well i woulnot consider it a force (such as i would gravity). The point is that it is expanding but that expansion started with the Big Bang and it wasn't constant, the uuniverse was not always expanding so i'd say No. But if you want to bring in dark matter and antimatter and all those things into the equation then i have't he foggiest idea.

7. Jun 16, 2008

### cristo

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

8. Jun 18, 2008

### robousy

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)
$$ds^2=dt^2-a^2(t)(dx^2+dy^2+dz^2)$$

The expansion of the universe is 'controlled' by the scale factor a(t). To find a(t) you need to solve Einsteins equation:
$$G_{\mu\nu}=\frac{8\pi G}{c^4}T_{\mu\nu}$$
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. Jun 19, 2008

### Fredrik

Staff Emeritus
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.

Last edited: Jun 19, 2008