Falsification of eternal inflation

offroff

Steinhardt makes a good point:

Now you should be disturbed. What does it mean to say that inflation makes certain predictions—that, for example, the universe is uniform or has scale-invariant fluctuations—if anything that can happen will happen an infinite number of times? And if the theory does not make testable predictions, how can cosmologists claim that the theory agrees with observations, as they routinely do?

I thought the theory can make testable predictions but maybe I was wrong. That's a serious problem

However, even if we're dealing with infinite sets I think we should be able to find a way to deal with probabilities. I mean; in some worlds we would see very very strange patterns and eternal inflation predicts that. It still doesn't make sense to let eternal inflation be the explanation for any problem (even if it might be true).

Chalnoth

I don't think that's a very good point. If it were, then quantum mechanics itself would make no predictions whatsoever. But it does: it makes probabilistic predictions. And when you have large numbers (the number of fluctuations in the early universe was very large), the statistics of those numbers becomes highly predictable, even if, in principle, anything can happen.

The primary issue that he does have a good point on is, to me, the measure problem. I am extremely skeptical of his discussion about the cyclical universe, however, as that seems to completely violate everything we know about thermodynamics.

Calrid

Has anyone pointed out that inflation is obviously eternal since nothing exists outside of time and there has never AFAIK been a time when the Universe was not inflating.

I'd like to think bangs and crunches were true because there should be evidence in this universe of them. But most likely it's just heat death and the slow demise of time itself. Or maybe in the quantum dominated almost void it starts all over again!

offroff

Chalnoth,
Yeah, we really need probabilistic predictions but one of the arguments in this thread was that we can't use that on an infinite set.

My initial argument was maybe flawed but in theory there must be a prediction how the typical evolution of life in the multiverse looks like. Do you agree?

Chalnoth

It doesn't work that way, actually. Basically, if you look at the way inflation works back into the past, unless you would like to impose an infinite degree of fine tuning, you end up with a singularity in the finite past. It seems clear that this singularity is rather unphysical, and thus inflation began as a result of some physical process in the finite past.

Even eternal inflation falls for this difficulty, and thus eternal inflation is generally considered to only be eternal in the future, not the past. There is nothing fundamental about inflation that requires it to have the properties for it to be eternal.

Chalnoth

I already mentioned that the problem is the measure problem, not the general property of quantum mechanics that "everything happens".

dm4b

offroff, yes that was the article, thanks! Maybe, since I am slow at work today, I will take the time to read it now ;-)

dm4b

Well, I just read that article. This paragraph stuck out for me:

"Its raison d'être is to fill a gap in the original big bang theory. The basic idea of the big bang is that the universe has been slowly expanding and cooling ever since it began some 13.7 billion years ago. This process of expansion and cooling explains many of the detailed features of the universe seen today, but with a catch: the universe had to start off with certain properties. For instance, it had to be extremely uniform, with only extremely tiny variations in the distribution of matter and energy. Also, the universe had to be geometrically flat, meaning that curves and warps in the fabric of space did not bend the paths of light rays and moving objects."

So, inflation was brought about so the Universe would not be dependent upon a set of highly unlikely initial conditions?

But, maybe the Universe really was born out of a set of highly unlikely initial conditions.

Sort of like buying a lottery ticket. Many folks think that with 180 million to 1 odds, it's a waste of time ... they say, "you can NEVER win". Well, every other month, or so, somebody does win. Those people usually wouldn't question whether or not they CAN win anymore .. because they just did.

Maybe questioning the fact that the Universe started off with unlikely properties, wasn't the best idea? Maybe, it really did. Well, we have to question, but perhaps it has brought us full circle, with a tour through a not-meant-to-be theory called Inflation? And, in doing so, we become faced with even worse scenarios along these lines - "bad" inflation being more likley than "good" inflation, and even no inflation being more likely than inflation, etc.

Anyhow, with all that said, I thought the original Big Bang theory had other gaps in it other than unlikely initial conditions? Is that correct?

Chalnoth

Well, it was. Inflation doesn't solve this problem. In fact, it basically has to be this way, because entropy has been increasing ever since.

The thing is, if you take the naive view of this, just using entropy, you run into the Boltzmann Brain problem: small fluctuations out of entropy are much more likely than large fluctuations. So it's much easier to get, for example, a single galaxy using this view than a whole universe. It's even easier to just get a single solar system. Easier still to just get a mind that merely thinks it sees an external universe, then immediately winks out of existence (the Boltzmann Brain).

But we know that's obviously wrong, so there had to be some interesting physics that let you get enough real universes produced that the real observers outnumber the Boltzmann Brains.

dm4b

Well, I've lost access to that article, so I can't comment on the first part. Sounds like maybe I've misunderstood something a bit though.

As far as the Boltzmann Brain - hadn't heard of it before. But, I just read this on wiki:

"our current level of organization, having many self-aware entities, is a result of a random fluctuation, it is much less likely than a level of organization which is only just able to create a single self-aware entity."

This logic seems over-stretched to me, because life, once started, appears to be a self-organizing "force". In some sense, it constantly fights against entropy. We're basically dying from the day we are born, but there are forces in the body that renew and heal us all the time, which keep us going.

In addition, a "single self aware entity", takes many millions of years of evolution to form. They don't just instantly pop out of nowhere from a random fluctuation. In addition, once the stage is set for one to evolve, it is set for many to evolve. That's just sorta how life seems to work. I doubt you will find a self aware entity anywhere in the Universe with a lonely history completely unique to itself.

So, the reasoning in the Boltzmann Brain problem makes sense to me right up to the part where it considers the "Brain", or until it considers life.

dm4b

Now, that I think twice about it, the Boltzmann Brain doesn't make sense to me at all, lol.

Could a Universe exists that actually consisted of a single galaxy. Or, a single solar system?

How could a solar system exist, w/o prior generation of stars that put out, in their deaths, some of the heavier elements that planets are made of. Could a galaxy exist, as we know it, without other galaxies?

In addition, it seems to argue that the order from most complex to most simple is, our Universe, a galaxy, a solar system, a self aware entity.

I would argue the order should be Our Universe with self-aware entities, Our Universe without self-aware entities, a galaxy, a solar system.

I think the whole idea ignores all the interdependencies that are required for the existence of the "objects" it takes under consideration.

Anyhow, I guess this is getting off topic ...

Chalnoth

That's sort of the point. It's an "argumentum ad absurdum", aka and argument from contradiction. If you consider a universe to simply be a particularly large fluctuation out of equilibrium, then you inevitably arrive at the conclusion that individual brains popping in and out of existence, as rare as they are, are far more common than real observers. And that is nonsense: it contradicts the observation that we are real.

So if your theory predicts that Boltzmann Brains are more common than real observers, that theory is wrong. That was my point.

dm4b

Well, I get that. I'm just not sure that our theories really predict that.

It seems to me it's assuming something is common, only because we are neglecting the fact that many other things are also required, and must come first, due to the interdependent nature.

If you look at it that way, perhaps the small fluctuations can't really bring about anything. It's only a large one, which can bring about everything we see that can, in reality, actually achieve anything. It's the whole show or nothing.

I don't know if this is the right way to think about things. It just seems like the other view is missing something too.

Calrid

I'm not sure everyone would agree with that assertion or that its even justifiable.

Eternal means existing always, from our perspective the universe has always existed since outside of time makes no practical sense. It does not I think mean what you think it does. The universe has always been in a stage of inflation since t>0 or when time and hence space exists if you like. The so called singularity or point of origin at t=0 is undefined for obvious reasons. If we prove that there was a before t = 0 empirically this may change. At the moment such a contention is still speculative so any conjecture is rather philosophical atm.

offroff

dm4b,
Boltzmann brains is not offtopic, my thoughts about ultraquick evolution is similar (my argument failed though). I think we need to be aware that there are a lot that needs to be explained. A cosmological theory that explains physics is not enough, it also needs to be in line with philosophy, psychology and biology as well. I think physicists should know that :)

Chalnoth

Boltzmann Brains are a definite prediction of a naive thermal fluctuation model, where one tries to explain the low entropy in the early universe by just saying, "Well, thermal fluctuations out of equilibrium happen all the time, maybe the early universe was just a particularly big thermal fluctuation."

Most think that the small physical size of the inflationary epoch likely has something to do with this, and there are some reasonable explanations that solve the problem. Eternal inflation, by the way, isn't one of them.

Chalnoth

Uh, what? There is no reason whatsoever to believe that inflation at the start of our observable universe was the beginning of all time and space.