dimanche 16 mars 2014

La rumeur d'un signe expérimental de l'inflation cosmologique (peut-elle faire autre chose qu')enfle(r?)

Vous avez dit ondes gravitationnelles primordiales?
Ce week-end la blogosphère scientifique bruisse d'une rumeur excitante car elle tourne autour d'une possible découverte majeure en astrophysique : la première mise en évidence expérimentale de la trace d'ondes gravitationnelles primordiales dans la polarisation du fond diffus cosmologique. Pour être précis remarquons que la rumeur porte a priori sur le contenu de la "découverte majeure en astrophysique" car cette dernière est déjà une annonce publique faite par une institution américaine sérieuse donc a priori une vrai promesse et demain à 18 heure un fait avéré :
March 17th Press Conference on Major Discovery at Harvard-Smithsonian Center for Astrophysics
Press Release Source: Harvard-Smithsonian Center for AstrophysicsPosted Wednesday, March 12, 2014
The Harvard-Smithsonian Center for Astrophysics (CfA) will host a press conference at 12:00 noon EDT (16:00 UTC) on Monday, March 17th, to announce a major discovery.
Video of the press conference will be streamed live beginning at 11:55 a.m. EDT from the link at http://www.cfa.harvard.edu/news/news_conferences.html

Pour savoir ce que pense la communauté des scientifiques professionnels qui partagent sur la toile leurs réactions et commentaire à propos de cette future nouvelle discutée aussi dans la presse grand publique anglo-saxonne (The Guardian) voici une liste non exhaustive : 

Derrières les ondes gravitationnelles, la question de l'inflation et de la cosmologie quantique...
We do not know the history of the observable Universe before the epoch of nucleosynthesis, but it is widely believed that there was an early era of cosmological inflation. During this era, the Universe was filled with a homogeneous scalar field φ, called the inflaton field, and essentially nothing else. The potential V(φ) dominated the energy density of the Universe, decreasing slowly with time as φ rolled slowly down the slope of V. 
The attraction of this paradigm is that it can set the initial conditions for the subsequent hot big bang, which otherwise have to be imposed by hand. One of these is that there by no unwanted relics (particles or topological defects which survive to the present and contradict observation). Another is that the initial density parameter should have the value Ω = 1 to very high accuracy, to ensure that its present value has at least roughly this value. There is also the requirement that the Universe be homogeneous and isotropic to high accuracy. 
All of these virtues of inflation were noted when it was first proposed by Guth in 1981 and very soon a more dramatic one was also noticed. Starting with a Universe which is absolutely homogeneous and isotropic at the classical level, the inflationary expansion of the Universe will ‘freeze in’ the vacuum fluctuation of the inflaton field so that it becomes an essentially classical quantity. On each comoving scale, this happens soon after horizon exit. Associated with this vacuum fluctuation is a primordial energy density perturbation, which survives after inflation and may be the origin of all structure in  the Universe. In particular, it may be responsible for the observed cosmic microwave background  (cmb) anisotropy and for the large-scale distribution of galaxies and dark matter.  Inflation also generates primordial gravitational waves as a vacuum fluctuation, which may contribute to the low multipoles of the cmb anisotropy. 
When it was first proposed in 1982, this remarkable paradigm received comparatively  little attention. For one thing observational tests were weak, and for another the inflationary density perturbation was not the only candidate for the origin of structure. In particular,  it seemed as if cosmic strings or other topological defects might do the job instead. This situation changed dramatically in 1992, when COBE measured the cmb anisotropy on large angular scales, and another dramatic change is now in progress with the advent of smaller scale measurement. Subject to confirmation of the latter, it seems that the paradigm of slow-roll inflation is the only one not in conflict with observation.
David H. Lyth et Antonio Riotto, Particle Physics Models of Inlfation and the Cosmological Density Perturbation, 16/03/1999

Le paradigme de l'inflation trouvera-t-il demain son meilleur modèle d'inflaton?
Is it possible to show convincingly that inflation is responsible for the large-scale homogeneity, isotropy and flatness of the universe, and the primordial spectrum of metric fluctuations that seeded galaxy formation and sourced the temperature and polarization variations in the cosmic microwave background (CMB)? Some would claim no, because there is too much freedom in constructing inflationary models. Even if one shows that the observations are consistent with the predictions of a particular inflationary model, this is unconvincing because, for virtually any given combination of observations, one can design many inflationary models that reproduce them. If a theory allows everything, it has no predictive power. In this paper, though, we describe how to combine observations into a sequence of “bootstrap tests” that, if any one of them is passed, will be the most direct confirmation possible that the universe underwent a brief period of acceleration followed by a long period of deceleration.
Latham Boyle et Paul J. Steinhardt, Testing Inflation: A Bootstrap Approach, 8/10/2010 

Un peu plus près du Big-Bang et de l'énergie de Planck (aux erreurs systématiques près)
ça-y-est ! L'article de la collaboration BICEP2 - à qui on doit à tout le moins vraisemblablement la naissance de l'astrophysique expérimentale des ondes gravitationnelles primordiales - est maintenant disponible :

La rumeur était donc vraie; voici les premiers échos sur la blogosphère :
At a presentation from the Harvard-Smithsonian the BICEP2 team have announced that they have the ”first direct evidence of cosmic inflation”. As rumoured they have detected what they believe to be primordial gravitational waves with a ratio or tensor to scalar modes of r=0.2 (+0.07 -0.05) which is 5 sigma over the null hypothesis. This is a game-changing result for inflationary cosmology and possibly for quantum gravity research because the result indicates that the scale of the inflation is only about a factor of 100 below the Planck scale.
Philip Gibbs, “first direct evidence of cosmic inflation” BICEP2 results, 17/03/2014

Until this measurement/discovery is confirmed by another experiment, you should consider it provisional. Although this is too large a signal to be likely to be due to a pure statistical fluke, it could still be due to a mistake or problem. The history of science is littered with examples; remember the 2011 measurement by OPERA that showed neutrinos moving faster than the speed of light was far too large to be a statistical fluke. Fortunately there will be other experiments coming and so we’ll have a chance for various experiments to either agree or disagree with each other in the very near future.
If this measurement is correct, and if indeed it reflects gravitational waves from inflation in the most conventional way, then it would tell us that inflation occurred with a dark energy per unit volume (i.e. dark energy density) that is comparable to the energy scales associated for decades with the energy and distance scale at which all the known non-gravitational forces would naively have about the same strength — the so-called “unification of coupling constants”, sometimes extended to “grand unification” in which the various forces actually turn out to be manifestations of just a single force. This would be very remarkable, but not necessarily evidence for unification. There are other ways to get the same scale, which is about 100 times lower in energy (100,000,000 times lower in energy per unit volume) than the scale of quantum gravity (the Planck scale, which, roughly is the energy density required to make the smallest possible black hole.)
Matt Strassler, BICEP2: New Evidence Of Cosmic Inflation!, 17/03/2014
Ajout du 15/05/2014
Pour découvrir des développements plus récents sur ce sujet voir ce billet.

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