Is Paul Steinhardt's Statement "Rather Pathetic"? Why?

[Chalnoth]

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

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]

But theories are never ruled out on account of their complexity. This is where I disagree with your reasoning. You are essentially performing a Bayesian model selection on your space of competing theories. They all satisfy the data equally well, however, some have additional structure than others that make them either less predictive, more complex, or both. The Bayesian evidence disfavors these models, but it does not exclude them! This is an incorrect interpretation of the statistical method.

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]

The point, however, is that despite the fact that a unique universe is strongly disfavored by the evidence,..

:

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 anyone 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]

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).
...

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]

:

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.

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

Now you say a unique universe strongly disfavored by evidence!

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.

On the contrary, a unique universe is the DEFAULT case that reflecting the traditional meaning of the word universe: everything that exists.

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]

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...

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 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.

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 I've 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. I've not heard any comments from the Loop guys, Ashketar, Bojowald, Singh etc argue that inflation is not eternal. Given the attention eternal inflation receives 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 that's 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]

Now when you talk about regions with different low-energy physics, you may be WEASELING by calling any such instance a "different universe".

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]

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.

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]

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.

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 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?

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]

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.

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

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.

This also seems different to me.


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

 

[alt]

Interesting thread. Aside from the multiverse (or not) issue, I'm trying to get my mind around the OP - universe accidental or not.

What do we really mean by use of the word 'accidental' ?

http://dictionary.reference.com/

ac·ci·dent
noun
1. an undesirable or unfortunate happening that occurs unintentionally and usually results in harm, injury, damage, or loss; casualty; mishap: automobile accidents.
2. Law. such a happening resulting in injury that is in no way the fault of the injured person for which compensation or indemnity is legally sought.
3. any event that happens unexpectedly, without a deliberate plan or cause.
4. chance; fortune; luck: I was there by accident.
5. a fortuitous circumstance, quality, or characteristic: an accident of birth.

Also, the etymology of the word ..

http://www.etymonline.com/
accident late 14c., "an occurrence, incident, event," from O.Fr. accident (12c.), from L. accidentem (nom. accidens), prp. of accidere "happen, fall out, fall upon," from ad- "to" (see ad-) + cadere "fall" (see case (1)). Meaning grew from "something that happens, an event," to "something that happens by chance," then "mishap." Meaning "unplanned child" is attested by 1932.

A more consise definition from the dictionary on my computer (Wordweb, downloadable) is ..

1)Anything that happens suddenly or by chance without an apparent cause
2) An unfortunate mishap; especially one causing damage or injury

So I don't feel that any of the definitions above, or even the etymology of the word 'accident', reflects the context of what is meant here when we discuss whether the universe is accidental or not.

Out of curiosity, I also translated the word to old Greek and Latin, then sought the meaning thereof in each case. I mostly came up with 'event, incident, occurrence', etc, be it good, bad or indifferent

All roads seem to basically end to 'good or bad event'. Surely that’s not what is been considered here.

So the question might be whether the universe is with cause, or without cause ? If so, how can anything be without cause ?

Or is the question merely restating the fact that science stops at the first cause (whatever that might be) ?

 

[Chalnoth]

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?

I don't get what your hangup is here. This is just the way spontaneous symmetry breaking works. There tends to be some finite distance across which the spontaneous symmetry breaking takes on the same value, beyond which there are different values. Since these different spontaneous symmetry breaking results lead to different low-energy laws of physics, these other regions can be thought of as separate universes (and are one of the main multiverse ideas).

 

[Chalnoth]

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.

And at some point more physicists will realize that making up additional dynamics to explain something that is already explained by the known and tested wavefunction dynamics is just plain engaging in a worthless endeavor.

 

[bapowell]

And at some point more physicists will realize that making up additional dynamics to explain something that is already explained by the known and tested wavefunction dynamics is just plain engaging in a worthless endeavor.

Totally agree. But simplicity is a statistical nicety; not necessarily a physical one. The universe simply might not work that way. Multiverses simply might not exist.

 

[marcus]

Totally agree. But simplicity is a statistical nicety; not necessarily a physical one. The universe simply might not work that way. Multiverses simply might not exist.

Exactly. The primary issue in the thread is whether it's appropriate to call Steinhardt's position (stated in post #1) is "rather pathetic".

I think it has been amply shown that it is not.

As you say "multiverses simply might not exist". Here's another way of saying it (let me know if it is not close enough to suit you.) I want to reflect the fact that cosmology is based on the construction of mathematical models rather than verbal/philosophical description:

"To get a good model of our big bang we don't need to include other big bangs having occurred."

 

[marcus]

It might be worthwhile for anyone who can't remember how the discussion started to review post #1 to see what we are talking about. Do we have to give up on the traditional reductionist program of explanation and resort to anthropics, or not?

The issue arose concisely in this post:
https://www.physicsforums.com/showthread.php?p=3725098#post3725098

I linked to Steinhardt's statement and Chalnoth replied that it was pathetic.

Here's the post we began the thread with:
===Originally Posted by Chalnoth===

==Originally Posted by marcus==
http://edge.org/response-detail/805/what-do-you-believe-is-true-even-though-you-cannot-prove-it

or google "Steinhardt annual question 2005"
==endquote==​

I've always found that response to anthropic arguments to be rather pathetic.
===endquote===

In fact Steinhardt's position has largely prevailed, physics has moved on and Chalnoth's complaint is out of date.
In scientific discussion (i.e. outside popular media) one does not hear much anthropics talk these days.
I think that's great and I'm grateful to Steinhardt for leading the attack on it.

There is a lot of obfuscation about this issue and natural confusion as well so I urge anybody who is not familiar with it to take a close look at post #1.

 

[Chalnoth]

Totally agree. But simplicity is a statistical nicety; not necessarily a physical one. The universe simply might not work that way. Multiverses simply might not exist.

That's extremely unlikely.

With a bit of hindsight, now that we know that the appearance of collapse was built into the wavefunction dynamics from the start, it is patently obvious that the entire process of attempting to explain wavefunction collapse was always an unnecessary enterprise. We were trying to explain something that was already explained by the wavefunction dynamics.

The whole enterprise of searching for another interpretation of quantum mechanics is akin to having Newtonian gravity before you and attempting to come up with some rule by which planets can have elliptical orbits. The very idea is nonsensical: Newtonian gravity explains it just fine. Why look for anything extra?

 

[Fuzzy Logic]

As I said in an earlier post, the only problem I have with Steinhardt's statement is his reason in that the universe was not accidental. Of course it was. Saying otherwise infers design. That answers nothing and complicates things immensely.

 

[Chalnoth]

In fact Steinhardt's position has largely prevailed, physics has moved on and Chalnoth's complaint is out of date.

Um, I guarantee you that nearly all theoretical physicists would find that a complete and utter surprise.

In scientific discussion (i.e. outside popular media) one does not hear much anthropics talk these days.

Only because these ideas are very difficult to nail down, and there are other areas where we have lots of good new experimental data to examine. As I said before, we can expect them to resurface once the LHC becomes mature and precision cosmology enters a lull in new data.

I think that's great and I'm grateful to Steinhardt for leading the attack on it.

I guarantee you he had nothing to do with it. Bad, irrational arguments like the one you quoted aren't likely to have had any impact on the scientific community.

 

[marcus]

As I said in an earlier post, the only problem I have with Steinhardt's statement is his reason in that the universe was not accidental. Of course it was. Saying otherwise infers design. That answers nothing and complicates things immensely.

That's great. Then you agree with the main body of the statement (we don't have to give up on the effort to find natural explanations) and just disagree verbally with how he chose to phrase the headline.

I'm sure you agree that Steinhardt was not arguing for "design"!
His basic message is there is no indication that we are through explaining yet. There are further layers of the onion.
We have not yet reached the end of physics where we have found the deepest explanation and where beyond that it "just is the way it is."

Notice how Steinhardt uses the word "desperation" in analyzing the motives of those who resort to anthropics. The pernicious thing is the cop-out: there are lots of different big bangs and this one just is the way it is, so don't ask why. He attributes it to an desperate attempt to excuse some theorist's failure to come up with a unique fundamental theory (by fundamental I mean good up to Planck scale.) A multiplicity of big bangs let's them off the hook.

Since 2006 it seems to me as an onlooker that the community has pretty much decided that theorists are NOT going to be let off the hook. The anthropic excuse for failure has been discarded or has lapsed into disuse.

 

[Fuzzy Logic]

That's great. Then you agree with the main body of the statement (we don't have to give up on the effort to find natural explanations) and just disagree verbally with how he chose to phrase the headline.

You miss the point. While our current model may be sufficient to explain what we already know, it doesn't explain everything. His statement is fine by scientific standards and I agree that the anthropic principle is not enough reason to justify a multiverse theory. I agree that the anthropic principle has no merit.
Would I say it's rather pathetic? No, but it is not a valid argument against multiverse theories, only the anthropic principle.

There are many compelling reasons to consider multiverse theories.
Most importantly, what happened at T=0.

 

[Chalnoth]

I agree that the anthropic principle has no merit.

That is taking things way way too far. The anthropic principle is, first of all, necessarily true. Because of its necessary truth, it manifests itself as a selection effect that must be applied whenever considering any theory for the birth of a new region of space-time. Failure to apply the selection effect that is the anthropic principle in such a situation will always lead you to incorrect conclusions.

Now, the anthropic principle can be used badly, as can most anything, but that's no reason to disregard it.

 

[Fuzzy Logic]

That is taking things way way too far. The anthropic principle is, first of all, necessarily true. Because of its necessary truth, it manifests itself as a selection effect that must be applied whenever considering any theory for the birth of a new region of space-time. Failure to apply the selection effect that is the anthropic principle in such a situation will always lead you to incorrect conclusions.

Now, the anthropic principle can be used badly, as can most anything, but that's no reason to disregard it.

No, I say it has no merit, because we don't know what the conditions for life really are. Life could just as easily prosper in a universe with completely different physics from our own. The fact that life is so prolific on Earth contradicts the idea of a finely tuned universe. The fact that we can't find life outside of our own planet only proves that the universe is vast, not that life is unique.

If you want to say that it's a necessary truth, then it is no different than saying "I think, therefore I am".

 

[marcus]

His statement is fine by scientific standards and I agree that the anthropic principle is not enough reason to justify a multiverse theory. I agree that the anthropic principle has no merit.

I agree. I think Steinhardt in 2005 led the attack that established that the anthropic principle has no place in fundamental physics. His statement was effective and influential. It was echoed and anthropics lost out e.g. as reflected in subsequent StringsXXXX conferences. Ridiculous to call it pathetic.

Would I say it's rather pathetic? No, but it is not a valid argument against multiverse theories, only the anthropic principle.

That was my point at the outset, specifically regarding the anthropic cop-out.
===Originally Posted by Chalnoth===
==Originally Posted by marcus==
http://edge.org/response-detail/805/...annot-prove-it

or google "Steinhardt annual question 2005"
==endquote==
I've always found that response to anthropic arguments to be rather pathetic.
===endquote===

BTW don't mind predictive testable theories that explain stuff involving subsequent big bangs as a byproduct. If a particular model of our big bang (as long as it is testable) just has to produce baby big bangs/reheats somewhere down the road, that is OK with me.
I just get disgusted when I see arguments like "it's the way it is because otherwise we wouldn't be here and we can't explain any more".

There are many compelling reasons to consider multiverse theories.
Most importantly, what happened at T=0.

Please explain, what do you think happened at T=0 that makes this or that compelling. Are you thinking of socalled rogue regions where inflation did not stop and is still going on---or where reheating occurred later? That was discussed in an interesting thread last summer. You might be intrigued by some of what Ben Crowell had to say regarding a conference at Perimeter about problems with the prevailing early universe pictures and related stuff
https://www.physicsforums.com/showthread.php?p=3403876#post3403876

 

[Fuzzy Logic]

Please explain, what do you think happened at T=0 that makes this or that compelling. Are you thinking of socalled rogue regions where inflation did not stop and is still going on---or where reheating occurred later? That was discussed in an interesting thread last summer. You might be intrigued by some of what Ben Crowell had to say regarding a conference at Perimeter about problems with the prevailing early universe pictures and related stuff
https://www.physicsforums.com/showthread.php?p=3403876#post3403876

I have no opinion on simultaneous multiverse theories, though because of intuition and the singularity problems I am inclined to a big-bounce theory. I don't think that we should be extrapolating back to exactly 0. I expect that the universe bounced slightly before. What we say is T=0 in the big-bang model is more like Planck time. What happened before that was the end of the collapse of the previous universe when it reached critical density and entropy surpassed gravity. If I say too much I'll surely paint myself into a corner. That is just my layman interpretation of what I think happens. I don't pretend to know enough details to argue for it.

I'll check out that thread, thanks.

Edit: sorry I didn't actually address your question. Why is it compelling.
I think it's a compelling reason because we can't explain the singularity event. Not only can we not explain what happened in the first moments, we can't explain how the universe came into existence at all. If I have to choose between saying the the universe just popped into existence and time began or saying that the universe has existed forever, without any direct evidence in favour of either argument the circumstantial evidence would suggest that the life cycle of the universe is closed system, just like the rest of nature. Nowhere do we have any evidence of spontaneous existence, but everywhere we can see examples of cyclic evolution.

 

[marcus]

Part of what causes confusion is that some people talk as if we have a settled theory of the mechanism behind inflation.
We don't. Smart people are still arguing about whether inflation even occurred (see Ben Crowell's condensed digest of the Perimeter conference last summer)
and there are several ideas of how it might have worked, if it did occur.
Here's a relevant Crowell post:
https://www.physicsforums.com/showthread.php?p=3404021#post3404021

Just to illustrate with an example, here is a recent paper by Perimeter's Laurent Freidel and others that proposes quite a different mechanism.

http://arxiv.org/abs/1201.5423
Dirac fields and Barbero-Immirzi parameter in Cosmology
G. de Berredo-Peixoto, L. Freidel, I.L. Shapiro, C.A. de Souza
(Submitted on 26 Jan 2012)
We consider cosmological solution for Einstein gravity with massive fermions with a four-fermion coupling, which emerges from the Holst action and is related to the Barbero-Immirzi (BI) parameter. This gravitational action is an important object of investigation in a non-perturbative formalism of quantum gravity. We study the equation of motion for for the Dirac field within the standard Friedman-Robertson-Walker (FRW) metric. Finally, we show the theory with BI parameter and minimally coupling Dirac field, in the zero mass limit, is equivalent to an additional term which looks like a perfect fluid with the equation of state p = wρ, with w = 1 which is independent of the BI parameter. The existence of mass imposes a variable w, which creates either an inflationary phase with w=-1, or assumes an ultra hard equation of states w = 1 for very early universe. Both phases relax to a pressureless fluid w = 0 for late universe (corresponding to the limit m→∞).
16 pages

From the conclusions section on page 15: "... the fermionic matter behaves effectively as a cosmological constant and creates an inflationary phase which is relaxed at late time into a pressureless fluid."

So here's another possible inflation mechanism just now proposed, which will quite possibly be worked on. At this point it isn't clear that it would for instance involve "rogue regions" where inflation does not turn off and which continue inflating. The idea needs to be explored and one does not know which problems it would or would not share with inflation mechanisms which people have speculated about earlier.

I guess the moral (which bears repeating) is "don't assume you know what you don't know and draw draw conclusions from it."

 

[salvestrom]

My thoughts on the subject solidified last night and rapidly reached two seemingly diametric dead ends.

If the expression of all possible outcomes is an intrinsic property of the universe then the first event was simply this: existence or non-existance (possibly even 'are all possile outcomes expressable or not). This automatically creates a universe and a non-universe. From here questions abound about the specific makeup of our early universe, requiring the acceptance of a trillion other alternatives with less or more of one thing or another. It gets messy quickly, but seems to offer an answer to the most fundamental question.

Here's the opposite:

All events today are the result of all the events over the last 13.5 billion years and so, far from being many posible outcomes, there is only one possible outcome for any event: that which happens. Although Fatalistic, the reality is we cannot know the future because we cannot know all prior actions in order to calculate 100% any future ones. Oddly enough, the line "the expression of all possible outcomes is an intrinsic property of the universe" still applies, it's just there's only ever one. This idea offers little revelation in the explanation of the existence of the universe, other than to accept that it couldn't have ever been anything else.

 

[Fuzzy Logic]

I guess the moral (which bears repeating) is "don't assume you know what you don't know and draw draw conclusions from it."

If I assumed, I wouldn't bother looking for answers. Satisfaction in ignorance is no better. The pursuit has and always will be fact, whether that coincides with my own expectations or not. I did not make a conclusion, I postulated an idea. Is it evidently wrong?

 

[marcus]

If I assumed, I wouldn't bother looking for answers. Satisfaction in ignorance is no better. The pursuit has and always will be fact, whether that coincides with my own expectations or not. I did not make a conclusion, I postulated an idea. Is it evidently wrong?

Fuzzy, I don't see the connection between that general "moral" and your posts. It wasn't directed towards them or reflecting on them.

But I would like to amplify and explain some, if I've got time.

All "inflation" means is exponential expansion e^Ht with a a high nearly steady Hubble rate H. The Hubble "constant" is a frequency, a reciprocal time.

There are various possible mechanisms that could cause inflation. But some people seem to have a fixed notion so when you say "inflation" all they can think of is a fixed range of notions mostly dating back 20 or 30 years which have been drummed into them.

That's the kind of thing I meant by "assuming you know what you don't, and drawing conclusions from it".

I'm short on time. I'll try to add some more clarification to this when I can.

BTW an interesting side aspect to all this is the "superinflation" that automatically occurs (and automatically ends) in the LQC bounce. It is faster than ordinary inflation and involves a rapidly INCREASING Hubble frequency H, so that you get faster than exponential growth. You just solve the bounce equations and this is what you get.

H is negative during the contraction phase, then crosses zero (that defines the moment of the bounce) and increases rapidly to something like Planck frequency, and then slacks off. This is not due to some imagined "quantum fluctuation" or anything not under the model's control. It is built in. Superinflation is a deterministic feature of the loop cosmology bounce by which the singularity is resolved and it happens only then. And it terminates deterministically and very quickly at that.

This is not to say LQC is right, that is something that will eventually be tested by observation. It may or may not be shown false. It fits observation so far---see Rinaldi's recent review of all the QC options. http://arxiv.org/abs/1201.4543

So i don't say anything about right/wrong. this is just an example to illustrate that the mechanisms underlying a brief period of exponential expansion don't have to correspond with anyone's (e.g. Chally's) preconceptions.
So drawing elaborate conclusions is really really premature.

 

[Chalnoth]

No, I say it has no merit, because we don't know what the conditions for life really are. Life could just as easily prosper in a universe with completely different physics from our own. The fact that life is so prolific on Earth contradicts the idea of a finely tuned universe. The fact that we can't find life outside of our own planet only proves that the universe is vast, not that life is unique.

If you want to say that it's a necessary truth, then it is no different than saying "I think, therefore I am".

It's not nearly so bad as that. We may not know the requirements for life in detail, but it is very easy to place limits based upon general, overall requirements. For example, if you want to have life, you are going to need structure formation. That is, you need galaxies. And simple limits like this are enough to make pretty powerful statements about the possible values of some parameters that any observer can potentially measure.