Steven Weinberg offers a way to explain inflation

marcus

Mtd2 spotted this paper by Steven Weinberg that just went on arxiv.
http://arxiv.org/abs/0911.3165
Asymptotically Safe Inflation
Steven Weinberg
13 pages
(Submitted on 16 Nov 2009)
"Inflation is studied in the context of asymptotically safe theories of gravitation. It is found to be possible under several circumstances to have a long period of nearly exponential expansion that eventually comes to an end."

It could be an important paper, and in any case it's kind of elegant because the inflation episode occurs naturally, by the running of constants, without having to dream up some exotic matter field.

Reuter and Bonanno already proposed something along these lines. The essential arithmetic is very simple: there is evidence of the existence of a UV fixed point for gravity where the dimensionless forms of G(k) the running Newton and Lambda(k) the running dark energy constant both converge to finite values as the length scale k -> 0

But one can see by simple dimensional reasoning that their dimensionless forms are
G(k)/k² and Lambda(k)k².

So for them to go to finite limits as k->0 we must have G(k) getting very small and Lambda(k) growing enormous.

That's just the thing to cause rapid expansion. The Newton constant is almost nothing, so nothing to hold the geometry together, and the cosmological constant---the dark energy that accelerates expansion---totally huge.

But as inflation proceeds the scale k increases, which increases G and reduces Lambda. So the process eventually (actually quite quickly) shuts itself off.

marcus

Two things about this idea seem extremely appealing.
One is the "graceful exit" from inflation. People like Andrei Linde who dream up "inflaton" inflation scenarios have a terrible problem arranging for inflation to stop.
First they have to fantasize some exotic matter field never before seen in the real world just to get inflation to begin in the first place, and then they have to go into contortions about special "slow roll" down a potential slope so that inflation will stop. It is all very ad hoc.
Weinberg (and also Reuter and Bonanno) seem to have an inflation mechanism that is the opposite of ad hoc. Everything happens naturally, has to happen, because of the running of the main constants. And you even get a "graceful exit" to inflation.

The other thing that seems so appealing is that it doesn't use any machinery that we don't have already. It is very OCCAM. Occam says not to increase the number of entities in your model (if you can avoid it.) And we already have these two entities Newton constant and Lambda. They are the two main constants that occur in Einstein's basic GR equation. Just those two and the speed of light c (if you want to count that instead of setting it to equal one.)

Weinberg spoke about this in his 6 July CERN talk, which is on video. He said he had a renewed research interest in Asymptotic Safe QG, and particularly its application to cosmology. So we have been waiting for exactly this paper to appear. And now a little over four months later, here it is.

That 6 July talk was really enlightening. Especially the last 12 minutes, which you can get to by dragging the time button to minute 58. I should get the link. Here's the video:
http://cdsweb.cern.ch/record/1188567/
and here's a link to get slides PDF if desired:
http://indico.cern.ch/conferenceDisplay.py?confId=57283

Sauron

It is curious how few attention has got a seemingly challenging paper made by such an eminent physicist.

Bob_for_short

It was made public three days ago. What do you want?

hamster143

The whole idea of an asymptotically safe QG is a big stretch. (Although, whether it's a bigger stretch than the 100+ parameter SUSY, I cannot say.)

marcus

Let me review. Weinberg lectured about asym.safeQG in 1976, published about it in 1979, the year he was awarded Nobel. Then early this year he posted a paper showing renewed interest in it and (as I recall) mentioning relevance to cosmo. That was in MARCH, http://arxiv.org/abs/0903.0568
(he described "his personal view" at that point.)
THEN PERIMETER decided to have a conference on on AsymSafe QG and discussed it with
Weinberg and Weinberg said he would attend.
THEN AT CERN ON 6 July he gave a talk opening a conference, with a lot of string theorists among others, which he
1. explained his renewed interest in AS, why he thinks it is worth investigating
2. said his own research was on AS and described the application to cosmo, explaining inflation
3. said string might not be needed for unification, and might not be how the world is.
4. sketched the history of "good old quantum field theory" and suggested that periodically it plateaus and people look at alternatives and then they abandon the alternatives and QFT advances to another plateau. He sketched a rough picture of the "stock market" for QFT over time and it looked like a flight of steps, ready to take another rise.
5. mentioned the Perimeter conference on AS that he would be attending in November.

Now I would say that the whole thing went very smoothly. Most of the "news" got around quietly between March and July, and whatever shift and adjustment happened without provoking any reaction. Now we are in a different situation, different mental climate. There has been a large recent increase in attention to AsymSafe QG. Attention in the form of people doing research and attending the conference, and visibility.

Hamster, I'm curious why you see it subjectively as a "big stretch". What about it seems to you difficult to accomodate mentally?

Essentially no new concepts or mathematical entities are required, it uses the machinery already in place as of, say, 1980.
The basic issue is whether or not the renormalization flow has a UV fixed point, on a finite dimensional critical surface.
Since Reuter's 1998 paper, evidence that it has such a fixed point gradually built up until, this year, the situation reached a tipping point.

Conceptually it seems pretty simple, so I don't understand why you see it as a big mental stretch. Please explain.

hamster143

It's not just that. It's the question whether we live in a universe that lies on the critical surface. Since the critical surface is most likely finite-dimensional and the space of all couplings is infinite-dimensional, the a priori probability that we actually live in such universe is zero. It would require either some not-as-of-yet-understood mechanism that puts the gravity in the UV fixed point, or the incredible amount of fine-tuning, to justify this scenario.

Secondly, it's skirting the big issue: even if QFT is valid beyond Planck scale, why is it set up the way it is (SU(3)xSU(2)x1, three generations, etc.).

Fortunately, the theory is, in principle, falsifiable, if we measure enough couplings and prove that the point we get is not on the trajectory that leads to the fixed point. Unfortunately, we only know one coupling with any degree of certainty (1 in 10^5) and most of them are suppressed by powers of Plank mass, and precise measurements of those couplings are not in the cards in the foreseeable future.

marcus

Sounds like you are imagining things in terms of a multiverse. AsymSafe doesn't connect with multiversy thoughts. The universe is not supposed to "lie on the critical surface".

The story here is basic 1970s Ken Wilson stuff, the renormalization group flow belonging to our one universe. There are no alternative universes or probabilities such as you seem to be imagining.


This doesn't connect with the discussion in this thread, so it does not call for a response.

hamster143

This has nothing to do with a multiverse. The question is, what happens if we take the set of coupling values in our universe and use renormalization group to take the UV limit? A priori, there's no reason to expect that we get into the fixed point, because the subspace of initial values that lead to the fixed point is measure zero in the space of all possible initial values. Contrast with QCD, where you can start with any value of coupling constant and you'll inevitably end up in the UV fixed point.

Weinberg is basically saying "look, gravity COULD be a renormalizable theory..." because, if the initial conditions are on the invariant surface, renormalization flow does not blow up near Planck mass. But it's hard to justify why it SHOULD be renormalizable, precisely because Wilson & such have disposed with the requirement.

Finbar

Hamster makes an interesting point. But the reason we know QCD flows to the fixed point is because we know the bare action and hence the underlying degrees of freedom. In the RG approach to asymptotic safety we only have average effective action and not the bare action.

If we had the bare action and then said "this is my theory of gravity and look its asymptotically safe" then we would be in the same situation as QCD.

Haelfix

Even more problematic, the critical surface could be spurious. You have to show universality, existence and stability across every type of approximation (1/epsilon expansions, truncation schemes etc) and inclusion of almost infinitely different matter terms. That is, the space of couplings most likely changes as you progressively refine your theory. No one knows how to even approach proving these things in general, so the state of the art is simply numerical investigations on various different types of simple toy models.

Even if you can show all of those things, and thats the way the world works, the problem for model builders remains the same. Eg you still lack general predictivity without actually doing the experiments that fixes the actual couplings perse (and determining whether you are or are not on the critical surface). And we are back to the problem of having to build galaxy size accelerators to pin down the dynamics of quantum gravity.

The original motivation for dropping field theory in the first place, was this generic futility argument. It didn't really matter if you found a plausible candidate theory of quantum gravity and figured out all the general principles. B/c unless you had some extra local symmetry group acting on your theory (say lots of SuSY or perhaps a conformal symmetry) you couldn't in general pin everything down uniquely. That was why string theory became so dominant, b/c there everything is fixed by consistency constraints and if you could figure out the solution, you had a unique theory across all energy scales.

Then there is the more theoretical issues (such as the wrong scaling behaviour of field theories in blackholes) and so forth.

hamster143

We don't know even that for sure. We know that a "toy QCD" with no dimension 5+ operators flows to the fixed point. There could be (there probably are) dimension 5+ operators suppressed by powers of some energy scale, and, once we reach that energy scale, QCD can do anything it wants. Below that scale the flow is in fact in the direction of a trivial UV fixed point. We tend to ignore higher-dimension operators because there's a lot of interesting dynamics even without them.

In GR, dimension 5+ operators are essential.

Finbar

unique theory? string theory? eh?

I don't follow you Haelfix?

And that black hole argument is rather silly...

Haelfix

Eh? String theory is completely unique. There are no independant adjustable free parameters. You can't 'tweak' the theory by adding new couplings or new matter content.

The blackhole scaling argument is not 'silly' either! You have a major problem when general thermodynamic arguments implies degrees of freedom that scale as the area, whereas your general prediction for any *local* field theory are volume degrees of freedom.

Finbar

Indeed, QCD should be viewed as an effective theory and so should asymptotically safe gravity(if it exists) otherwise we won't get unification. The point with asymptotic safety is that we don't need to go beyond QFT or the symmetries we are aware of in nature to quantise gravity. Its just a call to Occam's razor.

Finbar

So your telling me there is only one string theory, one vacuum? I'm not so sure.


It is a silly argument. The argument is that all QFT should be conformal at there UV fixed point. But an event horizon is not at the fixed point. Clearly an event horizon is an IR property as I can have an arbitrarily large black hole. If I could have a black hole with a radius of well under the Planck length then it would be a problem. But asymptotic safety predicts that black holes cannot have a mass of less than the Planck mass. So at the UV fixed point there is no horizon and hence no contradiction.

Haelfix

I can't decipher your last post. A few remarks.

1) There are many QFTs, an infinite space of possible theories. Now, each particular QFT typically has a single vacuum (or possibly multiple meta stable vacuums). String theory is the opposite. There is one single theory, but it has a large, possibly infinite amount of classical solutions. This isn't just pedantic, its a big difference. So for instance you can never talk about the critical surface of coupling constants in string theory, b/c that surface is a universal point.

2) "An event horizon is not at the fixed point", "At the UV fixed point there is no horizon", "But asymptotic safety predicts that black holes cannot have a mass of less than the Planck mass"

Those sentences don't make much sense... Nor do they have anything to do with the scaling properties of field theories.