Slava Mukhanov can stick to his guns

Rubrique : Curiositêtes (#2)
\\Ce billet a été révisé le 08/02/2015

How many time is history repeating ?

Slava Mukhanov, another old-school Russian physicist ... was one of the first to realise that inflation wouldn't just cause the universe to expand dramatically and to make it more homogeneous, it would also seed new fluctuations with a very small amplitude. These new, small, fluctuations arise from the stretching (and eventual amplification) of quantum fluctuations in the field driving inflation. This type of realisation was what took inflation from an interesting concept to a testable paradigm. With satellites like Planck those perturbations are now being ever more precisely examined. 

Mukhanov has a very different perspective to [Alexeï] Linde regarding what inflation can or cannot explain. To him the question of whether a theory is scientific or not comes down to one thing and one thing only: has it made unique a priori predictions that can then either be verified or used to rule out the theory? From that perspective his view is that inflation has only ever predicted one set of results and those are the predictions of the first, simplest models of inflation. He makes no distinction as to whether those models are well described by a quantum field theory model or not. 

[...] Muhkanov deserves credit for at the very least sticking religiously to his guns. He likes to show slides during talks like this that were written on overhead transparencies in the early 90's. This dates these slides to an era before the anisotropies in the CMB were discovered, before the late-time accelerated expansion was discovered and a time when the total observed mass in the universe was indicating that the curvature in the universe might be significant (i.e. in technical terms it would be "open"). These slides make a number of specific predictions for what inflation requires (by Mukhanov's definition of inflation).

• A flat universe (i.e. no curvature)
• Perturbations that had a Gaussian distribution
• Perturbations that were almost scale-invariant, but not quite (they would need to have a slightly larger amplitude at larger scales)
• Perturbations that were adiabatic (i.e. all the constituents of the universe were perturbed in the same way)
• A small, but not insignificant quantity of primordial gravitational waves

How many of these predictions have now been verified?

All but one.

The reason why Mukhanov deserves credit is that at two separate points in history at least one of these predictions has been in serious jeopardy. [...] when Mukhanov was first writing these predictions down, there seemed to be some evidence that the universe was open. At that time, some inflationary theorists (Linde amongst them) were trying to construct models of inflation that could generate an open universe. Mukhanov said in his talk that at this point of history he was considering leaving cosmology because he believed inflation could not survive as a predictive science if the universe was open. It turned out that those tentative hints of openness were actually the first evidence of the consequences of the accelerated expansion and that the universe is flat.  

Then, last decade the WMAP satellite was showing not insignificant evidence for a large degree of "non-Gaussianity" in the CMB. If that had been verified, Linde's inflation would have survived (after all it can explain anything), but Mukhanov would have pronounced inflation dead. Planck showed that WMAP's evidence was only a statistical fluctuation and that, to Planck's accuracy, there is no evidence for primordial non-Gaussianity.

Mukhanov's view of inflation seems to be surviving quite well.

There is that one missing piece though. These are primordial gravitational waves.

The universe as seen by Planck - Days Three and Four I

Posted by Shaun Hotchkiss April 8, 2013

(Having) Great expectations (but not too great)

Although primordial gravitational waves are not yet detected, the experimental confirmation of the flatness of the universe, adiabatic nature of nearly gaussian perturbations and the discovered (at 3,5 sigma level) logarithmic tilt of the spectrum unambiguously prove the quantum origin of the universe structure and the early cosmic acceleration. Needless to say that all these predictions, which were yet in conflict with observations about 15 years ago, are very nontrivial. Given that the quantum origin of the universe structure is experimentally confirmed, the precision measurements already now allow us to exclude many inflationary scenarios existing in the literature. Moreover, the improved accuracy of the determination of spectral index, the bound (or detection) on non-gaussianity and the bound (or possible future detection) on primordial gravitational waves will allow us to put further restrictions on the admissible inflationary scenarios. However, this seems will not help us too much in recovering the fundamental particle physics behind inflation. In fact, the observational data only allow us to measure only the effective equation of state and the rate of its change in a rather small interval of scales. Keeping in mind unavoidable experimental uncertainty, the effect of unknown physics right after inflation and degeneracy in the scenarios discussed above we perhaps will never be able to find out the microscopical theory of inflation without further very essential input from the particle physics. On the other hand, the remarkable property of the theory of quantum origin of the universe structure is that the gravity seems does not care too much about microscopic theory providing needed equation of state, and allows us to make experimentally verifiable predictions. 

Quantum Cosmological Perturbations: Predictions and Observations

Viatcheslav Mukhanov

(Submitted on 15 Mar 2013)

Working to avoid metaphysical problems

The Planck measurements have unambiguously confirmed the main predictions of the theory of quantum origin of the universe structure. Namely, the adiabatic nature and the Gaussian origin of primordial perturbations were established beyond any reasonable doubt. Even more amazing, more nontrivial infrared logarithmic tilt of the spectrum, first predicted in [2], was discovered at 6 sigma confidence level. The simplest way to amplify the quantum fluctuations is provided by the stage of inflation. Although nobody doubt the quantum origin of the primordial fluctuations, there are still claims in the literature that basically the same mechanism of amplification of quantum fluctuations can work also either in a bouncing universe on the stage of super slow contraction [18] or in conformal rolling scenario [19]. The generated spectra in the alternative theories are not the predictions of the theory, but rather postdictions which are constructed to be in agreement with observations. Nevertheless, this is not enough to rule out these possibilities at the level of a ”theorem”. Thus, at the moment the only robustly established experimental fact is the quantum origin of the universe structure with a little uncertainty left for the mechanism of amplification of quantum fluctuations. To firmly establish that namely inflation has provided us this mechanism one has to find the primordial gravitational waves the lower bound on which for the spectral index ns=0.96 corresponds to r about 0.003. According to [3, 4] one of the main motivations for looking the alternatives to inflation is the failure of predictability of so called ”postmodern inflationary paradigm”. Paradoxically this trouble seems to be due to the same successful quantum fluctuations with the red-tilted spectrum which lead to the galaxies. On one hand the quantum fluctuations explain the observed large scale structure of the universe, but on the other hand they are also responsible for the selfreproduction and produce eternal inflating multiverse where ”anything can happen and will happen an infinite number of times” [5]. In this paper I have shown how this problem can be avoided. Using the effective description of inflation I have found nearly unambiguous extension of inflation which avoids the selfreproduction. What is yet missing in this description is a justification of the model from the point of view of some fundamental theory. However, under circumstances when only effective description of inflation is needed to explain the observations and there are no even slightest experimental hints how the fundamental theory should look like at very high energies such an approach looks as the most plausible. Moreover, it can provide us with hints about fundamental theory, which can avoid even metaphysical problems.

Inflation without Selfreproduction

Viatcheslav Mukhanov

(Submitted on 8 Sep 2014)

Addendum 04/02/2015

Video: Critical review of alternatives to Inflation (6162) by Mukhanov

(Mardi 16 décembre 2014 18:00 - 18:30)