lundi 2 février 2015

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.
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
(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.
(Submitted on 8 Sep 2014)
Addendum 04/02/2015

samedi 31 janvier 2015

First, a (too) spectacular claim, then a spectacular {statistically} insignificant result!

First detection of inflationary gravitational waves probably did not occur in 2014

At the recombination epoch, the inflationary gravitational waves (IGW) contribute to the anisotropy of the CMB in both total intensity and linear polarization. The amplitude of tensors is conventionally parameterized by r, the tensor-to-scalar ratio at a fiducial scale. Theoretical predictions of the value of r cover a very wide range. Conversely, a measurement of r can discriminate between models of inflation. Tensor modes produce a small increment in the temperature anisotropy power spectrum over the standard [cosmological model] ΛCDM scalar perturbations at multipoles l<∼60; measuring this increment requires the large sky coverage traditionally achieved by space-based experiments, and an understanding of the other cosmological parameters. The effects of tensor perturbations on B-mode polarization is less ambiguous than on temperature or E-mode polarization over the range l<∼150...
Interstellar dust grains produce thermal emission, the brightness of which increases rapidly from the 100– 150 GHz frequencies favored for CMB observations, becoming dominant at ≥ 350 GHz even at high galactic latitude. The dust grains align with the Galactic magnetic field to produce emission with a degree of linear polarization [16]. The observed degree of polarization depends on the structure of the Galactic magnetic field along the line of sight, as well as the properties of the dust grains (see for example Refs. [17, 18]). This polarized dust emission results in both E-mode and B-mode, and acts as a potential contaminant to a measurement of r. Galactic dust polarization was detected by Archeops [19] at 353 GHz and by WMAP [2, 20] at 90 GHz. 
BICEP2 was a specialized, low angular resolution experiment, which operated from the South Pole from 2010 to 2012, concentrating 150 GHz sensitivity comparable to Planck on a roughly 1 % patch of sky at high Galactic latitude [21]. The BICEP2 Collaboration published a highly significant detection of B-mode polarization in excess of the r=0 lensed-ΛCDM expectation over the range 30 < l<150 in Ref. [22...]. Modest evidence against a thermal Galactic dust component dominating the observed signal was presented based on the cross-spectrum against 100 GHz maps from the previous BICEP1 experiment. The detected B-mode level was higher than that projected by several existing dust models [23, 24] although these did not claim any high degree of reliability.  
The Planck survey released information on the structure of the dust polarization sky at intermediate latitudes [25], and the frequency dependence of the polarized dust emission at frequencies relevant to CMB studies [26]. Other papers argued that the BICEP2 region is significantly contaminated by dust [27, 28]. Finally Planck released information on dust polarization at high latitude [29, hereafter PIP-XXX], and in particular examined a field centered on the BICEP2 region (but somewhat larger than it) finding a level of polarized dust emission at 353 GHz sufficient to explain the 150 GHz excess observed by BICEP2, although with relatively low signal-to-noise. [...] 
In this paper, we take cross-spectra between the joint BICEP2/Keck maps and all the polarized bands of Planck. [...]

Upper: BB spectrum of the BICEP2/Keck maps before and after subtraction of the dust contribution, estimated from the cross-spectrum with Planck 353 GHz. The error bars are the standard deviations of simulations, which, in the latter case, have been scaled and combined in the same way. The inner error bars are from lensed-ΛCDM+noise simulations as in the previous plots, while the outer error bars are from the lensed-ΛCDM+noise+dust simulations. Lower: constraint on r derived from the cleaned spectrum compared to the fiducial analysis shown in Figure 6.

[...] The r constraint curve peaks at r = 0.05 but disfavors zero only by a factor of 2.5. This is expected by chance 8% of the time, as confirmed in simulations of a dust-only model. We emphasize that this significance is too low to be interpreted as a detection of primordial B-modes. [...] 
In order to further constrain or detect IGW, additional data are required. The Planck Collaboration may be able to make progress alone using the large angular scale “reionization bump,” if systematics can be appropriately controlled [50]. To take small patch “recombination bump” studies of the type pursued here to the next level, data with signal-to-noise comparable to that achieved by BICEP2/Keck at 150 GHz are required at more than one frequency... During the 2014 season, two of the Keck Array receivers observed in the 95 GHz band and these data are under active analysis. BICEP3 will add substantial additional sensitivity at 95 GHz in the 2015, and especially 2016, seasons. Meanwhile many other ground-based and sub-orbital experiments are making measurements at a variety of frequencies and sky coverage fractions.
DataBICEP2/Keck and Planck Collaborations
30 January 2015

jeudi 22 janvier 2015

2015 : The Planck(-Bronstein) mass [concept] has more than 100 {only} 80 years

History of science can teach us something
Here is a long excerpt from a text by the researcher in philosophy and history of science Gennady Gorelik (available online) which illustrate the statement in the title:

Planck introduced his cGh values in 1899, without any connection to quantum gravity. Quantum limits to the applicability of general relativity (and, implicitly, their Planck scale) were first discovered in 1935 by the Soviet theorist Matvey P. Bronstein (1906-1938). It was not until the 1950s that the explicitly quantum-gravitational significance of the Planck values was pointed out almost simultaneously by several physicists.[...] In the fifth installment of his continuing study of irreversible radiation processes (Planck 1899), Max Planck introduced two new universal physical constants, a and b, and calculated their values from experimental data. The following year, he redesignated the constant b by the famous letter h(and in place of a, he introduced k = b/a, the Boltzmann constant).  
In 1899, the constant b (that is, h) did not yet have any quantum theoretical significance, having been introduced merely in order to derive Wien's formula for the energy distribution in the black-body spectrum. However, Planck had previously described this constant as universal. During the six years of his efforts to solve the problem of the equilibrium between matter and radiation, he clearly understood the fundamental, universal character of the sought-for spectral distribution. 
It was perhaps this universal character of the new constant that stimulated Planck, in that same paper of 1899, to consider a question that was not directly connected with the paper's main theme. The last section of the paper is entitled "Natural Units of Measure" ["Natürliche Maasseinheiten"]. Planck noted that in all ordinary systems of units, the choice of the basic units is made not from a general point of view "necessary for all places and times," but is determined solely by "the special needs of our terrestrial culture" (Planck 1899, p. 479). Then, basing himself upon the new constant h and also upon c and G, Planck suggested the establishment of

"units of length, mass, time, and temperature that would, independently of special bodies and substances, necessarily retain their significance for all times and all cultures, even extraterrestrial and extrahuman ones, and which may therefore be designated as natural units of measure." (Planck 1899, pp. 479-480)
[...]The quantum-gravitational meaning of the Planck values could be revealed only after a relativistic theory of gravitation had been developed. As soon as that was done, Einstein pointed out the necessity of unifying the new theory of gravitation with quantum theory. In 1916, having obtained the formula for the intensity of gravitational waves, he remarked:
"Because of the intra-atomic movement of electrons, the atom must radiate not only electromagnetic but also gravitational energy, if only in minute amounts. Since, in reality, this cannot be the case in nature, then it appears that the quantum theory must modify not only Maxwell's electrodynamics but also the new theory of gravitation." (Einstein 1916, p. 696).
For two decades after Einstein pointed out the necessity of a quantum-gravitational theory in 1916, only a few remarks about this subject appeared. There were too many other more pressing theoretical problems (including quantum mechanics, quantum electrodynamics, and nuclear theory). And, the remarks that were made were too superficial, which is to say that they assumed too strong an analogy between gravity and electromagnetism. For example, after discussing a general scheme for field quantization in their famous 1929 paper, Heisenberg and Pauli wrote:
"One should mention that a quantization of the gravitational field, which appears to be necessary for physical reasons, may be carried out without any new difficulties by means of a formalism wholly analogous to that applied here. (Heisenberg and Pauli 1929, p. 3)"
They grounded the necessity of a quantum theory of gravitation on Einstein's mentioned remark of 1916 and on Oskar Klein's remarks in an article of 1927 in which he pointed out the necessity of a unified description of gravitational and electromagnetic waves, one taking into account Planck's constant h. 
Heisenberg and Pauli obviously intended that quantization techniques be applied to the linearized equations of the (weak) gravitational field (obtained by Einstein in 1916). Being clearly approximative, this approach allows one to hope for an analogy with electromagnetism, but it also allows one to disregard some of the distinguishing properties of gravitation—its geometrical essence and its nonlinearity. Just such an approach was employed by Leon Rosenfeld, who considered a system of quantized electromagnetic and weak gravitational fields (Rosenfeld 1930), studying the mutual transformations of light and "gravitational quanta" (a term that he was the first to use). 
The first really profound investigation of the quantization of the gravitational field was undertaken by Matvey P. Bronstein. The essential results of his 1935 dissertation, entitled "The Quantization of Gravitational Waves," were contained in two papers published in 1936. The dissertation was mainly devoted to the case of the weak gravitational field, where it is possible to ignore the geometrical character of gravitation, that is, the curvature of space-time. However, Bronstein's work also contained an important analysis revealing the essential difference between quantum electrodynamics and a quantum theory of gravity not thus restricted to weak fields and "nongeometricness." This analysis demonstrated that the ordinary scheme of quantum field theory and the ordinary concepts of Riemannian geometry are not sufficient for the formulation of a consistent theory of quantum gravity. At the same time, Bronstein's analysis led to the limits of quantum-gravitational physics (and to Planck's cGh-values). [...] 
For two decades after Bronstein's work, there was calm in the field of quantum gravity. Only in the mid-1950s did the length l0 = (Gh/c3)1/2 appear almost simultaneously in a few different forms in a few papers. For example, in 1954, Landau pointed out that the length l= G1/2h/ce (= a-1/2l0, very near to the Planck length) is "the limit of the region outside of which quantum electrodynamics cannot be considered as a self-consistent theory because of the necessity of taking into account gravitational interactions" (Gm^2/r ~ e2/r, when m ~ p/c ~ h/lc) (Abrikosov, Landau, Khalatnikov 1954).[...] 
The term "Planck values," which is now generally accepted, was introduced later (Misner and Wheeler 1957). According to Wheeler, he did not know in 1955 about Planck's "natural units" (private communication).
A history of the Planck values provides interesting material for reflections on timely and premature discoveries in the history of science. Today, the Planck values are more a part of physics itself than of its history. They are mentioned in connection with the cosmology of the early universe as well as in connection with particle physics. In considering certain problems associated with a unified theory (including the question of the stability of the proton), theorists discovered a characteristic mass ~ 1016mp (mpis the proton mass). To ground such a great value, one first refers to the still greater mass 1019mp. In the words of Steven Weinberg:
"This is known as the Planck mass, after Max Planck, who noted in 1900 that some such mass would appear naturally in any attempt to combine his quantum theory with the theory of gravitation. The Planck mass is roughly the energy at which the gravitational force between particles becomes stronger than the electroweak or the strong forces. In order to avoid an inconsistency between quantum mechanics and general relativity, some new features must enter physics at some energy at or below 1019 proton masses." (Weinberg 1981, p. 71).
The fact that Weinberg takes such liberties with history in this quotation is evidence of the need to describe the real historical circumstances in which the Planck mass arose. As we saw, when Planck introduced the mass (ch/G)1/2 (~1019mp) in 1899, he did not intend to combine the theory of gravitation with quantum theory; he did not even suppose that his new constant would result in a new physical theory. The first "attempt to combine the quantum theory with the theory of gravitation," which demonstrated that "in order to avoid an inconsistency between quantum mechanics and general relativity, some new features must enter physics," was made by Bronstein in 1935. That the Planck mass may be regarded as a quantum-gravitational scale was pointed out explicitly by Klein and Wheeler twenty years later. At the same time, Landau also noted that the Planck energy (mass) corresponds to an equality of gravitational and electromagnetic interactions.
by Gennady Gorelik (1992)
Studies in the history of general relativity. [Einstein Studies. Vol.3].

To know more about Matvei Bronstein,  (an Ettore Majorana who came in from the cold, so to speak)

lundi 19 janvier 2015

Qu'est ce que mesure vraiment le nombre effectif de neutrinos dans le modèle cosmologique standard dit de concordance ?

Réponse de blogueur et commentaires d'experts 
Encore un billet sur les neutrinos ! Oui ça commence à faire beaucoup. On en parlait dans le billet précédent et il y a plus longtemps ici ou  -  mais que voulez-vous chers lecteurs-trices se sont les seules particules détectables à ce jour dont les propriétés nous permettent de sonder la physique au delà du Modèle Standard, brefs les seuls messagers fiables  porteurs d('un)e nouvelle( phy)s(iques) à se mettre sous la dent (pour le moment bien sûr). Alors il est toujours utile d'y revenir, particulièrement lorsque la lecture d'un billet de blog spécialisé (et de ses commentaires) comme ci-dessous nous permet de découvrir clairement les hypothèses implicites des modèles explicatifs de certaines données expérimentales pas toujours très explicites :
One interesting result [from the Planck satellite] is the new improved constraint on the effective number of neutrinos, Neff in short. The way this result is presented may be confusing. We know perfectly well there are exactly 3 light active (interacting via weak force) neutrinos; this has been established in the 90s at the LEP collider, and Planck has little to add in this respect. Heavy neutrinos, whether active or sterile, would not show in this measurement at all. For light sterile neutrinos, Neff implies an upper bound on the mixing angle with the active ones. The real importance of Neff lies in that it counts any light particles (other than photons) contributing to the energy density of the universe at the time of CMB decoupling. Outside the standard model neutrinos, other theorized particles could contribute any real positive number to Neff, depending on their temperature and spin. A few years ago there have been consistent hints of Neff much larger 3, which would imply physics beyond the standard model. Alas, Planck has shot down these claims. The latest number combining Planck and Baryon Acoustic Oscillations is Neff =3.04±0.18, spot on 3.046 expected from the standard model neutrinos. This represents an important constraint on any new physics model with very light (less than eV) particles.
Jester, Blog Résonaances,  Saturday, 13 December 2014
Anonymous comment:
Note that the standard value of N_eff = 3.046 for 3 active neutrinos relies on several assumptions:
*) 3 active neutrinos in the Standard Model
*) No partly thermalised lights species
*) Reheating happened above ~4 MeV
*) No entropy production between 1 MeV and today
*) No cooling of photons between 1 MeV and today (e.g. through Dark Sector mixing)
Only the first assumption was verified by LEP, so there were plenty of room for N_eff to be different from 3.046.

Jester reply:
I completely agree, maybe except that "Reheating happened above ~4 MeV" is independently confirmed by nucleosynthesis. I didn't mean that Neff is not useful, on the contrary. I meant that it is often presented as a measurement of the number of neutrinos, which may be misleading.

En attendant le retour du lièvre continuons à écouter se que nous raconte la tortue 
Autrement dit, approfondissons patiemment nos connaissances sur le rôle des neutrinos dans l'astrophysique des débuts de l'univers vue depuis nos modernes spectro-télescopes collecteurs-analyseurs de photons ayant voyagés plus de dix milliards d'années ... avant que les physiciens des particules ne relancent leur accélérateur géant en espérant trouver rapidement des réponses à leurs questions dans la collision-désintégration de quelques paires quark-antiquark ou gluons-gluons en une fraction de seconde (ou presque).
Physics Beyond the Standard Models (BSMs), i.e. beyond Electro-Weak Model and beyond Standard Cosmological Model (... also called λ Cold Dark Matter model) is required for the explanation of the astrophysical and cosmological observational data. Namely, the contemporary SCM, contains considerable BSMs components - the so called dark energy (DE) and dark matter (DM), both with yet unrevealed nature, alas. These constitute 96% of the universe matter today, and play a considerable role at the matter dominated epoch, i.e. at later stages of the Universe evolution! 
BSMs physics is needed also for revealing the nature and the characteristics of the inflaton (the particle/field responsible for inflationary expansion stage) and CP-violation (CPV) or/and B[aryon number]-violation (BV) mechanisms. These are expected necessary ingredients in the theories of inflation and baryon asymmetry generation, which are the most widely accepted today hypotheses providing natural explanations of numerous intriguing observational characteristics of our universe. 
The inflationary theory explains naturally and elegantly the initial conditions of the universe in the pre-Friedmann epoch, namely: the extraordinary homogeneity and isotropy at large scales of the universe at its present epoch; its unique isotropy at the Cosmic Microwave Background (CMB) formation epoch (when the universe was ∼ 380000 years old); its unique flatness and the pattern of structures it has. Besides, the inflationary early stage explains the lack of topological defects in the universe. While the baryon asymmetry generation models explain the locally observed matter-antimatter asymmetry of the universe. 
...we have been already the lucky witnesses of the experimental establishment of the BSM physics in the neutrino sector. Experimental data on neutrino oscillations firmly determined three neutrino mixing angles and three mass differences, corresponding to the existence of at least two non-zero neutrino masses. The concrete neutrino mass pattern and possible CPV mechanism are to be detected in near future. Thus, the neutrino experimental data ruled out the Standard Models assumptions about zero neutrino masses and mixing and about flavor lepton number (L) conservation. Cosmology provides complementary knowledge about neutrino and BSM physics in the neutrino sector, because neutrino had a considerable influence on the processes during different epochs of the universe evolution. At the hot early universe stage, radiation dominated (RD) stage, light neutrinos were essential ingredients of the universe density, determining the dynamics of the universe.
Neutrinos played also an essential role in different processes as for example Big Bang nucleosynthesis (BBN). In particular, electron neutrino participated in the pre-BBN neutron-proton transitions, that took place during the first seconds, and nucleons freezing, and thus they influenced considerably the primordial production of the light elements (BBN) during the first minutes of the universe. Hence, BBN is very sensitive to the number of the light neutrino types, neutrino characteristics, neutrino chemical potentials, the possible presence of sterile neutrino, etc. BBN is capable to differentiate different neutrino flavors, because νe participates into proton-neutron transitions in the pre-BBN epoch, essential for yields of the primordially produced elements, while νµ and ντ do not exert kinetic effect on BBN 
At later stages of the universe evolution (T < eV) relic neutrinos, contributing to the matter density, influenced CMB anisotropies, played a role in the formation of galaxies and their structures. CMB and Least Scattering Surface (LSS), being sensitive to the total neutrino density and provide information about the neutrino masses and number of neutrino species. Hence, although the relic neutrinos, called Cosmic Neutrino Background (CNB) are not yet directly detected, strong observational evidence for CNB and stringent cosmological constraints on relic neutrino characteristics exist from BBN, CMB and LSS data. In particular, the determinations of light elements abundances and BBN theory predictions are used to put stringent constraints on neutrino characteristics (the effective number of relativistic particles, lepton asymmetry, sterile neutrino characteristics, neutrino mass differences and mixings) while CMB and LSS data provide constraints on neutrino masses and neutrino number density corresponding to CMB and LSS formation epochs.
Daniela Kirilova (Submitted on 7 Jul 2014)

jeudi 4 décembre 2014

D'un Noël à l'autre le nombre de neutrinos effectifs se rapproche de celui des Rois Mages

Noël 2012 
Just before Christmas [2013], the WMAP collaboration posted the 9-years update of their Cosmic Microwave Background [CMB] results...
The effective number of relativistic degrees of freedom at the time of CMB decoupling, the so-called Neff parameter, is now Neff = 3.26 ± 0.35 3.84 ± 0.40, compared to Neff= 4.34 ± 0.87 quoted in the 7-years analysis. For the fans and groupies of this observable it was like finding a lump of coal under the christmas tree...

So, what is this mysterious Neff parameter? According to the standard cosmological model, at the temperatures above 10 000 Kelvin the energy density of the universe was dominated by a plasma made of neutrinos (40%) and photons (60%). The photons today make the CMB about which we know everything. The neutrinos should also be around, but for the moment we cannot study them directly. However we can indirectly infer their presence in the early universe via other observables. First of all, the neutrinos affect the energy density stored in radiation... which controls the expansion of the Universe during the epoch of radiation domination. The standard model predicts Neff equal to the number of known neutrinos species, that is Neff=3 (in reality 3.05, due to finite temperature and decoupling effects). Thus, by measuring how quickly the early Universe was expanding, we can determine Neff. If we find Neff≈3 we confirm the standard model and close the store. On the other hand, if we measured that Neff is significantly larger than 3, that would mean a discovery of additional light degrees of freedom in the early plasma that are unaccounted for in the standard model. Note that these new hypothetical particles don't have to be similar to neutrinos, in particular they could be bosons, and/or have a different temperature (in which case they would correspond to non-integer increase of Neff). All that is required from them is that they are weakly interacting and light enough to be relativistic at the time of CMB decoupling. Theorists have dreamed up many viable candidates that could show up in Neff : additional light neutrinos species, axions, dark photons, etc... 
The interest of particle physicists in Neff come from the fact that, until recently, the CMB data also pointed at Neff≈4 with a comparable error. The impact of Neff on the CMB is much more contrived, and there are many separate effects one needs to take into account. For example, larger Neff delays the moment of matter-radiation equality, which affects the relative strength and positions of the peaks. Furthermore, Neff affects how the perturbations grow during the radiation era, which may show up in the CMB spectrum at l ≥ 100. Finally, the larger Neff, the larger is the effect of Silk damping at l ≥ 1000. Each single observable has a large degeneracy with other input parameters (matter density, Hubble constant, etc.) but, once the CMB spectrum is measured over a large range of angular scales, these degeneracies are broken and stringent constraints on Neff can be derived. That is what happened recently, thanks to the high-l CMB measurements from the ACT and SPT telescopes, and some input from other astrophysical observations. The net result [Neff = 3.84 ± 0.40] ... using [the CMB data] in addition [with] an input from Baryon Acoustic Oscillations and Hubble constant measurements... can be well accounted for by the three boring neutrinos of the standard model.
Jester, Friday, 18 January 2013
Noël 2014
Les nouveaux résultats de la collaboration Planck portent aussi sur un autre type de particules très élusives : les neutrinos. Ces particules élémentaires « fantômes », produites en abondance dans le Soleil par exemple, traversent notre planète pratiquement sans interaction, ce qui rend leur détection extrêmement difficile. Il n’est donc pas envisageable de détecter directement les premiers neutrinos, produits moins d’une seconde après le Big-Bang, qui sont extrêmement peu énergétiques. Pourtant, pour la première fois, Planck a détecté sans ambiguïté l’effet de ces neutrinos primordiaux sur la carte du rayonnement fossile.

Les neutrinos primordiaux décelés par Planck ont été libérés une seconde environ après le Big-Bang, lorsque l’univers était encore opaque à la lumière mais déjà transparent à ces particules qui peuvent s’échapper librement d’un milieu opaque aux photons, tel que le cœur du Soleil. 380 000 ans plus tard, lorsque la lumière du rayonnement fossile a été libérée, elle portait l’empreinte des neutrinos car les photons ont interagi gravitationnellement avec ces particules. Ainsi, observer les plus anciens photons a permis de vérifier les propriétés des neutrinos.
PRÉLIMINAIRE - Contraintes et lien entre le nombre d’espèces de neutrinos, la vitesse d’expansion de l’univers aujourd’hui H0 et le paramètre σ8 qui caractérise la structuration de la matière à grande échelle. Les points de couleur correspondent aux contraintes température + effet de lentille gravitationnelle uniquement, les contours noirs en ajoutant la polarisation à toutes les grandes échelles angulaires et les oscillations acoustiques de baryons. Les lignes verticales correspondent à la valeur de Neff prédite par divers modèles : la ligne pleine correspond au modèle standard, les lignes pointillées à des modèles avec une quatrième espèce de neutrino (selon le type de neutrino, actif ou stérile, et l'époque de leur découplage). © ESA - collaboration Planck
Les observations de Planck sont conformes au modèle standard de la physique des particules. Elles excluent quasiment l’existence d’une quatrième famille de neutrinos auparavant envisagée d’après les données finales du satellite WMAP, le prédécesseur américain de Planck. Enfin, Planck permet de fixer une limite supérieure à la somme des masses des neutrinos, qui est à présent établie à 0.23 eV (électronvolt).
Les données de la mission complète et les articles associés qui seront soumis à la revue Astronomy & Astrophysics (A&A) seront disponibles dès le 22 décembre 2014 sur le site de l’ESA. Ces résultats sont notamment issus des mesures faites avec l’instrument haute fréquence HFI conçu et assemblé sous la direction de l’Institut d’astrophysique spatiale (CNRS/Université Paris-Sud) et exploité sous la direction de l’Institut d’astrophysique de Paris (CNRS/UPMC) par différents laboratoires impliquant le CEA, le CNRS et les universités, avec des financements du CNES et du CNRS.
Communiqué de presse du CNRS, Lundi, 1 Décembre 2014

Premier Noël n’est précisé nulle part dans la Bible le nombre de ces « Rois » mages, et encore moins leur nom! Cela reste donc sujet à interprétation suivant les auteurs: ils sont seulement deux sur les ornements muraux des catacombes de Saint-Pierre, trois dans les catacombes de Priscille ou quatre dans les catacombes de Domitille. La tradition syrienne considère même qu’il étaient au nombre de douze! ... 

Pourtant, au fil des siècles, la coutume tend à les considérer au nombre de trois… Pourquoi? Tout simplement parce que l’Évangile de Matthieu mentionne l’existence de trois cadeaux donnés à Jésus: l’or (symbole de la royauté – les Rois mages voyaient en Jésus-Christ le futur roi des Juifs...), l’encens (symbole de la divinité) et la myrrhe (très employée dans les rites d’embaumement, elle symbolise l’humanité de Jésus – même si cette interprétation ne fait pas l’unanimité)... 

Les noms de Melchior, Gaspard et Balthazar apparaissent pour la première fois au VIe siècle après Jésus Christ dans un Évangile apocryphe... Mais il y a pire! Les « Rois-Mages » n’étaient en réalité pas rois! Ils étaient seulement mages, c’est-à-dire spécialistes d’astronomie et de divination.
Les Rois Mages n’étaient pas trois. D'ailleurs, ils n'étaient même pas rois...
Djinnzz, le 16/07/2013

mercredi 3 décembre 2014

Pourquoi le blogueur Jester (physicien des particules) est-il si (raisonnablement) "méchant" (avec ses collègues astrophysiciens)?

Hypothèse 1 : parce qu'il sait que toute preuve expérimentale est probablement fausse (sans estimation correcte de son incertitude) jusqu'à preuve du contraire (sa reproductibilité)
There indeed seems to be an excess in the 2-4 GeV region. However, given the size of the error bars and of the systematic uncertainties, not to mention how badly we understand the astrophysical processes in the galactic center, one can safely say that there is nothing to be excited about for the moment. 

It is well known that sigmas come in varieties: there or more significant 3 sigmas, less significant 3 sigmas, and astrophysical 3 sigmas. 

Notice that different observations of the helium abundance are not quite consistent with each other, but that's normal in astrophysics; the rule of thumb is that 3 sigma uncertainty in astrophysics is equivalent to 2 sigma in conventional physics. 

Although the natural reaction here is a resounding "are you kidding me", the claim is that the excess near 3.56 keV ...  over the background model is very significant, at 4-5 astrophysical sigma. It is difficult to assign this excess to any known emission lines from usual atomic transitions. If the excess is interpreted as a signal of new physics, one compelling (though not unique) explanation is in terms of sterile neutrino dark matter. In that case, the measured energy and intensity of the line correspond to the the neutrino mass 7.1 keV and the mixing angle of order 5*10^-5, see the red star in the plot. This is allowed by other constraints and, by twiddling with the lepton asymmetry in the neutrino sector, consistent with the observed dark matter relic density.
Clearly, a lot could possibly go wrong with this kind of analysis. For one thing, the suspected dark matter line doesn't stand alone in the spectrum. The background mentioned above consists not only of continuous X-ray emission but also of monochromatic lines from known atomic transitions. Indeed, the 2-10 keV range where the search was performed is pooped with emission lines: the authors fit 28 separate lines to the observed spectrum before finding the unexpected residue at 3.56 keV. The results depend on whether these other emission lines are modeled properly. Moreover, the known Ar XVII dielectronic recombination line happens to be nearby at 3.62 keV. The significance of the signal decreases when the flux from that line is allowed to be larger than predicted by models. So this analysis needs to be confirmed by other groups and by more data before we can safely get excited.

Hypothèse 2 : Parce qu'il est un peu las de ne pas trouver dans son laboratoire la nouvelle physique (que les autres voient déjà dans leur observatoire)
There is no evidence of new physics from accelerator experiments (except, perhaps, for the 3-3.5 σ discrepancy of the muon (g-2) 7a, 7b, 8)). Most of the experimental evidence for new physics comes from the sky like for Dark Energy, Dark Matter, baryogenesis and also neutrino oscillations (that were first observed in solar and atmospheric neutrinos). One expected new physics at the electroweak scale based on a ”natural” solution of the hierarchy problem 4). The absence so far of new physics signals casts doubts on the relevance of our concept of naturalness. 
(Submitted on 8 Jul 2014 (v1), last revised 17 Jul 2014 (this version, v2))

mardi 25 novembre 2014

Il faut sauver le physicien John Bell (des griffes d'un blogueur polémiste)

Une réaction personnelle à un billet de Lubos Motl  
J. Bell : a mediocre physicist ? Do you talk about the same guy who discovered with R. Jackiw (and independently S. Adler) chiral anomaly, such an important phenomenon in quantum field theory? I can agree with all your technical arguments to support QM against classical zealots but the pedagogical value of your post would be undermined in my opinion if you would not recognize the pedagogical usefulness of the Bell's theorem and if you would not make the difference between necessarily old-fashioned conceptions or terminology used by Bell in the sixties and the loosely-defined concepts of a large number of QM's contenders nowadays.

John Bell et sa plus grande contribution à la physique 
... John Bell codiscovered the mechanism of anomalous symmetry breaking in quantum field theory. Indeed, our paper on this subject is his (and my) most-cited work. The symmetry breaking in question is a quantum phenomenon that violates the correspondence principle; it arises from the necessary infinities of quantum field theory. Over the years it has become evident that theoretical/mathematical physicists are not the only ones to acknowledge this effect. Nature makes fundamental use of the anomaly in at least two ways: the neutral pion’s decay into two photons is controlled by the anomaly [1, 2] and elementary fermions (quarks and leptons) arrange themselves in patterns such that the anomaly cancels in those channels to which gauge bosons – photon, W, Z – couple [3]. (There are also phenomenological applications of the anomaly to collective, as opposed to fundamental, physics – for example, to edge states in the quantum Hall effect.)
R. Jackiw, november 2000

Le mot de la fin à Richard Feynman et Alain Aspect
Chaque fois que l’on se replonge dans le problème que nous venons de présenter, on ne peut s’empêcher de se poser la question : y a-t-il un problème réel ? Il faut reconnaître que la réponse à cette question peut varier, même pour les plus grands physiciens. En 1963, R. Feynman donnait une première réponse à cette question dans son fameux cours de physique48 : « Ce point ne fut jamais accepté par Einstein… il devint connu sous le nom de paradoxe d’Einstein-Podolsky-Rosen. Mais lorsque la situation est décrite comme nous l’avons fait ici, il ne semble pas y avoir quelque paradoxe que ce soit … ». Deux décennies plus tard, Feynman exprimait une opinion radicalement différente, toujours sur la situation EPR : « nous avons toujours eu une très grande difficulté à comprendre la vision du monde que la Mécanique Quantique implique … Il ne m’est pas encore apparu évident qu’il n’y ait pas de problème réel… Je me suis toujours illusionné moi même, en confinant les difficultés de la Mécanique Quantique dans un recoin de plus en plus petit, et je me retrouve de plus en plus chagriné par ce point particulier. Il semble ridicule de pouvoir réduire ce point à une question numérique, le fait qu’une chose est plus grande qu’une autre chose. Mais voilà : – elle est plus grande … »
Alain Aspect
We must be grateful to John Bell for having shown us that philosophical questions about the nature of reality could be translated into a problem for physicists, where naive experimentalists can contribute. 
Alain Aspect (Submitted on 2 Feb 2004)