Universe not accidental: Is this Steinhardt statement "rather pathetic"? If so, why?

martinbn

But is not an answer to my question, where in the symmetry breaking is the need for many universes!

salvestrom

Is it perhaps derived from not treating any given probability as special, so it is considered that they all playout.

Chalnoth

Spontaneous symmetry breaking is an accidental event which occurs differently in different regions, and which leads to different low-energy laws of physics when it occurs differently.

Chalnoth

The point, however, is that despite the fact that a unique universe is strongly disfavored by the evidence, many people seem to expend all of their vitriol on the most likely models: multiverse models.

marcus

:

You amaze me Chally

First you say the multiverse approach to cosmology requires fewer assumptions than any of the alternatives, which it clearly doesn't, as I've shown by examples.

Now you say a unique universe strongly disfavored by evidence! On the contrary, a unique universe is the DEFAULT case that reflecting the traditional meaning of the word universe: everything that exists.

Obviously it is unique by definition. Other cases could be imagined but this is the one (uni) that exists. So you have to start changing the meaning of words if you want to claim what you say.

And there is no evidence for multiplicity whatsoever. All we have to look at is the universe we've got.
The rest is fantasy and speculation.

Haelfix

There are three different things that are logically distinct.

1) The case for a multiverse
2) The case for the strong anthropic principle (eg scanning the multiverse and selecting out the universe that has the parameters tuned for life) in the sense of an explanatory principle.
3) The multi worlds interpretation of quantum mechanics

They are all logically distinct, you can believe in any one of the three without having to agree with the other (with the exception that 2 requires 1 obviously but the converse is not true)

Ok!

The case for 1 is very strong theoretically and experimentally. Essentially WMAP disfavored a number of simple variants of inflation, and strongly supported eternal Inflation (which made a bunch of predictions that were subsequently verified).

Even if eternal inflation is incorrect, it now seems necessary for some type of inflation to have existed at some time. Thats great, except the problem is that it implies length and time scales that are astronomically larger than what we are used too in our visible universe... Indeed even basic assumptions about physics starts to have problems with numbers of that magnitude (that's why people worry about ridiculously improbable events, like vacuum decay). Think numbers like 10^120 (in whatever units you want).

The issue there is that if you believe in statistical mechanics, all sorts of crazy phenomenon start to be logically possible over timescales of that magnitude. In particular, it is ridiculously difficult to contrive a scenario where inflation happens once and only once.

You can spout all sorts of philosophy about falsifiability, but the mathematics doesn't care, and numbers like 10^120 don't care either. In that sense, models where inflation only happens once are incredibly fine tuned and implausible. At the end of the day, the issue is exponential expansion, and not details of the physics!

The case for 2 is also unfortunately relatively strong. And it pains me to say it, b/c it is an annoying argument! The exact details for why exactly anthropic selection seems necessary is non trivial, and come from a number of different lines of reasoning. For instance, it might not be obvious to nonphysicists but the fact that the Higgs particle seems to be centered at 125 MeV is a relative boost to the anthropic argument!

It wouldn't surprise me if there was a mechanism or some new physics that made the case for 2 go away, but at this time no such compelling model exists.

The case for 3 is a subject that I don't pay much attention too. All of the interpretations are presumably mathematically identical in their consequences!

marcus

That's pretty vague Haelfix and I don't think it holds water. Ashtekar has written a couple of papers and given talks mentioning how well the 7 year WMAP data compats with Loop cosmology and its associated inflation. I'll get a link to one or more of his papers. I doubt the inflation he has in mind was disfavored by WMAP---he says the opposite. And it certainly involves no multiplicity of universes!

So I doubt WMAP disfavored everything besides some multifarious type of inflation (like eternal). You aren't very specific about that claim--maybe better give us a link. If it is just departmental scuttlebut it could well be overlooking something.

Here's a link I said I'd get: http://arxiv.org/abs/1103.2475 This refers directly to WMAP data and inflation. Also there is an even more recent invited review artice by Ashtekar http://arxiv.org/abs/1108.0893 See
pages 98-101 and references therein.

Chalnoth

No, you haven't. It's very, very simple. Uniqueness is always an additional assumption. There is no way to get uniqueness "automatically".

Yes, it is. Very strongly. Because it is looking more and more like many aspects of physical law are due to accidents in our past.

That's just silly word play. What I mean by uniqueness is that our big bang event was the only big bang event, and the low-energy physical laws that we observe are the same everywhere. Both positions are highly unlikely.

marcus

Well it's nice that you say what you mean. About the first. Cosmology is a mathematical science, not philosophy. A mathematical model of the universe does not need to include a statement THIS IS THE ONLY UNIVERSE! It is simply a math model that one fits to the data. A simple model with superior fit wins and is used to make predictions.

So if I have a model which accounts for the big bang, inflation and the present structure and that model does not happen to predict other big bangs, then FINE. That is neither an additional assumption nor an additional conclusion. The issue doesn't even come up among reasonable people.

Now when you talk about regions with different low-energy physics, you may be WEASELING by calling any such instance a "different universe". I've heard that we could have regions with a different QCD θ angle and perhaps different dark energy density within the same universe, all stemming from the same big bang. I think a bounce cosmology should be able to accommodate some regional variation of low-energy physics depending on when and how the spontaneous symmetry cookie crumbles. You don't need a whole other big bounce just to get some measly variation in some measley parameter. You can have different regions in the same universe which we know is the same universe because it stems from the same bounce, or if you prefer, "bang".

Haelfix

Marcus, the reason WMAP was viewed as a victory for EI and more particularly certain breeds of chaotic inflation, was b/c they made very specific and technical predictions twenty years a priori (regarding such things as the tensor to scalar ratios and spectral index etc). When WMAP came out, huge swaths of inflationary models were ruled out, and yet the ones that fit the details to a T happened to be certain types of chaotic inflation models (yet others in the subclass were ruled out).

Anyway, there are currently hundreds of models that fit WMAP, and i'm sure the LGC groups are well aware of the constraints. Of course, many of these people are postdicting results, which is of course much easier!

As far as the papers you linked. They say more or less exactly the opposite of what you think they do.

They argue for a specific measure regarding the probability for inflation to occur. Ashtekar thinks that this probability is nearly unity! Well, that's great, except that it makes the case for a multiverse much more likely! If the conditions necessary for inflation is generic, then a Hamiltonian system will graciously proceed to recur those identical conditions many times over. Once you have that, the power of exponential expansion takes over and the plurality of the volume of spacetime will be dominated by inflating regions.

So to reiterate to argue against a Multiverse, you have to contrive it so that inflation is astronomically unlikely! But then that defeats the purpose of inflation in the first place (which was to save the big bang model from the mother of all finetuning problems).

marcus

I don't think you get what Ashtekar is saying. The loop bounce has something unusual a brief period of superinflation--faster than exponential expansion. (Inflation is just ordinary exponential or nearly but not quite exponential). Ashtekar says the probability of adequate inflation ensuing is ~ 1 at the loop bounce.

Not at other times. In a typical model there is one bounce which the entire universe universe undergoes at the same moment. (Other versions are cyclic but for simplicity just think of the one-time bounce.)

Its a model that fits the data including WMAP and has only one inflation opportunity, one which the whole universe participates in at once, and which makes an adequate inflation episode highly probable without fine tuning.

I'm not saying it's right. but I note that it has an increasing number of people interested.

Here is a current invited review article (string cosmology, loop, Horava, Einstein-aether...) by a guy at Belgian institution that discusses the trends. In case you are interested and don't have other sources this may give you an idea what other people outside the department coffee-room are thinking.

http://arxiv.org/abs/1201.4543
Aspects of Quantum Gravity in Cosmology

the author is not a Loop theorist, indeed before 2007 most of his papers were string/M or stringy cosmology. From 2007 he seems to be doing straight cosmology without the branes. In any case I wouldn't expect him to have any pro-Loop bias. So you probably get a clear balanced view covering a halfdozen approaches to resolving the singularity and giving historical perspective on the field.

skydivephil

I have a question for you Marcus of how one gets a singualr universe in LQC inflation , I hope you can help me clear up some issues.
If Ive understood Guth correctly inflation is eterrnal becuase of the way that it ends, not becuase of the way that it starts.
http://arxiv.org/pdf/hep-th/0702178v1.pdf
"In fact, in any successful
inflationary model the rate of exponential expansion is always much faster than the
rate of exponential decay. Therefore, even though the false vacuum is decaying, it
never disappears, and in fact the total volume of the false vacuum, once inflation starts,
continues to grow exponentially with time, ad infinitum"

So why is this process different in LQC? is it different?
Also my reading of the field is the main promoters of inflation: Guth, Linde, Vilenkin, Aguirre etc and the main detractors Steinhardt, Turok etc all agree inflation is eternal. Ive not heard any comments from the Loop guys, Ashketar, Bojowald, Singh etc argue that inflation is not eternal. Given the attention eternal inflation recieves and that they all agree inflation occurs I would expect them to say something. Hence it seems to me that there is somehting of a consensus (maybe thats too strong a word but somehting aproaching that) that inflaiton is eternal.
Have I read this wrong? If so on what basis would you say that?

Chalnoth

Weaseling? This is the primary point of interest where "other universes" are concerned. How the hell is cutting straight to the heart of the matter weaseling?

martinbn

You more or less repeat what you've already said, and I think I understand you say, but I don't understand why you say it. Can you give me a reference for the spontaneous symmetry breaking mechanism where many universes are needed for it?

bapowell

Yeah, I get that, but my intent is not to debate most likely models. I'm aware that there's plenty of cross fire going on in this thread so it's hard to keep things straight. I am referring back to our discussion regarding the multiverse as being part of objective reality because the theory from which it emerges is well established and accepted -- this is your stance. My point is while many-worlds is perhaps the most likely theory from a Bayesian perspective, it is not definitively so because competing theories are not ruled out using these kinds of simplicity arguments. So, I reject your claim that the multiverse is necessarily part of objective reality because many-worlds has not been singularly identified as the correct theory of quantum mechanics.

bapowell

You're missing his point. He's not saying that multiple universes are required for SSB. What he's saying is that given a large enough universe in which SSB occurs, it is inevitable that different causal regions will be characterized by different order parameters (the SSB field will evolve to different vacua.) This conclusion requires only a universe larger than the one we observe and a continuously distributed field facilitating the SSB.

Alternatively, according to many-worlds QM, the SSB that occurs right in our Hubble patch resulted in only one possible low energy vacuum among many. Other vacua in our Hubble patch are selected and evolve in other branches of the wavefunction.

martinbn

Hm, that is very different than what I think he said. He said that SSB leads to that, and I don't understand why.

This also seems different to me.


Anyway, I am probably lost and need to reread the whole thread.