# Why does the universe expand?

1. Oct 1, 2004

### Mike2

The Newtonian view assumes instant communication of the gravitational field. In such a case, one could pose that the kinetic energy of all the matter in the universe speeding off in all directions is equally balanced by gravitational potential energy that would bind all matter together. I suppose this works when the universe was very close to the size of a singularity. However, the force of gravity travels at the limited speed of light by force cariers called gravitons. So by the time a graviton travels from one side of the universe to the particles on the other side, the particles on the other side have travelled even further away. I suppose this would make the gravitational force of a particle felt by particles on the other side of the universe seem weaker than in the pure Newtonian scheme. This is like slowly reducing the force of gravity as the universe expands. Wouldn't this have the tendency to make the particles fly apart more rapidly since you are slowly eliminating the opposition of gravity? Or at least it would insure that the universe expands forever. Right?

If we throw in a particle horizon where some particles have not yet even felt the gravitational force of other particles very distant from us, this would contribute to expansion, maybe even acceleration.

And if we also have an event horizon where more and more distant objects are accelerated to the speed where we will never again feel their gravitational force, then that would only contribute to even faster acceleration, right?

The question is: can we explain the expansion of the universe without supposing that space itself is expanding, where instead expansion and acceleration can be explained in terms of gravitational forces that are delayed or removed from consideration? Thanks.

Last edited: Oct 1, 2004
2. Oct 2, 2004

### Chronos

No. The apparent effects of propogation delay of gravity is an illusion. When you factor in conservation of angular momentum, there is no problem. Gravity obeys causality and propogates at no more than the speed of light.

3. Oct 2, 2004

### Mike2

I fail to see what angular momentum has to do with anything. Since, I assume, the expansion is uniform throughout the universe, then if there is an event horizon for one point in space, then for the same reason there is an event horizon for every other point in space that is the same size and is growing at the same rate for each point. So if every point is losing touch with the gravitational force of galaxies crossing the event horizon, then does this decrease in gravitational force equal an increase in a repulsive force (a.k.a. dark energy)?

4. Oct 4, 2004

### Chronos

Objection! Once causally connected in this universe, you are forever causally connected. No object, however distant, will ever be able to escape from our observable universe.

5. Oct 4, 2004

### turbo

Do you believe in the standard big bang theory and do you believe that the expansion of the universe is accelerating? Then you must accept that there are many things in the Universe that are not in our observable universe, and that there are objects that are currently in our observable universe that will not be observable at some time in the future.

6. Oct 4, 2004

### Nereid

Staff Emeritus
Let's assume it for this discussion.
Yes, that's clearly the case.
IOW, some distant galaxy in the HUDF, or some CMBR micro-K fluctuation will, at some time in the future, 'wink out' (not simply become more red-shifted, even into the ELF radio spectrum? Hmm, are you sure that's a prediction of the LCDM (including DE) models?

7. Oct 4, 2004

### turbo

Yes, I'm pretty sure. It's a matter of geometry and mass. If the universe is flat (and it appears to be) or hyperbolic, and there is insufficient mass to reverse the presumed accelerating expansion, then galaxies must leave our observable universe. I'm not at home (with my 100's of bookmarks), but here is a very basic explanation of the process.

Last edited by a moderator: May 1, 2017
8. Oct 4, 2004

### Mike2

Evidence for Horizons

What evidence can there be for any kind of horizon, whether it be the particle horizon or the event horizon? I mean, if things are outside our ability to observe, then how can we suppose that there is more if we cannot see it? It sounds like a contradiction of terms, evidence we cannot observe. Galaxies would have to travel faster than light in order that we never see their light and thus suppose there is an event horizon. Are there galaxies that we observe that are moving close to the speed of light?

9. Oct 4, 2004

### turbo

From Ned Wright's FAQ:

http://www.astro.ucla.edu/~wright/cosmology_faq.html#FTL

In the accelerating-expansion BB model, many of the galaxies we see (as they were long ago) are receding from us at faster than the speed of light. Not in the sense of real proper motion, only in terms of universal expansion.

10. Oct 4, 2004

### Mike2

Do you mean that those galaxies are NOW receding from us faster than light? How could you possibly know that? What percent of the universe has receeded beyond our view? And how is it that we can see the most distant object, the CMB, but not things closer? Thanks.

11. Oct 4, 2004

### turbo

If the standard big bang theory (with accelerating expansion) is to be believed, there are distant galaxies that we see (via light emitted very early in the life of the Universe) that are currently being separated from us by expansion at rates faster than the speed of light. I'm not a huge fan of the standard big bang model, so don't ask me to defend it to the death, but this is my understanding of the effects of expansion. There are lots of cosmology tutorials that can explain this much more cogently than I can.

12. Oct 5, 2004

### Mike2

I read an good summary of the various horizons in
http://bat.phys.unsw.edu.au/~charley/papers/0310808.pdf

Though I am left unsure which horizon I am more interested in. Which horizon is it that tells us which galaxies we can still presently see? Or, what horizon do galaxies cross where we can no longer see their light emitted in the past?
Thanks.

Last edited by a moderator: Apr 21, 2017
13. Oct 6, 2004

### Makiavel

Why does the universe expand?
well, because density of universe is small, so the gravitation force doesn't have "force" to atract this mass and with time density will smaller. and the gravitation force will go too small to atract this mass and universe will expand forever.

and we pass of this critical density that make the atraction of universe, so this is my only explication for your question.

i didn't read all posts, sorry if i post stupid coments!

14. Oct 7, 2004

### meteor

Yes, there are galaxies receding faster than c. the locus of the points of spacetime that are receding exactly at the velocity of c is called the Hubble sphere. Those points have a redshift of z=1.46, but there have been discovered galaxies with z greater that this cipher. These galaxies are receding faster than c. A redshift of 1.46 means that the comoving radial distance to the Hubble sphere is 14.1 billion light years (courtesy of Ned Wright's calculator)

15. Oct 7, 2004

### Chronos

Consider this. We can currently see galaxies that were receding from us faster than c at the time the light we now see was emitted - which includes any galaxy with a red shift above z = 1.66 [using the current values omega=.27 lambda=.73 and H=71]. At z=10, as reported for the Rosso Peller galaxy, that particular galaxy was receding at nearly 4 times the speed of light when it emitted the light we are now seeing. It is therefore probably reasonable to conclude the observable universe is not limited to objects with subluminal recession velocities at the time the light was emitted. Try this calculator and see what you get.
http://www.earth.uni.edu/~morgan/ajjar/Cosmology/cosmos.html [Broken]

Last edited by a moderator: May 1, 2017