# B Expansion of the universe

1. Jun 19, 2017

### mohau tshekoeng

It is been said that the universe is expanding but the question is,where will the universe end?
Are we the only universe?

2. Jun 19, 2017

### Staff: Mentor

First, a moderator's note: I have relabeled this thread as "B".

The universe is spatially infinite, so it doesn't "end" anywhere. And our current best model says it will continue expanding forever, so it will have no end in time either.

We have no way of knowing by observation, and we don't have a good model for how our universe originally began; our best current model is inflation, but that model still doesn't tell us how inflation started, and there are a number of speculative hypotheses that say different things. This is an open area of research.

3. Jun 19, 2017

### mohau tshekoeng

Well OK,...it is expanding and that means that galaxies are moving away from each other...what are the chances of them colliding?

4. Jun 19, 2017

### Staff: Mentor

The expansion refers to the average motion of all the matter in the universe, and "all the matter" is better thought of as galaxy clusters on this scale. Individual galaxies within clusters can still be moving towards each other and collide, since the clusters are gravitationally bound systems.

5. Jun 19, 2017

### kimbyd

Nearby galaxies collide pretty frequently. I can't give you probabilities, but, for example, our own Milky Way will collide with the Andromeda galaxy in about a billion years.

This video gives a good, beginner level description of galaxy mergers, with a bunch of images of different mergers:

6. Jun 19, 2017

### bahamagreen

Wiki says collision in 4.5 billion years... galaxies can "collide" without bumping a single pair of stars together.

7. Jun 20, 2017

### rootone

Yes the stars in galaxies are very far apart except in the core region, and the core region is a fairly small part of the whole galaxy.
This makes head on collisions of stars unlikely, though there will be a lot of gravitational disturbance making their trajectories chaoctic.

8. Jun 20, 2017

### Trollfaz

There has been many different theories of multiple universes. The problem is, it is impossible to test them since we can never directly witness a parallel universe

9. Jun 21, 2017

### kimbyd

That's an overly-simplistic and largely false statement. It is not at all difficult to imagine a theory that unambiguously predicts a multiverse and is experimentally testable. Verification of spontaneous symmetry breaking would do this, for instance.

10. Jun 21, 2017

### Staff: Mentor

How so?

11. Jun 21, 2017

### Staff: Mentor

If the "predicts a multiverse" part is not experimentally testable, then testing the parts that are does not establish the multiverse part. If you have a theory that makes two predictions, A and B, and only A is testable, then confirming A does not confirm B, unless you can prove that it is impossible to have a theory that predicts A but does not predict B.

12. Jun 21, 2017

### kimbyd

Because spontaneous symmetry breaking means that the results of the symmetry breaking event depend upon random chance. You can't have a universe whose low-energy physical laws depend, in part, upon random chance without a multiverse.

13. Jun 21, 2017

### Staff: Mentor

Why not?

14. Jun 22, 2017

### kimbyd

While my statement was a little strong, it's still a near impossibility, because as long as you have spontaneous symmetry breaking, there are a multitude of ways that different regions can wind up with different low-energy physical laws.

The first point is that if it's truly spontaneous symmetry breaking, then it's virtually impossible for the symmetry breaking event to be universal, because the domain walls will only be able to spread so far before horizons prevent them from spreading further. You could contrive a scenario where this could happen, but it would be very contrived indeed, especially in the context of inflation.

The second point is that there's no reason to believe that our own "big bang" event was a one-off event. Without knowing the full details of exactly what precipitated this event, the default assumption should be that it was not a singular event. As long as this kind of thing can happen multiple times, then multiple regions with different low-energy physics are guaranteed.

The scenarios where the entire universe has the same low-energy laws of physics depend instead upon explicit symmetry breaking, where there is a single ground state towards which the universe inevitably evolves regardless of where you start from.

15. Jun 23, 2017

### Staff: Mentor

In other words, spontaneous symmetry breaking is compatible with a multiverse (which, btw, is already a vague term that can have more meanings than the one you are implicitly using--see below), or suggests that a multiverse is very likely. That's not quite the same as spontaneous symmetry breaking predicting or requiring a multiverse.

Notice here that you have subtly shifted meanings for the terms "universe" and "multiverse", since "the entire universe has the same low-energy laws of physics" implies that an alternate possibility is "different regions of the universe have different low-energy laws of physics", which now uses the term "universe" in a way that makes it perfectly compatible with having multiple regions with different results of spontaneous symmetry breaking--i.e., that no "multiverse" is required by spontaneous symmetry breaking. On this interpretation, "multiverse" would refer to something like the $10^{500}$ possible vacua of string theory; each of those would be a "universe", but in each "universe" you could still have regions with different low energy physics due to spontaneous symmetry breaking.

16. Jun 23, 2017

### kimbyd

Many predictions are probabilistic in nature. Statistical mechanics predicts that the air in a room will not ever be observed to collect itself into a single cubic centimeter in the corner of said room. You can use statistical mechanics to predict precisely how often that will occur, and it is so rare as to be disregarded as a possibility.

The same is true here. Having a single domain in a universe with spontaneous symmetry breaking is just as absurd. And if you think the universe is infinite, it's flat-out impossible.

The only interesting multiverse is one where low-energy physical laws differ. Merely having more Hubble volumes where the overall statistics and behavior are identical doesn't really fit what most people think of when they hear the term "multiverse". Perhaps more importantly, without the differing physical laws what you would be describing would be materially indistinct from a universe that is merely larger than the observable universe (which is certainly true).

17. Jun 23, 2017

### Staff: Mentor

But you said that we could have a "universe" (not a "multiverse") in which the low energy physical laws were different in different regions. So you appear to me to be shifting the meanings of your terms, which is not going to help the discussion. Can you give a precise definition of the term "multiverse"? Or, even better, point us to a standard reference in cosmology that gives a standard definition of that term as it is used in the literature?

18. Jun 25, 2017

### Drakkith

Staff Emeritus
Indeed. I've always disliked the use of the word "multiverse" to describe different regions of the "universe", as it usually muddies up the definition of the two words in my opinion.

19. Jun 25, 2017

### rootone

I guess that about just over a century ago, the term 'multiverse' could have been applied as description of a theory proposing that more than one galaxy exists.

20. Jun 26, 2017

### timmdeeg

https://en.wikipedia.org/wiki/Multiverse

According to Wikipedia there is no clear definition, rather the term "multiverse" means a smorgasbord of thinkable universes including Everett' s many worlds.