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

[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? :-)

 

[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.)

 

[Garth]

Inflationary paradigm in trouble after Planck2013 Anna Ijjas, Paul J. Steinhardt, Abraham Loeb

7. Discussion
In testing the validity of any scientific paradigm, the key criterion is whether measurements agree with what is expected given the paradigm. In the case of inflationary cosmology, this test can be divided into two questions: (A) are the observations what is expected, given the inflaton potential X?; and (B) is the inflaton potential X that fits the data what is expected according to the paradigm?. In order to pass, both questions must be answered in the affirmative.
For Question A, the standard approach is straightforward: compare the data to the model predictions (assuming classical evolution and ignoring eternal inflation). The literature is full of analyses of this type. Question B is considered less often, though it is no less important. It is answered by a combination of internal logic and quantifiable tests. In this paper, we have focused on tests that are quantifiable using Planck2013 data.
The Planck2013 analysis, like many previous analyses of cosmic parameters, focused on Question A. Based on tighter constraints on flatness, the power spectrum and spectral index, and non-Gaussianity, the conclusion from Planck2013 was that single-field plateau-like models are the simplest that pass and they pass with high marks.
However, our focus in this paper has been Question B – are plateau-like models expected, given the inflationary paradigm? Based on the very same tightened constraints from Planck2013, we have identified three independent issues for plateau-like models: a dangerous new type of initial conditions problem, a twist on the multiverse problem, and, for the first time, an inflationary unlikeliness problem. The fact that a single data set has been able to expose three new problems is a tribute to the quality of the experiment and serious trouble for the paradigm, for the only reasonable conclusion from Planck2013 is that the answer to Question B for inflationary cosmology is negative.
The situation suggests a new view of future data opportunities. At present, the three new problems arise because of conflicts between prediction and observation at the 2−3 level. Future data can amplify or diffuse them. Detecting tensor modes and pushing the limits on non-Gaussianity further downward would ease the problems. On the other hand, not detecting tensor modes or detecting non-Gaussianity would each represent yet another threat which, combined with the three problems identified in this paper, spell doom for the inflationary paradigm and encourage consideration of alternatives.

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.

In sum, by favoring only plateau-like models, the Planck2013 data creates a serious new challenge for the inflationary paradigm: the universally accepted assumption about initial conditions no longer leads to inflation; instead, inflation can only begin to smooth the universe if the universe is unexpectedly smooth to begin with!

2. A twist on the multiverse problem.

Planck2013 results lead to a new kind of multiverse problem that is independent of the initial conditions and unlikeliness problems described above. The plateau-like potentials selected
by Planck2013 are in the class of eternally inflating models, so the issue cannot be avoided. That being the case, it is surprising in a multiverse that data agrees so precisely with the naive predictions derived by totally ignoring the multiverse and assuming purely uniform slow-roll down the potential.

3. An inflationary unlikeliness problem.

All inflationary potentials are not created equal. The odd situation after Planck2013 is that inflation is only favored for a special class of models that is exponentially unlikely according
to the inner logic of the inflationary paradigm itself. The situation is independent of the initial conditions problem described above; even assuming ideal conditions for initiating inflation, the fact that only plateau-like models are favored is paradoxical because inflation requires more tuning, occurs for a narrower range of parameters, and produces exponentially less plateau-like
inflation than the now-disfavored models with power-law potentials. This is what we refer to as the inflationary “unlikeliness problem.”

Garth

 

[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.

https://www.youtube.com/watch?v=EYPapE-3FRw

 

[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

 

[TrickyDicky]

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

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:
http://books.google.com/?id=OIG0F37QrmQC&pg=PT237&lpg=PT237 dq=%22flatness+problem+was%22


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.

 

[Chalnoth]

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? :-)

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.

 

[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...

 

[Garth]

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.

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

 

[TrickyDicky]

Without inflation we are left with a great coincidence...

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.

 

[bapowell]

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.

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?

 

[Garth]

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.

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

 

[TrickyDicky]

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?

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.

 

[bapowell]

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.

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.

 

[TrickyDicky]

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...

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 which one determines the other.

 

[TrickyDicky]

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.

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?

 

[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.

 

[bapowell]

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)

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?

 

[TrickyDicky]

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?

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.

 

[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.

 

[TrickyDicky]

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.

I myself found that statement in the quote from the WP a bit strong when I read it.

 

[John86]

http://arxiv.org/abs/1304.3122
Planck 2013 results support the simplest cyclic models
Jean-Luc Lehners, Paul J. Steinhardt
(Submitted on 10 Apr 2013)
We show that results from the Planck satellite reported in 2013 are consistent with the simplest cyclic models for natural parameter ranges i.e., order unity dimensionless coefficients, assuming the standard entropic mechanism for generating curvature perturbations. With improved precision, forthcoming results from Planck and other experiments should be able to test the parameter ranges by confirming or refuting the core predictions - i.e., no observable primordial B-mode polarization and detectable local non-gaussianity. A new prediction, given the Planck 2013 constraints on the bispectrum, is a sharp constraint on the local trispectrum parameter g_{NL}; namely, the simplest models predict it is negative, with g_{NL} < -1700.

 

[marcus]

http://arxiv.org/abs/1304.3122
Planck 2013 results support the simplest cyclic models
Jean-Luc Lehners, Paul J. Steinhardt
(Submitted on 10 Apr 2013)
We show that results from the Planck satellite reported in 2013 are consistent with the simplest cyclic models for natural parameter ranges i.e., order unity dimensionless coefficients, assuming the standard entropic mechanism for generating curvature perturbations. With improved precision, forthcoming results from Planck and other experiments should be able to test the parameter ranges by confirming or refuting the core predictions - i.e., no observable primordial B-mode polarization and detectable local non-gaussianity. A new prediction, given the Planck 2013 constraints on the bispectrum, is a sharp constraint on the local trispectrum parameter g_{NL}; namely, the simplest models predict it is negative, with g_{NL} < -1700.

This is a useful and highly cogent paper. Among the helpful clarifications was this short bit on page 2:
==quote Lehners Steinhardt==
The big bounce has been modeled in two inequivalent ways so far: in the first class of models the bounce is classically non-singular in the sense that the scale factor a reverses from contraction to expansion before reaching zero. Such models typically rely on higher-derivative kinetic terms [12–15]. In the second class of models, which were originally motivated by the braneworld picture suggested by string theory, the bounce corresponds to the collision of orbifold branes...
==endquote==
It appears to me that Steinhardt is distancing himself from the "stringy" or string-inspired editions of cyclic/ekpyrotic (throwing brane-bump under the bus, one could say) and emphasizing the version where the scale-factor a(t) turns around before reaching zero. This is more like what happens in the LQG cosmology bounce, where contraction rebounds when a critical density is reached.
We should list Lehners Steinhardt references [12-15] since they have to do with the non-branebump class of cyclic models.[12] E. I. Buchbinder, J. Khoury, and B. A. Ovrut, Phys. Rev.D76, 123503 (2007), http://arxiv.org/abs/hep-th/0702154.
[13] Y.-F. Cai, D. A. Easson, and R. Brandenberger, JCAP1208, 020 (2012), http://arxiv.org/abs/1206.2382.
[14] M. Osipov and V. Rubakov (2013), http://arxiv.org/abs/1303.1221.
[15] T. Qiu, X. Gao, and E. N. Saridakis (2013), http://arxiv.org/abs/1303.2372.

New Ekpyrotic Cosmology
Evgeny I. Buchbinder, Justin Khoury, Burt A. Ovrut
(Submitted on 20 Feb 2007)
In this paper, we present a new scenario of the early Universe that contains a pre big bang Ekpyrotic phase. By combining this with a ghost condensate, the theory explicitly violates the null energy condition without developing any ghost-like instabilities. Thus the contracting universe goes through a non-singular bounce and evolves smoothly into the expanding post big bang phase. The curvature perturbation acquires a scale-invariant spectrum well before the bounce in this scenario. It is sourced by the scale-invariant entropy perturbation engendered by two ekpyrotic scalar fields, a mechanism recently proposed by Lehners et al. Since the background geometry is non-singular at all times, the curvature perturbation remains nearly constant on super horizon scales. It emerges from the bounce unscathed and imprints a scale-invariant spectrum of density fluctuations in the matter-radiation fluid at the onset of the hot big bang phase. The ekpyrotic potential can be chosen so that the spectrum has a "red'' tilt, in accordance with the recent data from WMAP. As in the original Ekpyrotic scenario, the model predicts a negligible gravity wave signal on all observable scales. As such "New Ekpyrotic Cosmology" provides a consistent and distinguishable alternative to inflation to account for the origin of the seeds of large scale structure.
41 pages, 4 figures, published in PRD

Towards a Nonsingular Bouncing Cosmology
Yi-Fu Cai, Damien A. Easson, Robert Brandenberger
(Submitted on 11 Jun 2012)
We present a nonsingular bouncing cosmology using single scalar field matter with non-trivial potential and non-standard kinetic term. The potential sources a dynamical attractor solution with Ekpyrotic contraction which washes out small amplitude anisotropies. At high energy densities the field evolves into a ghost condensate, leading to a nonsingular bounce. Following the bounce there is a smooth transition to standard expanding radiation and matter dominated phases. Using linear cosmological perturbation theory we track each Fourier mode of the curvature fluctuation throughout the entire cosmic evolution. Using standard matching conditions for nonsingular bouncing cosmologies we verify that the spectral index does not change during the bounce. We show there is a controlled period of exponential growth of the fluctuation amplitude for the perturbations (but not for gravitational waves) around the bounce point which does not invalidate the perturbative treatment. This growth induces a natural suppression mechanism for the tensor to scalar ratio of fluctuations. Moreover, we study the generation of the primordial power spectrum of curvature fluctuations for various types of initial conditions. For the pure vacuum initial condition, on scales which exit the Hubble radius in the phase of Ekpyrotic contraction, the spectrum is deeply blue. For thermal particle initial condition, one possibility for generating a scale-invariant spectrum makes use of a special value of the background equation of state during the contracting Ekpyrotic phase. If the Ekpyrotic phase is preceded by a period of matter-dominated contraction, the primordial power spectrum is nearly scale-invariant on large scales (scales which exit the Hubble radius in the matter-dominated phase) but acquires a large blue tilt on small scales.
21 pages, 11 figures, JCAP08(2012)020

Galileon bounce after ekpyrotic contraction
M. Osipov, V. Rubakov
(Submitted on 5 Mar 2013)
We consider a simple cosmological model that includes a long ekpyrotic contraction stage and smooth bounce after it. Ekpyrotic behavior is due to a scalar field with a negative exponential potential, whereas the Galileon field produces bounce. We give an analytical picture of how the bounce occurs within the weak gravity regime, and then perform numerical analysis to extend our results to a non-perturbative regime.
15 pages, 5 pictures

Towards Anisotropy-Free and Non-Singular Bounce Cosmology with Scale-invariant Perturbations
Taotao Qiu, Xian Gao, Emmanuel N. Saridakis
(Submitted on 10 Mar 2013)
We investigate non-singular bounce realizations in the framework of ghost-free generalized Galileon cosmology, which furthermore can be free of the anisotropy problem. Considering an Ekpyrotic-like potential we can obtain a total Equation-of-State (EoS) larger than one in the contracting phase, which is necessary for the evolution to be stable against small anisotropic fluctuations. Since such a large EoS forbids the Galileon field to generate the desired form of perturbations, we additionally introduce the curvaton field which can in general produce the observed nearly scale-invariant spectrum. In particular, we provide approximate analytical and exact semi-analytical expressions under which the bouncing scenario is consistent with observations. Finally, the combined Galileon-curvaton system is free of the Big-Rip after the bounce.
13 pages, 9 figures

 

[John86]

http://arxiv.org/abs/1304.2785
Inflationary paradigm in trouble after Planck2013
Anna Ijjas, Paul J. Steinhardt, Abraham Loeb
(Submitted on 9 Apr 2013)
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.

 

[John86]

Here is a paper that comes to some similar conclusions, as described in the publications by Steinhardt en co.

http://arxiv.org/abs/1304.4358
Reexamination of inflation in noncommutative space-time after Planck results
Nan Li, Xin Zhang
(Submitted on 16 Apr 2013)
An inflationary model in the framework of noncommutative space-time may generate a nontrivial running of the scalar spectral index, but usually induces a large tensor-to-scalar ratio simultaneously. With the latest observational data from the Planck mission, we reexamine the inflationary scenarios in a noncommutative space-time. We find that either the running of the spectral index is tiny compared with the recent observational result, or the tensor-to-scalar ratio is too large to allow a sufficient number of $e$-folds. As examples, we show that the chaotic and power-law inflation models with the noncommutative effects are not favored by the current Planck data.

 

[skydivephil]

Can anyone explain what the Galilean cosmology is? I seem to see this phrase popping up from time to time and in the above cited paper .

 

[John86]

what could follow when cosmological models inhabetating inflation will not show up in the coming years ?

Do we have reasonable worked out alternative ideas that will do the job ?

 

[Garth]

what could follow when cosmological models inhabetating inflation will not show up in the coming years ?

Do we have reasonable worked out alternative ideas that will do the job ?

If it is concluded that inflation cannot be made to work then other answers to the horizon, density, smoothness and magnetic monopole problems will have to be found.

As Tricky has suggested if space is always made to be flat because of some as yet unknown mechanism then the density problem would not exist, the actual density would be forced to be the critical density.

However if the universe had not decelerated over 10⁷⁰ decades of its existence, from 5 X 10^-44 secs. after BB to ~ 10²⁸ secs. then the horizon, density, smoothness and magnetic monopole problems would not have existed in the first place.

One such model is called A coasting cosmology model. (Aka Linearly Expanding model)

To get such an expansion you would have to have either:

1) An empty universe - the Milne model - obviously only an approximation to the real universe! (But possibly an asymptotic limit of the expanding universe)

2) A universe in which the gravitational attraction of matter is canceled out by (hypothetical) gravitational repulsion of and equal amount of anti-matter - the Dirac-Milne universe: Introducing the Dirac-Milne universe.

3) A universe in which the equation of state (eos) is $p = - \frac {1}{3} \rho c^{2}$.
Kolb - in the paper A coasting cosmology suggested the universe might be dominated by a form of Dark Energy he called "K matter".

However the problem would be to get such a form of DE to always produce $p = - \frac {1}{3} \rho c^{2}$. In general the eos would change with expansion, such as when the universe changed from a radiation dominated to a matter/DM/DE dominated phase.

A suggestion made by another published theory, A New Self Creation Cosmology, connects a Brans-Dicke type scalar field to matter and then connects the creation of matter to the scalar field. This appears to produce the required eos at all epochs.

Although there are problems fitting the coasting cosmology model to observations, some believe otherwise, Cosmological Constraints on a Power Law Universe.

Linearly coasting cosmology is comfortably concordant with a host of cosmological observations.

I would also point out that there were problems getting the standard model to fit, which was only resolved (if it has been) by the invocation of Inflation, DM and DE, which are all unknown in laboratory physics.

If an equal amount of such speculative imagination were applied to the coasting cosmology model then perhaps that would fit as well!

Just a thought,
Garth

 

[Chalnoth]

One such model is called A coasting cosmology model. (Aka Linearly Expanding model)

This model isn't consistent with the observational evidence. It has problems with structure formation, the matter density in the observable universe, and the expansion history.

The reason why the Planck results are potentially troubling for some inflation models is because no tensor perturbations were detected (yet). Polarization data from Planck should shine a brighter light on this issue, and dedicated polarization missions like EBEX may do an even better job (EBEX flew pretty recently, and I'm really interested to see what the results are).

But regardless, the fact of the matter is that there are a lot of inflation models which don't predict much of any tensor perturbations, and there are some alternatives to inflation which also do not (e.g. the LQC bouncing models).

 

[skydivephil]

From people that I have spoken to that work on PLanck they don't expect to see the B mode because its not designed to detect it. They hope they will get lucky but no one is betting on it.
Waht they really need is a dedicated mission for that , see here for example:
http://arxiv.org/abs/1102.2181
they have a website here:
http://www.core-mission.org/documents.php
I don't think there's any hint of money for such a mission yet and even then its usally 10 years between approval and flight.
So let's hope Planck does get luck or one of these baloon experiments can do something, I certainly hope so but think we might be waiting longer a fiar bit longer.

 

[Chronos]

Two CMB b mode polarization experiments are currently underway - EBEX and POLARBEAR.

 

[Chalnoth]

From people that I have spoken to that work on PLanck they don't expect to see the B mode because its not designed to detect it. They hope they will get lucky but no one is betting on it.

Yes, this is correct. A rather high tensor amplitude is at the very limit of detectability of the instrument, but the simulations which say that don't take into account realistic systematic errors or the problems of component separation. So a detection of the B modes would be exceptional indeed.

What I was mostly referring to was the E-mode polarization, which Planck should be able to detect at a higher precision than previous experiments. While this is a dirty measurement of the tensor amplitude, it should provide at least some improvement in the constraints.

 

[mitchell porter]

Andrei Linde strongly criticizes "Inflationary paradigm in trouble after 2013", in this talk.

 

[skydivephil]

Two CMB b mode polarization experiments are currently underway - EBEX and POLARBEAR.

Do you know when we can expect from results from these?

 

[Chronos]

The first science run of EBEX was launched in late December 2012. POLARBEAR became operational in January 2013 and CMB polarization measurements are scheduled to begin in April 2013. No schedule for data release has been announced for either project, AFAIK.