# Why the multiverse?

1. Jun 5, 2013

### JohnLuck

According to Leonard Susskind the "finetuning" of the constants of this universe is evidence of many universes with different random constants. The most remarkable of the constants he says is the energy of empty space because of its tiny size (10^113 J/m) and the fact that we couldn't live without it being this very close to this tiny size.
First of all I don't see that the size or any size is special for what we at the moment must assume is a fundamental constant of nature. To be able to say that one value is more remarkable than another we must know the probability distribution for whatever caused the value. Not only do we not know this distribution, we do not even know if one exists.

The second part of the argument for a multiverse is that if the fundamental constants changed much we wouldn't exists. A possible explanation to why we do in fact exist is then seen as the possibility that many universes exists, but people are only here to observe it in the ones with the right conditions for them to live (Anthropic principle). This principle in my opinion can only be used to explain why the sample (this universe) of a population (all existing universes) could be non representative of the population. It cannot be used to determine the size or the nature of the population. The population size could still be 1. If there was only one universe and we didn't exist in it we wouldn't know about it. Now if only one universe exist and we live in it, we cant just assume that it is special. And if the constants of a universe are only special if there is someone to inquire about them, would this universe still be special if humans hadn't evolved?

2. Jun 5, 2013

### Staff: Mentor

Technically, yes.
On the other hand: if a parameter has a range from 0 to 1, a value between 0.2 and 0.4 is not so surprising as a value of 0.00.... (120 zeros)...00335 with an allowed deviation of less than 1% (arbitrary numbers).
Sure. It is a probabilistic argument only.

3. Jun 5, 2013

### phinds

I would add to what mfb said, the following:

There are, as I recall, something over 20 different physical constants each of which COULD have wide ranges but ALL of which have a value such that we can exist.

This does not in any way invalidate your argument (which by the way I agree with) but it sure bothers the hell out of me that there are so MANY of these things that are all fine-tuned.

4. Jun 5, 2013

### VantagePoint72

Personally, I agree with you on this. I don't think it makes any sense to speculate about why particular parameters in physics take on certain values when we don't yet know if those values are contingent or necessary facts. That is, the question, "How do we explain how it just happens that all these parameters have the just the right value for (whatever)?" doesn't make much sense when we don't even know if they're free parameters. They may well be fixed by whatever deeper theory comes next.

I've never really bought the "horizon problem" argument for inflation for the same reason. Don't get me wrong, I understand there are other reasons for using the inflationary model—and ultimately it's an empirical claim that will stand or fall on the basis of observation—but the horizon problem was the original reason for introducing the model. I don't see why there is something surprising in need of explanation about the fact that causally disconnected regions of space had/have the same temperature. Given we don't have a theory of quantum gravity to tell us what the conditions of the Big Bang at time zero were, it doesn't make much sense to me that we should assume the temperatures of these causally disconnected regions could have been anything other than what they were. It seems to me a bit like observing that objects fall with (nearly) the same acceleration on opposite sides of the earth and saying, "What an extraordinary coincidence!" It happens, of course, because the acceleration is fixed by the underlying physical laws, which are obeyed on both sides of the earth. I don't see why whatever physical law gave the CMB its initial temperature couldn't have fixed that temperature to be homogeneous. In fact, not only do I not see why that couldn't be the case, I can't see how it's not the most obvious conclusion from homogeneity.

5. Jun 5, 2013

### Staff: Mentor

@LastOneStanding: As far as I know, the horizon problem is not related to the overall homogeneity of the CMB. It is related to temperature fluctuations larger than the horizon(s) of a non-inflationary universe. Without a causal connection, the fluctuations should be independent of each other.
I guess theories with "something" "before" the big bang can give a solution, too, but inflation fits really well to observations.

6. Jun 5, 2013

### VantagePoint72

I have never heard the horizon problem defined other than in terms of the homogeneity of CMB temperature, including across causally disconnected regions. The typical formulation I've always seen is like the one here. Temperature fluctuations don't enter into it.

7. Jun 5, 2013

### Staff: Mentor

I heard some interesting CMB talks recently, I would have to check if I can get the slides somewhere.
Anyway, correlated temperature fluctuations looks like a stronger argument - if you can get those, getting a correlated average temperature is easy (as there has to be some causal connection). On the other hand, a uniform average temperature might have the same common cause somewhere else.

8. Jun 5, 2013

### VantagePoint72

Interesting, I'd like to see them if you can find those slides. As I said, I'm aware there are other reasons for introducing inflation; however, temperature homogeneity was the original justification and still seems to get thrown around casually as the reason for it.

9. Jun 6, 2013

### Chronos

The multiverse argument has a convenient escape clause - other universes are causally disconnected from our observable universe. I like the concept, but, this apparent unfalsifiability clause really turns me off.

10. Jun 6, 2013

### MathematicalPhysicist

Why does it bother you?

Obviously there should be such a fine-tuning otherwise we and anything else that we observe wouldn't exist, perhaps something else would have existed, and in that case you could also ask why is it tuned that way.

Philosophical questions rarely get answered, and this is a philosophical quandary.

Turtles all the way down...

11. Jun 6, 2013

### MathematicalPhysicist

I don't understand this clause, it's not verifiable idea in that case. I might as well believe in fairies...

If on the other hand you'd argue that there's someway (obviously nowadays, theoretically speaking) that you can visit another universe then it's simple matter of experiment.

Sci-Fi discuss this issue, and as we all know, today's Sci-Fi tomorrow may be a reality.

12. Jun 6, 2013

### DimReg

That's basically the main reason to dislike it. If we can somehow convince ourselves that we are for sure in a multiverse, some previously unexplained values will be explained by our "location". For many physicists our difficulties explaining these constants is evidence enough to say that we must be in a multiverse. But many others aren't convinced.

Also, it should be noted that string theory, at least in our current understanding, lends itself well to a multiverse picture. A lot of what we see as parameters are actually vacuum expectation values in string theory, and there's a huge landscape of possible vacuum states. Each "universe" could be in a different vacuum state, so would have different parameters. So it's not too unreasonable an idea, it's just like how there are lots of planets, but only some allow life (and thus we find ourselves on one of those planets).

Personally, I don't like the multiverse picture, but there are good reasons to at least think about it. I also never understood how a value of a constant could be 'surprising' or 'unexpected', since our best picture of what nature is like has always been, and always will be, what we observe it to be. I suppose that's an unfashionable belief these days.

13. Jun 7, 2013

### George Jones

Staff Emeritus
I think "the reason" now is often taken to be generation of perturbations via quantum fluctuations.

Last edited: Feb 8, 2014
14. Jun 7, 2013

### VantagePoint72

Interesting, thanks for the citations.

15. Jun 8, 2013

### kurros

The question makes perfect sense. Presumably such parameters ARE fixed by a deeper theory; the questions are about the nature of this deeper theory. "Weird" values of parameters may be clues as to what sort of deeper theory is necessary. The parameters don't have to be random variables for us to be legitimately surprised about the values they have. One can think about the typical sorts of deeper theory one might expect, and if these typically do not produce parameter values in the effective theory like the ones you see then it is a good reason to be surprised and look for some other classes of theories more likely to produce the values you see.

16. Jun 8, 2013

### Chronos

This is pretty much the point of finding a GUT that works. In principle, it should largely explain why the fundamental constants have the values they do. They then may not seem so finely tuned.

17. Jun 8, 2013

### MathematicalPhysicist

But it will just give us new parameters, which are more basic than our current fundamental constants.

18. Jun 8, 2013

### kurros

The exercise will not JUST give us new parameters, it will give us a full physical model that tells us new and interesting things about the way Nature is. Of course we still have to do experiments to really find out if this new model is correct, but it helps a lot to know what to look for.

19. Jun 9, 2013

### Staff: Mentor

The new theory could have less free parameters - or it could predict many different regions ("worlds" or whatever) with different parameters.

20. Jun 10, 2013

### MathematicalPhysicist

The problem with such models that as of yet they aren't testable, it's not physics, it's mathematics.

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