Inflation is very unlikely less than 10^-6.6×10^7

Chalnoth

This is why the evidence in favor of inflation is primarily from WMAP observations (but also other balloon-borne and ground-based CMB observations). Hopefully Planck will have some interesting things to say further in this regard in a couple of years (First data release should be two years from next month).

Haelfix

That is certainly true as stated however in this context I would disagree with 1, namely I disagree that model X predicts that inflation is highly unlikely.

Penrose's (and others like Sean) argument is simple (or at least part of it is) eg: The subset of all possible initial conditions that allows for subsequent inflation is of measure zero, ergo in the standard story inflation is vanishingly unlikely, ergo it cannot be inflation that accounts for flatness/homogeniety/whatever.

But lets go back to the billiard ball analogy. We have B, PHI and we use those two to calculate for A and thus we can bound these initial conditions to a small epsilon ball (or even a point if certain mathematical conditions on Phi are satisfied). Manifestly this set, is also of measure zero in the space of all initial conditions.

What is wrong would be to claim that b/c of this fact, the observed billiard ball scattering event couldn't have happened as stated or that PHI is somehow incorrect!!!

Chalnoth

A better way of saying it is that the model seems to predict that in the space of all observers, a vanishingly small fraction will observe a universe that began through inflation. Now, the paper doesn't say this alone, but that is the direction in which (in my opinion) it is attempting to point.

Well, it's not an entirely wrong comparison to make. If one is to consider the space of all initial conditions to consider one ball striking another, one is simply wrong. All initial conditions do not occur equally, because the way this happens in reality is that you have a person that strikes the cue ball with the intention of hitting the cue ball into a certain ball, so that it goes at a particular angle. One could, with a bit of work, predict with reasonable accuracy the outcome of such a strike after watching a player play the game for a while.

But if we consider the perfectly randomized case with the pool table, we end up with nonsensical results when compared against reality. This isn't surprising, because the perfectly randomized case is wrong.

In a similar way, discovery of nonsensical results in our models of the formation of our universe is an indication that we are missing something about how the early universe behaved. For instance, as Marcus pointed out, some calculations that include LQG appear to show that inflation is actually very likely, which would indicate that the "something missing" is not taking into account the quantum nature of gravity (I'm skeptical of this result, but it illustrates the point).

Haelfix

"In a similar way, discovery of nonsensical results in our models of the formation of our universe is an indication that we are missing something about how the early universe behaved. "

Again, just b/c an initial condition seems less than generic, doesnt mean that it wasn't the seed that created us physically. You cannot use that particular information (the unlikeliness of the particular ic) in order to discriminate against the dynamics (say the existence of inflation) so long as the dynamics correctly reproduces the observed final state. That's why it irrates me to say that 'inflation is very unlikely', when instead one should say inflation is nearly 100% certain b/c its the only set of dynamics possible that logically links A to B -I am exagerating of course, but you get the point.

There's another issue that troubles me. While its true that by far the majority of initial conditions has the inflaton far from the top of its potential, in the context of eternal inflation you can always wait awhile (or a very long while, since we are presumably talking about infinities) and simply invoke the anthropic principle after that point and it wouldn't be all that wrong. There is nothing that automatically implies that inflation has to happen immediately.

heusdens

As of the billiard ball analogy, suppose we witness only a static end condition of a billiard shot, and didn't witness any fragment of the dynamic situation nor the initial condition. How in the world are we to figure out - even when all the dynamics is known that could have occured - what possible initial conditions caused that later condition? And for sure there isn't a unique possible initial condition.

Same as the situation prior to the Big bang. We might find all kinds of clues of what could have happened, and what not. But I think we'll never find a unique match for it, since all kind of things could have occured.

The question is then: what difference does it make? For the sake of what are we wanting to find that out?

Chalnoth

Why wouldn't we want to know where we came from?

Anyway, I don't think things will turn out to be as bleak as you paint them. We have quite a bit more investigation to do. There may be roadblocks we don't yet see, but we don't know that yet. And we also don't need to demonstrate a unique set of initial conditions in order to learn a lot about how our universe began.

heusdens

Well my remark was more of a societal sort of remark then a scientific one, as a scientists, one wants to know that. But as a society we could think that there are better things to explore that have more of a practical purpose. But that is probably a whole new discussion in and of it's own.

Chalnoth

These things aren't mutually exclusive. It is entirely possible for some scientists to study the early universe, while other scientists work on antibiotics or cures for cancers.

Still, from a purely practical standpoint, pure science investigations tend to have benefits that far exceed their costs. One way in which this occurs is that answering these hard questions often requires the construction of specialized devices that themselves require brand-new technologies to be developed. These new technologies then often make their way into the private sector where they, on average, prove to have economic benefits far in excess of the original research cost to develop them.

Then there's the point to be made that all of science is interconnected, and we never know what we'll learn by investigating one area that will apply in others.

So no, even from a societal point of view, there are good reasons to investigate inflation, even if it isn't personally interesting to everybody (though, from my experience, this sort of investigation actually is interesting to a very large number of people).

Chronos

I like the billiard analogy. What would the table look like without rails or pockets? A lot like, IMO, the universe we observe today. If we run all the ball positions backwards, they appear to be packed together at infinite density before being flung apart at near infinite velocity. Heck, we can even throw in a slight curvature to the table top to mimic accelerated expansion without overcomplicating the model.

bapowell

Yeah, I agree. And get rid of literature, art, theater, poetry, and history while you're at it!