# Combined affects of gravity?

1. Jul 16, 2011

### YoungDreamer

I am wondering about the gravity of loose groupings of matter, such as galaxies. Stars and other objects have their own gravity but when you look at groups of galaxies you see that the galaxies have the combined gravity of all the matter that makes up the galaxy.

My question is not about the combined affects of gravity in galaxies but of the Universe as a whole.

So what I want to know is if it is possible for all the gravity from all the matter in the Universe to be combined.
Could this have an event horizon effect at the "edge" of the Universe, forming an invisible unbreachable barrier around the Universe that doesn't allow light or matter to "escape"
and instead just sends light or matter around the "edge" of the Universe?

I don't know if I have formed my thoughts into the proper words so that you understand what I am trying to say, but I figure some of you will get it.

2. Jul 16, 2011

### mathman

If I understand you correctly, you are asking if the universe is a gigantic black hole. Off hand, I don't think the question has been settled one way or the other.

3. Jul 16, 2011

### zhermes

The universe is not dense enough to have such an external event horizon, like a black-hole. There are (numerous) other types of event horizons which border our universe, but they're very different.

4. Jul 17, 2011

### siiix

i would expect that there is no such battier do to the fact that the universe is increasingly expanding... if so i doubt light would be pulled back as other matter seems not to at all

5. Jul 18, 2011

### Drakkith

Staff Emeritus
Youngdreamer, the key with gravity is not its total strength, it is the density of its strength. I don't know if those words are the correct ones to use, but I don't know how to describe it any other way. (The gradient perhaps?)

For example, the galaxy has much much more mass than any black hole, yet the galaxy isn't a black hole itself, why is that? It is because the galaxy is not very dense as a whole. In a typical nebula of hydrogen gas has many times more mass than an average star, or even a black hole, yet it is so spread out that at any point there is only a small force of gravity. Only when all that mass is compressed into a small area does a black hole form.

Last edited: Jul 19, 2011
6. Jul 19, 2011

### qraal

Yes and maybe no. Yes, gravity does combine from the Universe as a whole in most theories, but usually the Universe is not assumed to exist in some larger void. Instead when it is closed, topologically, space-time folds back on itself. Thus it has no edge, yet it doesn't go on forever. But at present there hasn't been enough time for light to retrace a path all the way around, and if it keeps expanding there may never be enough time to do so.

The maybe no answer is from the fact that some cosmological assumptions mean space-time is infinite/endless in all directions except the past. Thus there can never be a closed path followed at any time in the Universe. We have insufficient evidence for or against the Universe being finite to know for sure.

7. Jul 19, 2011

### Nabeshin

Well, it's not really density though. Supermassive black holes have average densities less than water. The quantity which is constant when you transition into a blackhole regime is something like $M^2 \bar{\rho} \sim \textrm{constant}$ with $\bar{\rho}$ being the average density.

(Don't want to talk about singularities because it's irrelevant here)

8. Jul 19, 2011

### Cosmo Novice

While I agree the Universe has no tangible "edge", you cannot irrefutably state that it does not go on forever. In spatially flat and infinite cosmological models the Universe is temporally finite and spacially infinite.

9. Jul 19, 2011

### Drakkith

Staff Emeritus
Interesting. Nevertheless, I don't know what other term would apply more accurately. Got any in mind?

10. Jul 19, 2011

### YoungDreamer

I know the Universe has no edge and that light would not be pulled in, but in theory doesnt light get sent on a neverending trip through Universe because of expansion (and because of the properties of light itself)?

Anyways, my thought in relation to what Drakkith was saying about density, galaxies as you said are not dense relative to black holes, although I didnt know they had an average density of less than water, maybe because most of the mass is thought to exist in or closely around the singularity? Anayways, galaxies nonetheless exert enough gravity to form clusters and to completely pull galaxies into themselves, so that is why I wondered about the colllective gravity of the Universe.

Could the properties of the Universe that warp it back on itself include gravity, I guess would be my main question?

11. Jul 19, 2011

### YoungDreamer

I guess the low density is not because of mass in a sungularity.

12. Jul 19, 2011

### Nabeshin

I think gradient is probably the best. If you imagine an embedding diagram, basically the rubber-sheet analogy, the point of criticality in transition to a black hole is when this warped surface has a certain gradient, or steepness.

13. Jul 19, 2011

### Nabeshin

Right. Essentially we're just concerned with a mass M inside a volume V. Since we cannot ever obtain information about how the mass is actually distributed inside the volume V, it is irrelevant (could be in elephants, still doesn't matter). So although the mathematical solution for a black hole does extend into this regime, we don't care and are talking only about the solution exterior to the event horizon.