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

[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.

 

[Fuzzy Logic]

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.

It is interesting how fickle life seems to be but it doesn't matter how many times you roll the dice, the odds of rolling a 1 are always the same. You can maximize chance with iterations but not odds. It is just as likely that life emerged on the first iteration or the trillionth.

All of nature is uncanny how it manages to work, not just life. I don't think that uncanny is evidence of anything.

 

[Chalnoth]

It is interesting how fickle life seems to be but it doesn't matter how many times you roll the dice, the odds of rolling a 1 are always the same. You can maximize chance with iterations but not odds. It is just as likely that life emerged on the first iteration or the trillionth.

All of nature is uncanny how it manages to work, not just life. I don't think that uncanny is evidence of anything.

What are you trying to say here? Because as near as I can tell it has nothing to do with anthropic arguments. Anthropic arguments are, at their heart, arguments about what aspects of nature we have a right to be surprised about. If a certain aspect of nature seems, on its face, highly unlikely, but it turns out that something like it is required for life to exist, then we don't have any right to be surprised to see it. The cosmological constant is a good example here. Sure, the number 10^{-120} seems fantastically small, but it can't possibly have been much bigger and still allowed the existence of galaxies. Because a cosmological constant this small is required for us to exist, we can't, by rights, be surprised about it.

 

[MathematicalPhysicist]

I am not sure, my message was really understood.

How do you define "universe"?

If it's:"All existing matter and space considered as a whole; the cosmos."

Then multiverse=universe, multiverse is just another name for "all that exists".

If on the other hand you say that a universe is all that is made of the same matter as we are, then there might be other universes with different type of matter different than ours.

But it depends on your definition.

 

[Chalnoth]

I am not sure, my message was really understood.

How do you define "universe"?

If it's:"All existing matter and space considered as a whole; the cosmos."

Then multiverse=universe, multiverse is just another name for "all that exists".

If on the other hand you say that a universe is all that is made of the same matter as we are, then there might be other universes with different type of matter different than ours.

But it depends on your definition.

The only way in which it's an interesting topic of discussion is in regard to the low-energy laws of physics varying from place to place within the universe.

 

[Fuzzy Logic]

Ok in that sense sure, but do we really need a principle to define it? That is just elementary deduction. "I think, therefore I am"

I was referring to the anthropic principle being a justification for a multiverse.
As far as I understand, there is no evidence that any of the fundamental conditions must or even could change, only predictions.

 

[Chalnoth]

Ok in that sense sure, but do we really need a principle to define it? That is just elementary deduction. "I think, therefore I am"

The fact that so many people argue so vociferously against it seems to indicate that yes, yes we do.

I was referring to the anthropic principle being a justification for a multiverse.

I don't think that's an entirely correct way of thinking about it. Rather, as I said earlier, the anthropic principle must be taken into account when considering any law for how new regions of space time are born, or how the low-energy laws of physics might vary.

As far as I understand, there is no evidence that any of the fundamental conditions must or even could change, only predictions.

Any spontaneous symmetry breaking event causes a change in the low-energy laws of physics. The electro-weak symmetry breaking is one we know of. There are probably many more.

 

[marcus]

The only way in which it's an interesting topic of discussion is in regard to the low-energy laws of physics varying from place to place within the universe.

The way I think of it can connect with this. I think of the universe as the unique whole of nature with a unique set of fundamental laws.

Spontaneous symmetry-breaking may have resulted in regional variation in some constants that emerge at lower energy. I think that is extremely interesting and I think it is something we humans may be able to study and understand.

At present I don't see any compelling reason to involve multiple big bangs in our model of the big bang---the start of expansion. Pending evidence to the contrary I expect one start of expansion, operating under one set of fundamental highenergy laws of physics will probably fit the data.

It would be quite interesting if we got some evidence of other big bangs having happened, of course. But absent such evidence *shrug*.

So my view is similar to the one expressed in post #1---the reductionist program is on track, no need to give up on the program of finding ever deeper explanations for what we see in terms of one universe, one start of expansion, one set of fundamental physics laws.

 

[Chalnoth]

At present I don't see any compelling reason to involve multiple big bangs in our model of the big bang---the start of expansion. Pending evidence to the contrary I expect one start of expansion, operating under one set of fundamental highenergy laws of physics will probably fit the data.

Well, it kinda has to. But that same model may unambiguously predict other bangs, other low-energy laws of physics (our current model already predicts other low-energy laws of physics...and other bangs are the natural expectation of any model that produces at least one).

Now, I don't think there is any conceivable way that we will ever be able to obtain direct evidence of universes with different fundamental laws. Though I do think it may be interesting to think about the possibility.

 

[marcus]

Since we just turned a page, I'll recall the post of mine (#112) you were responding to just now

==quote post #112==

The only way in which it's an interesting topic of discussion is in regard to the low-energy laws of physics varying from place to place within the universe.

The way I think of it can connect with this. I think of the universe as the unique whole of nature with a unique set of fundamental laws.

Spontaneous symmetry-breaking may have resulted in regional variation in some constants that emerge at lower energy. I think that is extremely interesting and I think it is something we humans may be able to study and understand.

At present I don't see any compelling reason to involve multiple big bangs in our model of the big bang---the start of expansion. Pending evidence to the contrary I expect one start of expansion, operating under one set of fundamental highenergy laws of physics will probably fit the data.

It would be quite interesting if we got some evidence of other big bangs having happened, of course. But absent such evidence *shrug*.

So my view is similar to the one expressed in post #1---the reductionist program is on track, no need to give up on the program of finding ever deeper explanations for what we see in terms of one universe, one start of expansion, one set of fundamental physics laws.

==endquote==

Then continuing the discussion with your post #113, which think was mainly in response to what I just highlighted blue:

Well, it kinda has to. But that same model may unambiguously predict other bangs, other low-energy laws of physics (our current model already predicts other low-energy laws of physics...and other bangs are the natural expectation of any model that produces at least one).

Now, I don't think there is any conceivable way that we will ever be able to obtain direct evidence of universes with different fundamental laws. Though I do think it may be interesting to think about the possibility.

That seems pretty reasonable to me.

 

[Radrook]

I understood Hawking to mean that the universe is as it is because we can't help but perceive it that way because of what we are-human.