# Inflationary paradigm in trouble after Planck2013 ? I doubt it, but

1. Apr 10, 2013

### mitchell porter

"Inflationary paradigm in trouble after Planck2013"? I doubt it, but..

After all those reports that Planck2013 gave us a picture of the early universe perfectly consistent with a simple form of inflation, slow-roll inflation, now comes a preprint saying that the fine print is problematic:

http://arxiv.org/abs/1304.2785
Title: Inflationary paradigm in trouble after Planck2013
Authors: Anna Ijjas, Paul J. Steinhardt, Abraham Loeb

Abstract: The recent Planck satellite combined with earlier results eliminate a wide spectrum of more complex inflationary models and favor models with a single scalar field, as reported in the analysis of the collaboration. More important, though, is that all the simplest inflaton models are disfavored by the data while the surviving models -- namely, those with plateau-like potentials -- are problematic. We discuss how the restriction to plateau-like models leads to three independent problems: it exacerbates both the initial conditions problem and the multiverse-unpredictability problem and it creates a new difficulty which we call the inflationary "unlikeliness problem." Finally, we comment on problems reconciling inflation with a standard model Higgs, as suggested by recent LHC results. In sum, we find that recent experimental data disfavors all the best-motivated inflationary scenarios and introduces new, serious difficulties that cut to the core of the inflationary paradigm. Forthcoming searches for B-modes, non-Gaussianity and new particles should be decisive.

I have only skimmed the paper, but I'm going to go out on a limb and say that the arguments will turn out to be dubious. They don't seem to be based just on calculation, it's more like "here are our theoretical prejudices, and our theoretical prejudices imply that inflation should work differently from what we see... so let's forget inflation and look for a new cosmology".

Could someone better versed in inflationary theory have a look and confirm my prejudices? :-)

2. Apr 10, 2013

### marcus

Thanks for posting this, Mitchell. I was just now checking the new arxiv postings, saw it, and came over here directly. I have a lot of respect for Abraham Loeb, Steinhardt too.

I think the paper is likely to be valuable/important even if they are only showing severe constraint on SOME inflation scenarios. There may be other scenarios that they are ignoring which evade their strictures. But that is all to the good! At least they would be doing us a favor by thinning out the field of stuff that has to be considered (if their critique is sustained.)

3. Apr 11, 2013

### Garth

Inflationary paradigm in trouble after Planck2013 Anna Ijjas, Paul J. Steinhardt, Abraham Loeb
I have always favoured consideration of alternatives!

Three interesting problems:
1. A dangerous new type of initial conditions problem.
2. A twist on the multiverse problem.
3. An inflationary unlikeliness problem.

1. A dangerous new type of initial conditions problem.
2. A twist on the multiverse problem.
3. An inflationary unlikeliness problem.
Garth

Last edited: Apr 11, 2013
4. Apr 11, 2013

### TrickyDicky

Yeah, well the truth is that inflation was from the start more of a wishful thinking problem-solver prejudice for cosmologists than a serious scientific paradigm. And even before the Planck data Steinhardt(that was one of the initial proponents of inflation) was making evident the problems of the inflationary paradigm.
There are few things more dangerous for scientists than having "pet" theories. Fortunately observations have the last word.

5. Apr 11, 2013

### Garth

If it is the case that the Inflationary paradigm is "doomed" how are we to resolve the horizon, smoothness, density and magnetic monopole problems of GR cosmology?

Garth

6. Apr 11, 2013

### TrickyDicky

Flatness and horizon problems are seen as no-problems by a few cosmologists since they were formulated 40 years ago, these are "why" type of problems. Why was density, or homogeneity, or .... so finely tuned? They dissolve if you take the cosmological principle as a given, and if one is to take the FRW metric from GR as starting point to build a cosmology, those critical parameters should be taken as given parameters. Besides "why" type of questions are usually frowned at in physics, I don't know why these particular problems worry cosmologists so much.
See for instance in WP "Flatness problem" page:
"Some cosmologists agreed with Dicke that the flatness problem was a serious one, in need of a fundamental reason for the closeness of the density to criticality. But there was also a school of thought which denied that there was a problem to solve, arguing instead that since the universe must have some density it may as well have one close to $\rho_{crit}$ as far from it, and that speculating on a reason for any particular value was "beyond the domain of science". And reference therein:

The monopoles is a different thing, but note that this problem is dependent on a very specific narrative of the hot initial conditions that is built on a highly speculative part of particle physics, rather than a direct prediction of GR.

Confront these "problems" with the missing mass problem and its mainstream solution cold dark matter. Now that is what I'd call a real problem of GR cosmology.

Last edited: Apr 11, 2013
7. Apr 11, 2013

### Chalnoth

I'd honestly be surprised if we didn't simply end up with a simple $\phi^2$ inflationary model in the end, even including Planck's latest results.

8. Apr 11, 2013

### bapowell

There's also the possibility that the inflaton generates only a small fraction of the perturbation amplitude, so that measurements of the power spectrum do not directly constrain V. Also, the cited Planck constraints apply to single field slow roll inflation -- broaden your zoology to include non-canonical models, those non-minimally coupled to gravity, those with spectator fields, etc. and you'll likely find that there's absolutely nothing wrong most polynomial potentials.

EDIT: I'll add that there is a certain other leading early universe model that is altogether (suspiciously) absent from this paper that is "also" on the ropes after Planck...

9. Apr 11, 2013

### Garth

Hi Tricky,

I cannot see how the flatness problem is a "no-problem".

In a universe that decelerates as it expands the actual density of the universe at a particular epoch diverges from the critical density at that epoch, if it is greater than $\rho_{crit}$ then the actual density will grow larger, if it is less then it will decrease from the critical density at the same epoch.

If the universe accelerated in its expansion (throughout the greater part of its history and not just in the present DE epoch) then the actual density would have approached the critical density as the universe expands.

The universe has expanded by something of the order of 1060 since Planck time and so one expects the actual density to be a factor of something of that order away, higher or lower depending on whether the initial density was higher or lower than $\rho_{crit}$ at Planck time.

For example, if the numbers, for the sake of argument, were inverted and the critical density today worked out to be about 10+30 gms/cc and the actual density was measured at about 10-30 gms/cc then that would be explained by decelerating expansion since Planck time. The fact that they are equal, or thereabouts, is a great problem of coincidence.

Inflation solved the problem by reversing the effect by having a tremendous burst of accelerating expansion of the order of over 1060 in a fraction of a second thus forcing the actual density down onto the critical density at that time so closely that subsequent decelerating expansion has not been able to force them apart.

Without inflation we are left with a great coincidence, which either is another Anthropic coincidence (we wouldn't be here if the universe were much denser, it would all be BHs and too short-lived or something,) or the decelerated expansion did not happen in the first place. The present DE acceleration helps but has not gone on long enough to resolve the coincidence.

That is why I am also prepared to consider the coasting model as an alternative to $\Lambda$CDM model.

Garth

Last edited: Apr 11, 2013
10. Apr 11, 2013

### TrickyDicky

But coincidences have to do with a -priori expectations and these may be wrong. Ultimately science is not about solving the reason for a coincidence, it's about modelling what we observe in theories with predictive power.
The model we use, the FRW universe is compatible with a flat spatial universe for instance, if we take that as a given, the universe has always had the critical density, No need for finetuning and no flatness problem.

11. Apr 11, 2013

### bapowell

So are you saying that science is not concerned with understanding why the universe is flat simply because we happen to have an effective model that presumes it?

12. Apr 11, 2013

### Garth

Yes, if you could fix the curvature to be always flat then there would be no problem.

However it is normally assumed it is cosmological density that determines the curvature, not the other way round....

Garth

Last edited: Apr 11, 2013
13. Apr 11, 2013

### TrickyDicky

Actually it is not me who says that, it is one of the mottos of these forums as I have had the oportunity to verify that "why" questions or ultimate causes questions and ontological and epistemological questions in general belong to the realm of philosophy (a discipline not so popular around here) and to qualify as science should be converted to "how" questions and answered by effective models that basically describe and predict instead of indulging in "why" philosophies.

14. Apr 11, 2013

### bapowell

I don't see how "how" questions can only be answered via effective models. I don't see why you presuppose that questions regarding the flatness of the universe -- which is a dynamical property -- are somehow beyond the epistemological purview of science. Yeah, dragons before the big bang? Who knows. But why (or if you would prefer, "how is it that...") the curvature of the universe today is so close to flat is absolutely an empirical question.

15. Apr 11, 2013

### TrickyDicky

Yes you are right about that in general except in the specific case of density parameter exactly equal to one where it is not relevant wich one determines the other.

16. Apr 11, 2013

### TrickyDicky

How are questions about the pertinence of certain supposed(but not known and maybe not knowable) initial conditions of the universe absolutely empirical? They are not inmediately empirical (although they can be tackled with the help of actual empirical questions and always depending on the theoretical model applied) . Empirical questions are: what is the current universe energy density?, or what is the curvature of the spatial hypersurface?

17. Apr 11, 2013

### TrickyDicky

The bottom line is that turning puzzlement about initial conditions into a paradigm like the inflationary one (or the ekpyrotic for that matter) in a model that is "doomed" by an initial singularity which is like the mother of all unexplainable initial conditions is like putting the cart before the horse. Even more so when the "solution" creates many more initial conditions problems than it solves as Steinhardt and others before make clear.

18. Apr 11, 2013

### bapowell

Well, yeah. Making an observation today and comparing it with the predictions of a theoretical model is what I have in mind. Using this approach to understand why today's universe is so flat given the instability of this solution has implications for the nature of the early universe. If inquiry into the nature of the initial curvature is off limits, then so should be questions regarding the initial perturbations. Should we just "accept" the spectrum as an initial condition that we plug into our effective model? Or should we try to understand it?

19. Apr 11, 2013

### TrickyDicky

I'm not saying that initial curvature inquiry is off limits. We should always try to understand empirical data of course. The starting point of this thread is the Planck data. I simply warned against putting too much hope in certain maybe too artificial models and that some authors that I referenced in a previous post consider that the perceived flatness and horizon "problems" are out of the scope of science. I don't think that implies that we shouldn't try to understand anything.

20. Apr 11, 2013

### bapowell

No, I get your gripe, and I mostly agree with the spirit. I'm just trying to understand why, say, the horizon and flatness problems are maybe out of the scope of science while the origin of the density perturbation is not.