Why the Multiverse? Evidence of Many Universes

[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 can't 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?

 

[mfb]

To be able to say that one value is more remarkable than another we must know the probability distribution for whatever caused the value.

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).

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.

Sure. It is a probabilistic argument only.

 

[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.

 

[VantagePoint72]

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.

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.

 

[mfb]

@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.

 

[VantagePoint72]

@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.

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.

 

[mfb]

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.

 

[VantagePoint72]

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.

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.

 

[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.

 

[MathematicalPhysicist]

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.

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...

 

[MathematicalPhysicist]

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.

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.

 

[DimReg]

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

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.

 

[George Jones]

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.

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

The most serious of the above three problems is the horizon problem. As we have seen, there are possible solutions of the flatness and monopole problems that do not rely on inflation.

The most exciting aspect of the inflationary cosmological theories described in chapter 4 is that they provide a natural quantum mechanical mechanism for the origin of the cosmological fluctuations observed in the cosmic microwave background and in the large scale structure of matter, and that may in the future be observed in gravitational waves.

In the modern view, by far the most important function of inflation is to generate the primordial curvature perturbation ... It may generate other primordial perturbations too, including the isocurvature and tensor perturbations ... However, the historical motivation for inflation was rather different, and arose largely on more philosophical grounds concerning the question of whether the initial conditions required for the unperturbed Big Bang seem likely or not.

Originally inflationary scenarios were suggested as a pseudo-solution to certain pseudo-problems; these are only of historical interest today and the only reason to take the possibility of an inflationary phase in the early universe seriously is because it provides a mechanism for generation the initial perturbations.

 

[VantagePoint72]

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

Interesting, thanks for the citations.

 

[kurros]

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.

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.

 

[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.

 

[MathematicalPhysicist]

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.

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

 

[kurros]

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

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.

 

[mfb]

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

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

 

[MathematicalPhysicist]

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.

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

 

[kurros]

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

Maybe this is true of models that try to solve all the problems of physics at once, but there are plenty of more targeted and testable models that are still motivated by tuning arguments. Besides, your argument has nothing to do with the question of whether such models describe the way the world really is or not. Not to mention that mathematics is, at it's heart, systematic logic, so if it is telling us something then probably we should listen.

 

[skydivephil]

As a historical note, inflation was not designed to solve the horizon problem at all.. You can read Alan Guths account of how inflation was developed in his book
“The Inflationary Universe”. He is very clear inflation was developed for one reason alone, to solve the monopole problem.
It may well be that because it’s believed to solve the horizon problem too that was what sold it to the community, but that’s a separate question. The origin of cosmic structure via inflation may be the best motivated reason for accepting inflation now and of course the evidence in favour of it in the CMB.

I don’t see why we have to invoke a multiverse to give us the cosmological constant (CC), if QFt gives the wrong value for the CC, then there’s a problem with QFT or our interpretation of vacuum energy. I don’t see that as a reason to invoke a multiverse.
However inflationary cosmology has been invoked as a reason to invoke a multiverse. I find this slightly more compelling. If we want to know if there really is a multiverse w need to address 2 questions.
1 did inflation really happen? The evidence looks good so far, but I don’t think it’s a done deal yet with the lack of a gravity wave spectrum.
2 Is inflation really eternal or not?, this can only be answered by theorists unless we get lucky and see some sign in bubble collision in the CMB. If theorists agree inflation is eternal and experimenters agree inflation happened that makes the idea of a multiverse plausible to me.

Without a direct measurement of course we can’t be so sure the theorists are correct. They can bungle things too. But that’s life. Science doesn’t have to give us yes or no answers, some answers from science are compelling and others are just suggestive. The multiverse arguments may never be compelling but they may be strongly suggestive especially if the two above criteria can be established.

 

[pforeman]

I'll "believe" you if you say that we live in a multiverse.
What is science fiction for me is when people talk about parallel universes that are just like ours, only in that one I'm left handed, or Hitler won...
Isn't this just one more huge leap into sci fy?
Paul

 

[kurros]

I'll "believe" you if you say that we live in a multiverse.
What is science fiction for me is when people talk about parallel universes that are just like ours, only in that one I'm left handed, or Hitler won...
Isn't this just one more huge leap into sci fy?
Paul

Lol well I would only say that it is quite possible that we live in a multiverse, not that I have confidence that we do. As for universe like ours but only slightly different, well, I agree it is farfetched, but there are several ways it is plausible. 1- if our current universe is truly infinite in extent, you would expect all possible disconnected causal pieces of it to occur infinitely many times. So somewhere there are zillions of Hubble volumes that look almost exactly like ours. 2- if some eternal inflation type scenario causes infinitely many universe to be born, the scenario of 1 is realized in a slighly different way. 3- if some many-worlds quantum mechanics style multiverse exists, then the slight variations on our universe are just relatively "nearby" pieces of the universal wavefunction.

You have to take infinities extremely seriously in 1 and 2 though, since it would take truly gargantuan numbers of realisations before you get something like our universe again (presumably). If many-worlds is real, though, then all those slight variation universes cannot help but be there.

 

[pforeman]

I understand the math says at some point it's going to possibly happen, but do you really think with enough monkeys tapping on typewriters, at some point you'll get shakespears complete sonnets (in order)? The math says it'll happen, but it won't. Isn't this when you get rid of the mathematician in you and use common sense.
Besides, there are 350 billion (?) galaxies in the observable universe. Even if this is a fraction of the real number, the number is real, not infinity. Yes, perhaps the space of the universe may be infinite,(not likely ((?)) ) The number of universes must be finite. There must be a correlation between the (finite) size of the universe and the number of possible multiverses.
Right(?)
paul

 

[kurros]

I understand the math says at some point it's going to possibly happen, but do you really think with enough monkeys tapping on typewriters, at some point you'll get shakespears complete sonnets (in order)? The math says it'll happen, but it won't. Isn't this when you get rid of the mathematician in you and use common sense.

Well, monkeys aren't true random typewriter-letter-pressers so even with infinite time they probably won't write Shakespeare. A quantum random number generator typing letters would, though, eventually.

Besides, there are 350 billion (?) galaxies in the observable universe. Even if this is a fraction of the real number, the number is real, not infinity. Yes, perhaps the space of the universe may be infinite,(not likely ((?)) ) The number of universes must be finite. There must be a correlation between the (finite) size of the universe and the number of possible multiverses.
Right(?)

Nope. The finiteness of the observable universe need have nothing to do with the infiniteness of the extended universe, or of the multiverse. As for bowing to intuition rather than mathematics, well, we are talking about the grandest scales we know of in this reality; there is no reason to expect that your intuition is of any help here, unless it has been well trained in the relevant mathematics. This scale is far beyond the scope our primitive ape-minds are equipped to handle without the tools of science, of which mathematics is the most powerful.

 

[audioloop]

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.

no totally disconnected.

Alexander Vilenkin.
http://www.2physics.com/search/label/5-Breakthroughs

3. Evidence for the multiverse. Inflationary cosmology leads to the multiverse picture, with multiple "bubble universes" expanding and occasionally colliding with one another. Collisions of our bubble with others may have observational signatures in cosmic microwave background and in gravitational waves. A discovery of such a collision would provide a direct evidence for the existence of the multiverse..

 

[phinds]

... Not to mention that mathematics is, at it's heart, systematic logic, so if it is telling us something then probably we should listen.

Well, FIRST we should listen to nature, then if some system of math happens to described it well, and gives us a good tool to work with our understanding of nature, that's great but never forget that the map is not the territory.

 

[audioloop]

However inflationary cosmology has been invoked as a reason to invoke a multiverse. I find this slightly more compelling. If we want to know if there really is a multiverse w need to address 2 questions.
1 did inflation really happen? The evidence looks good so far, but I don’t think it’s a done deal yet with the lack of a gravity wave spectrum.
Without a direct measurement of course we can’t be so sure the theorists are correct. They can bungle things too. But that’s life. Science doesn’t have to give us yes or no answers, some answers from science are compelling and others are just suggestive. The multiverse arguments may never be compelling but they may be strongly suggestive especially if the two above criteria can be established.

right.
But slowly, step by step it will be done

Detection of B-mode Polarization in the Cosmic Microwave Background with Data from the South Pole Telescope
http://arxiv.org/abs/1307.5830v1-------
http://news.uchicago.edu/article/20...ts-big-bang-may-hold-clues-universe-s-infancy

"B modes from inflation are caused by gravitational waves. These ripples in space-time are generated by intense gravitational turmoil, conditions that would have existed during inflation. These waves, stretching and squeezing the fabric of the universe, would give rise to the telltale twisted polarization patterns of B modes. Measuring the resulting polarization would not only confirm the theory of inflation—a huge scientific achievement in itself—but would also give scientists information about physics at very high energies—much higher than can be achieved with particle accelerators".
.