Falsification of eternal inflation

[marcus]

...
So it seems like the idea has a contradiction built-in: if it's genuinely generic, then it predicts there will be an increase in entropy into the past, but that increase in entropy into the past requires massive fine tuning.

I'm curious, Chalnoth. How do you define the entropy of the universe? The quantity which you think would be increasing as you go back in time through and beyond the bounce?

 

[dm4b]

Well, that depends upon what you mean by "t=0". If you mean the time of the singularity if you naively extrapolate a Lambda-CDM model back in time, yes, it is correct that inflation started before then. That time t=0 would have just been a particular time late in inflation, just before reheating, not any special time. The start of inflation would have come earlier. We don't know exactly when so it's not so easy to label a specific time t=0 that would have been the beginning of inflation.

Inflation is still responsible for the generation of the observable universe.

hmmm, doesn't really sound like you answered my questions, but that's okay, I kinda figured there may not be an answer for those particular ones.

Sounds like eternal inflation leaves as many questions as answers, as far as a "where did the Universe come from" theory.

 

[Chalnoth]

I'm curious, Chalnoth. How do you define the entropy of the universe? The quantity which you think would be increasing as you go back in time through and beyond the bounce?

I don't see how it matters in this case, because we define entropy as increasing into the future in our universe. If the bounce is generic, then that is enough to show that the theory predicts entropy increasing into the past before the bounce.

 

[Chalnoth]

hmmm, doesn't really sound like you answered my questions, but that's okay, I kinda figured there may not be an answer for those particular ones.

Sounds like eternal inflation leaves as many questions as answers, as far as a "where did the Universe come from" theory.

Well, I felt I'd go with the first one, since the rest depended on that.

 

[marcus]

I don't see how it matters in this case, because we define entropy as increasing into the future in our universe. If the bounce is generic, then that is enough to show that the theory predicts entropy increasing into the past before the bounce.

I don't understand, entropy has to be defined or there is no argument.
BTW I assume that by "generic" you mean what Loop cosmology people call "robust". That is it happens pretty much with any choice of parameters, does not require fine adjustment or special choices.

Indeed that has been found. The bounce is a robust feature.

However the theory does not predict that the pre-bounce collapse looks just like the post-bounce run backwards.

I suspect that is the source of your misunderstanding.

 

[Chalnoth]

I don't understand, entropy has to be defined or there is no argument.

I don't see that it is necessary. This is a point about the symmetry of physical law. As long as LQC is time reversal invariant, then it should equally describe the universe's behavior backward in time as forward.

Nevertheless, you could use the simple definition used to describe the entropy during inflation of any Hubble volume as being proportional to the horizon scale. With this definition, entropy would indeed be decreasing as the universe collapses.

BTW I assume that by "generic" you mean what Loop cosmology people call "robust". That is it happens pretty much with any choice of parameters, does not require fine adjustment or special choices.

Indeed that has been found. The bounce is a robust feature.

If the previous universe doesn't look like a universe that came about as a result of inflation with the time coordinate set the wrong way, then the bounce isn't robust. If it does look like that, then it doesn't explain the low entropy boundary.

 

[marcus]

I never heard anyone say that LQC model of bounce is time reversal invariant. What Ashtekar says is that you have a classical universe collapse (often called a "big crunch") leading up to the bounce. A crunch does not look like a bang movie run backwards. The Loop bounce has an obvious "arrow of time" directionality.

About microscopic reversibility of laws, note that the distinction between microscopic and macroscopic law becomes hazy during what A. calls the "quantum regime" or "Planck regime" phase of the bounce. It is definitely something for theorists to explore and get sorted out.

 

[marcus]

About the need to have a definite mathematical meaning for the entropy. You do need this. Otherwise you have no assurance that the word has any meaning at all in the regime under consideration.

 

[Chalnoth]

I never heard anyone say that LQC model of bounce is time reversal invariant. What Ashtekar says is that you have a classical universe collapse (often called a "big crunch") leading up to the bounce. A crunch does not look like a bang movie run backwards. The Loop bounce has an obvious "arrow of time" directionality.

About microscopic reversibility of laws, note that the distinction between microscopic and macroscopic law becomes hazy during what A. calls the "quantum regime" or "Planck regime" phase of the bounce. It is definitely something for theorists to explore and get sorted out.

That's not what I'm saying. What I am saying is that the expansion of our space-time would look like a collapse run backwards. Because of that, one should be able to do the same analysis backwards in time with our universe as the "starting" point.

 

[marcus]

What I am saying is that the expansion of our space-time would look like a collapse run backwards.

I think that is wrong, thought I made that clear earlier. I'm too lazy to look back and find where I made that point.

You are welcome to think that a classic universe collapse looks like the start of expansion run backwards. You must suit yourself. I don't want to argue with somebody's deep-set mental images.

 

[marcus]

If anybody else besides Chalnoth (who seems to have a conceptual hangup) is following this thread, the obvious reason that classical expansion from a smooth intitial condition does not look like a classical crunch run backwards is basically just what Two-Fish said a few posts back.

As long as grav. is universally attractive the forward progression of time tends towards the grav field---the geometry of the U---becoming more lumpy and pock-marked.

The geometry gets more and more inhomogeneous as stuff coagulates and as structure forms.

I've said this before in this and parallel threads.

So the picture of a crunch has lots and lots of inhomogeneity.

On the other hand starting when the classical regime takes over in the big bang you have lots of uniformity, for 100s of thousands of years. Lots of homogeneity. It takes time for structure to begin to nucleate and start gathering.

So the two movies are quite different. One movie is not going to bear any resemblance to the other movie run backwards.

I will try to discuss this, and how this happens in bounce cosmology ( a research area that seems to be taking off) in that other thread.

"Eternal Inflation" is a much older idea (1980s? 1990s?) and this thread is supposed to be about "Falsification of eternal inflation". So I think to be considerate of people interested in that kind of thing we ought to discuss bounce cosmology, how it implements this, and the definition of entropy, and so forth in that other thread.

Here is the "Definition of entropy of the universe?" thread's link, so we can get out of this "eternal inflation" one:
https://www.physicsforums.com/showthread.php?t=487703

 

[eloheim]

Hey just a quick (edit: okay maybe not) question in regards to this issue and the OP's question: (And I have to admit I haven't made it through this whole thread yet so I hope I'm not repeating what others have already said.)

Is the "youngness paradox" an example of the anthropic principle? My understanding (via Brain "String-bean" Greene) is that Steven Weinberg suggested in his '87 paper on the topic that, should there be a multitude of different universes with different cosmological constants, then the probabilities say we should inhabit one with a constant that has a value just low enough for us to be able to exist, and not much smaller.

Does the OP's criticism of eternal inflation follow the same path, by pointing out that, if true, then, given the odds, we should inhabit a universe that is just [STRIKE]young[/STRIKE] old enough for life to exist?

(Hoping I'm on the right track) I take it that the reason this argument is presented as evidence against E.I. is that it seems we could have come about a lot earlier? And that is because there have been stars and galaxies, presumably with planets like ours, for a long time now?

My question then is, "How long do universes consistent with E.I. allow for the emergence of life?" If the answer were something huge, like a trillion, trillion, trillion, trillion, etc. years, then wouldn't the "youngness paradox" actually bolster the case for E.I. because, relatively speaking, we did evolve extremely quickly?

 

[offroff]

eloheim,
You understand me I think. My original post and the youngness paradox is very similar but according to the other guys in this thread it's flawed reasoning.

But if the youngess paradox is valid after all, then I don't think your scenario helps E.I. at all. You just have to consider that there will be so many universes like ours out there. Almost all of them one second or more younger than ours. The universes older than ours is just not relevant because there are so few of them.

Anyway I think the youngness paradox is too naive. And I think it shows that physical cosmology is ridicoulously naive (because guys like Guth takes the youngness paradox seriously).