samedi 23 janvier 2016

Gravityational waves (direct detection on Earth and its) reception spectrum on the blogosphere

From the bright side ...
Masses of both black holes exceed 10 solar masses
... many of us eagerly expect the announcement of a hugely exciting discovery (direct detection) of the gravitational waves...

There are several "traps" that may make you think that the {Laser Interferometer Gravitational Waves Observatory} LIGO  shouldn't work at all (I was tempted to be confused by several such traps) and for decades after the discovery of General Relativity, people felt uncertain whether the gravitational waves could have been physical at all (Mach's principle was the primary misconception that drove those who wanted to say that they were unphysical) but at the end, all of them are wrong. Gravitational waves do exist and LIGO-like detectors may detect them. Note that the lengths of the 4-kilometer arms are measured with the accuracy 100,000 times better than the radius of the atomic nucleus. Because it's so, a LIGO discovery will eliminate all conceivable theories that claim that Nature has an unavoidable error margin in positions that would be longer than 10−2010−20 meters (e.g. the nuclear radius). Nature only prevents you from measuring the positions and momenta simultaneously; but it surely keeps track of the position separately and the position makes sense with an arbitrary accuracy – at most, at the Planck scale, there may be some issues (but not the issues that non-stringy quantum gravity babblers are sometimes imagining). 
... A commenter whose name is known to us has told us that there have been two events (short periods with gravitational waves) detected by the LIGO. A new rumor I got yesterday says that LIGO has "heard" a merger of two black holes into one bigger black hole. (I don't know whether this event is one of the two events from the first sentence of this paragraph.) Because black holes are among the heaviest "stellar mass" objects and allow the shortest orbital radii (and therefore strongest gravitational waves), this situation is obviously an event that creates strong enough gravitational waves, especially when both black holes are said to be heavier than 10 Suns (and most black holes in the Universe probably are: the stellar black holes' masses are believed to be between 3 and 50 Suns or so). 
The frequency of the gravitational wave from the two black holes that the LIGO has supposedly detected is comparable to the aforementioned 100 hertz. It's hard to say how many periods of the radiation they will be able to detect. The truly final moments of the merger only correspond to the number of orbital periods comparable to one. But before the black holes merge, they orbit one another for a very long time. The radiation only gets strong enough during the final stages of the merger and whether the LIGO may "hear" the waves long before the final moment (and how long) depends on its actual sensitivity. LIGO got "advanced" so the sensitivity has been improved by an order of magnitude but I don't want to collect all the engineering data and calculate how good the sensitivity has become. 
...At the end, if the rumor is true, what they should have heard is very similar to this LIGO example of inspiral gravitational waves except that the newly actually observed frequency is "somewhat" lower than the frequency in the example (a deeper sound). Note that the frequency of the vibrations as well as their intensity increases with time up to the final moment when the waves almost abruptly disappear.
Posted on monday, january 11, 2016 by Lubos Motl on his blog The Reference Frame

 
... to its dark one
On the Theory of Gravitational Wave Rumour Sources 
There has been a great deal of excitement almost nowhere in the astrophysics community since it was announced recently that rumours of the detection of gravitational waves had yet again begun to circulate, so I thought I would add here a brief discussion of the theoretical background to these phenomena. 
The standard theoretical model of such rumours is that they are produced from time to time during the lifetime of a supermassive science project after periods of relative quiescence. It is thought that they are associated with a perceived lack of publicity which might threaten funding and lead to financial collapse of the project. This stimulates a temporary emission of hype produced by vigorous gossip-mongering which acts to inflate the external profile of the project, resisting external pressures and restoring equilibrium. This general phenomenon is not restricted to gravitational wave detection, but also occurs across many other branches of Big Science, especially cosmology and particle physics. 
However, observations of the latest outburst suggest support for a rival theory, in which rumours are produced not by the project itself but by some other body or bodies in orbit around it or even perhaps entirely independent of it. Although there is evidence in favour of this theory, it is relatively new and many questions remain to be answered. In particular it is not known what the effect of rumours produced in this way might be on the long-term evolution of the project or on the source itself.
Posted in AstrohypeThe Universe and StuffUncategorized with tags gossipgravitational wavesRumours on January 12, 2016 by Peter Coles aka telescoper on his blog In the Dark

...with several shades of grey (another drill before the next thrill)

An improbable rumour has started that the observatory has already made a discovery — but even if true, the signal could be a drill.
Davide Castelvecchi 30 September 2015 (NATURE | NEWS: EXPLAINER)

Gravitational-wave rumours in overdrive 
Could a signal be a false alarm?

That is also possible. The LIGO detectors have sophisticated systems for reducing unwanted vibrations, but the team needs to carefully check that any detection is not a false alarm — vibrations produced by a passing lorry, for instance. 
But the rumoured signal could also be the result of a deliberate drill. Three members of the LIGO team have access to systems that can secretly nudge the mirrors and simulate all the hallmarks of an astrophysical phenomenon — a procedure called a ‘blind injection’. Only when researchers are ready to reveal that they have spotted something will the blind-injection team announce whether it has created a deliberate signal. Two such exercises occurred in earlier runs of the LIGO, in 2007 and 2010.

But Krauss, at Arizona State University in Tempe, tweeted that he has heard rumours that the team detected a signal during tests of the upgraded detectors last summer, before official data-collection began — and before the blind-injection system was in place. (He did not know any more details when asked by Nature.)

The LIGO researchers would still take their time analysing such an early event, says Laura Cadonati, a physicist at the Georgia Institute of Technology in Atlanta who heads LIGO's data-analysis team; scientists would not be able to assess such an early signal before gaining a better understanding of how often particular types of false alarm are likely to show up in the data.
The LIGO collaboration declined to comment on whether there was any time when both interferometers were active but no blind injection was possible. 
Are LIGO physicists concerned about the rumours?

González is a little miffed. “I am concerned about creating false expectations in the public and the media,” she says.

But Cadonati says that the buzz around the experiment has been “energizing”. “The fact that leaks started early on meant that they were something we had to learn to live with,” she says. “It means that there is excitement around what we are doing.”
Davide Castelvecchi 12 January 2016 (NATURE | NEWS: EXPLAINER)

Steven T. Corneliussen of Physics Today wrote a somewhat critical text about the wave of LIGO rumors that escalated one week ago:
Cosmologist Lawrence Krauss is often cited as a source, mainly because his rumors are relatively quickly spread through a large number of his Twitter followers. Your humble correspondent is often quoted as the propagator of the most precise rumors. Wait for February 11th to hear the announcement. And you will learn about at least two events. And at least one of them will be the detection of a merger of two black holes each of which weighs at least 10 solar masses. ;-) 
... As far as I can say, there are three basic reasons to be concerned when it comes to similar rumors:

  • Accuracy and balance between hype and underlying evidence
  • Fair distribution of fame and credit
  • Discipline and secrecy for their own sake 
Concerning the accuracy, well, people are told that it's "just rumors". But the track record of similar rumors has been extremely good in recent years. On Friday, March 14th, 2014, rumors spread that BICEP2 was going to claim the discovery of primordial gravitational waves on the following Monday. And it did. 



BICEP2 no longer believes that their 2014 paper was quite good but because the rumor was about the announcement, not the perfection of their actual analysis, the controversy about the BICEP2 results doesn't imply that the rumors were inaccurate.
Posted on wednesday, january 20, 2016 by Lubos Motl on his blog The Reference Frame


Even if all this gives you some feeling of déjà-vu let me wish you a happy new year 2016!

samedi 17 octobre 2015

The theoretical prediction that did not fit a wrong experimental finding and was probably right

The superluminal group velocity difference of neutrinos that was NOT measured by Opera
I take advantage of the Physics Nobel Prize 2015 rewarding the discovery of neutrino oscillations to shine a different light on a famous experiment that made big headlines in september 2011 (and gave me opportunity to start this blog!)
we discuss the possibility that the apparent superluminality is a quantum interference effect, that can be interpreted as a weak measurement [2, 3, 45]. Although the available numbers strongly indicate that this explanation is not correct, we consider the idea worth exploring and reporting – also because it might suggest interesting experiments, for example on electron neutrinos, about which relatively little is known. Similar suggestions, though not interpreted as a weak measurement [6, 7] or not accompanied by numerical estimates [6, 8], have been proposed independently. 
The idea, following analogous theory and experiment [9] involving light in a birefringent optical fibre, is based on the fact that the vacuum is birefringent for neutrinos. We consider the initial choice of neutrino flavour as a preselected polarization state, together with a spatially localized initial wavepacket. Since a given flavour is a superposition of mass eigenstates, which travel at different speeds, the polarization state will change during propagation, evolving into a superposition of flavours. The detection procedure postselects a polarization state, and this distorts the wavepacket and can shift its centre of mass from that expected from the mean of the neutrino velocities corresponding to the different masses. This shift can be large enough to correspond to an apparent superluminal velocity (though not one that violates relativistic causality: it cannot be employed to send signals). Large shifts, corresponding to states arriving at the detector that are nearly orthogonal to the polarization being detected, are precisely of the type considered in weak measurement theory. 
It seems that only muon and tau neutrino flavours are involved in the experiment... The initial beam, with ultrarelativistic central momentum p, is almost pure muon, which can be represented as a superposition, with mixing angle θ, of mass states |+> and |- >, with m> m- ... The two mass states evolve with different phases and group velocities neglecting the spreading and distortion [10] of the individual packets – both negligible in the present case. E± and v± are the energies and group velocities of the two mass states, and we write E±=E±1/2ΔE, v±=v±1/2Δv, x=vt+ξ... in which the new coordinate ξ measures deviation from the centre of the wavepacket expected by assuming it travels with the mean velocity. In the experiment, the detector postselects the muon flavour [1]... thus the shift in the measured final position of the wavepacket [can be interpreted]... as an effective velocity shift, that is  
[where the prefactor, tΔv is the relative shift of the two mass wavepackets, expected from the difference of their group velocities (it is small compared with the width of the packet ... in the neutrino case). The main factor represents the influence of the measurement-that is of the pre- and postselection and the evolution]... The possibility of superluminal velocity measurement arises because the amplification factor in (8) can be arbitrarily large if sin22θ and sin2(tΔE/2ℏ) are close to unity, corresponding to near-orthogonality of |pre> and |post>. 
For neutrinos with momentum p, ... the group velocity [difference Δv is given by -ΔE/p]. Thus Δv<0, so, in order for the apparent velocity to be superluminal, Δveff in (8) must be positive; this can be accommodated by making cos2θ negative. 
Note also that v+ and v-- are less than c if both neutrino masses are nonzero, so the individual mass eigenstate wavepackets move with subluminal group velocities; any superluminal velocity arising from (8) is a consequence of pulse distortion ... associated with the postselection, i.e. considering only arriving muon neutrinos. In the more conventional superluminal wave scenario [10], group velocities faster than light, and the pulse distortions that enable them to occur, are associated with propagation of frequencies near resonance, for which there is absorption, i.e. non-unitary propagation. That is also true in the optical polarization experiments [9] and in the neutrino situation considered here, with the difference that the nonunitarity, which gives rise to the superluminal velocity, is not continuous during propagation but arises from the sudden projection onto the postselected state.
In the [Opera] experiment, the energies of the neutrinos varied over a wide range, with an average of cp = 28.1GeV. For the difference in the squared masses, with electron neutrinos neglected and m+ and m- identified with the standard m2 and m3, a measured value [13] is m+2c4-m-2c42.43×10-3eV2. This gives
Δv/c=-1.5×10-24.                  (16) 
 The apparent velocity measured in the experiment [1] was (1+2.5×10-5)c . Comparison with the quantum velocity shift Δveff in (8) would require knowlege of m+ and m-, not just their squared difference, and the individual masses are not known. But even on the most optimistic assumption, that m-=0, it is immediately clear that it is unrealistic to imagine that the quantum amplification factor in (8) can bridge the gap of 19 orders of magnitude between (16) and the measured superluminal velocity.
(Submitted on 13 Oct 2011 (v1), last revised 14 Nov 2011 (this version, v2))


Remark: for the anecdote the abstract of this article by the distinguished mathematical physicist Michael Berry and his collaborators might be the shortest one ever written since it answered laconically to the question asked in the title : "probably not". And time has proved that it was right...


A superluminal group velocity of photons that was effectively measured
While the theoretical prediction from the last paragraph has not been tested by the Opera experiment and will stay quite hard to test empirically given the smallness of the effect, the physics behind it is pretty sound and falsifiable in other contexts. I think the article below is a nice illustration:
The physics of light propagation is a very timely topic because of its relevance for both classical [1] and quantum [2] communication. Two kind of velocities are usually introduced to describe the propagation of a wave in a medium with dispersion ω( k): the phase velocity vph=ωk and the group velocity vg=∂ω/∂k . Both of these velocities can exceed the speed of light in vacuum c in suitable cases [3]; hence, neither can describe the speed at which the information carried by a pulse propagates in the medium. Indeed, since the seminal work of Sommerfeld, extended and completed by Brillouin [4], it is known that information travels at the signal velocity, defined as the speed of the front of a square pulse. This velocity cannot exceed c [5]. The fact that no modification of the group velocity can increase the speed at which information is transmitted has been directly demonstrated in a recent experiment [6]. Superluminal (or even negative) and, on the other extreme, exceedingly small group velocities, have been observed in several media [7]. In this letter we report observation of both superluminal and delayed pulse propagation in a tabletop experiment that involves only a highly birefringent optical fiber and other standard telecom devices. 
Before describing our setup, it is useful to understand in some more detail the mechanism through which anomalous group velocities can be obtained. For a light pulse sharply peaked in frequency, the speed of the center-of-mass is the group velocity vg of the medium for the central frequency [3]. In the absence of anomalous light propagation, the local refractive index of the medium is nf , supposed independent on frequency for the region of interest. The free propagation simply yields vg=L/tf where L is the length of the medium and tf =nL/c is the free propagation time. One way to allow fast- and slow-light amounts to modify the properties of the medium in such a way that it becomes opaque for all but the fastest (slowest) frequency components. The center-of-mass of the outgoing pulse appears then at a time t = tf+<t>, with <t> the mean time of arrival once the free propagation has been subtracted; obviously <t><0 for fast-light, <t>>0 for slow-light. If the deformation of the pulse is weak, the group velocity is still the speed of the center-of-mass, now given by  
                          vg=Ltf+<t>.                                                                         (1) 
This can become either very large and even negative (<t>→−∞) or very small (<t>→∞) — although in these limiting situations the pulse is usually strongly distorted, so that our reasoning breaks down.



 

(Submitted on 20 Jul 2004 (v1), last revised 10 Jan 2005 (this version, v2))

mercredi 7 octobre 2015

Neutrino oscillations : experiment validated and awarded the 2015 Physics Nobel Prize ...

... but theory is still under [discuss]{construct}ion


Neutrino physics is one of the most interesting and vividly discussed topics in high-energy physics today. Especially the question whether the neutrinos can oscillate or not (i.e. different neutrinos can change into each other) gave rise to a huge number of experiments to actually observe these oscillations. At least since the results from the Super Kamiokande ... and the SNO experiment ... are published, it is widely believed that neutrino oscillations are an experimentally verified fact. However, the first hint has already been found in 1964 when the Homestake experiment ... discovered the solar neutrino problem. That is, the number of measured electron neutrinos from the sun is by a factor of 2-3 less than the number of neutrinos predicted by the standard solar model (SSM). 
Since within the standard model (SM) of particle physics the neutrinos are massless, and consequently cannot oscillate, their measurement shows that new physics beyond the SM exists. And indeed nowadays the experiments on neutrino oscillations are important to measure the unknown parameters of the SM and its minimal extensions. In particular, these unknown parameters are the neutrino masses and the entries in the neutrino mixing matrix. 
From all the measurements made to discover neutrino oscillations one should think that the theory behind [them] is well established and understood. But surprisingly this is not the case. The first who mentioned the idea of neutrino oscillations, though he assumed neutrino-antineutrino oscillations, was Pontecorvo in 1957 [Pon57, Pon58]. A few years later Maki, Nakagawa and Saka were the first to consider oscillations between the electron and the muon neutrino [MNS62]. Then it took around 20 years before Kayser in 1981 showed that the up to that point used plane-wave approximation cannot hold for oscillating neutrinos and he proposed a wave packet treatment [Kay81], which then has again not been discussed for around 10 years. In the early 90s the discussion on the theoretical description of neutrino oscillations finally started with several seminal papers. First, Giunti, Kim and Lee explicitly calculated the oscillation probability for the neutrinos in a wave packet model [GKL91] and then showed that the state vectors used for the quantum mechanical description are, in general, ill-defined [GKL92]. In 1993 they published together with Lee a calculation of the probability in a quantum field theoretical framework without using state vectors for the neutrinos [GKLL93]. And finally, in 1995 Blasone and Vitiello showed that the description of mixed particles in quantum field theory (QFT) yields unexpected problems for the interpretation of neutrinos as particles. By only using exact—without perturbation—QFT methods they calculated an oscillation probability which differs significantly from the other results [BV95]. All these different approaches are even today still under discussion, but however under the assumption of relativistic neutrinos which have tiny mass squared differences, all approaches give the same result. Thus, the theoretical discussion on the right description of the neutrinos does not spoil the experimental results, because today we are only able to measure ultra-relativistic neutrinos whose energy is at least a few orders of magnitude higher than their mass. 
Diploma Thesis On Theories of Neutrino Oscillations (Summary and Characterisation of the Problematic Aspects) Daniel Kruppke September 2007

A quantum field theory for flavor states ...
The study of mixing of fields of different masses in the context of Quantum Field Theory (QFT) has produced recently very interesting and in some sense unexpected results ... The story begins in 1995 when in Ref.[1], it was proved the unitary inequivalence of the Hilbert spaces for (fermion) fields with definite flavor on one side and those (free fields) with definite mass, on the other. The proof was then generalized to any number of fermion generations [7] and to bosonic fields [2, 5]. This result strikes with the common sense of Quantum Mechanics (QM), where one has only one Hilbert space at hand: the inconsistencies that arise there have generated much controversy and it was also claimed that it is impossible to construct an Hilbert space for flavor states [16] (see however Ref.[6] for a criticism of that argument). In fact, not only the flavor Hilbert space can be consistently defined [1], but it also provide a tool for the calculation of flavor oscillation formulas in QFT ..., which exhibit corrections with respect to the usual QM ones [20, 21]. From a more general point of view, the above results show that mixing is an “example of non-perturbative physics which can be exactly solved”, as stated in Ref.[13]. Indeed, the flavor Hilbert space is a space for particles which are not on-shell and this situation is analogous to that one encounters when quantizing fields at finite temperature [22] or in a curved background [23]. In the derivation of the oscillation formulas by use of the flavor Hilbert space, both for bosons and for fermions, a central role is played by the flavor charges [9] and indeed it was found that these operators satisfy very specific physical requirements [6, 8]. 
(Submitted on 23 May 2003 (v1), last revised 10 Jun 2003 (this version, v2))


... with an unfinished taste
Blasone and Vitiello (BV) have attempted to construct a Fock space for neutrino flavor states [4]... Giunti conclude that “the Fock spaces of flavor neutrinos are ingenuous mathematical constructs without physical relevance” [3]. 
... there is another issue that plagues the scheme in [5]. The problem is that the neutrino flavor vacuum defined in [5] is time-dependent and hence Lorentz invariance is manifestly broken. Recently, BV and collaborators attempted to tackle this issue by proposing neutrino mixing as a consequence of neutrino interactions with an external non-abelian gauge field [7]. Under this framework, the Lorentz violation of the neutrino flavor vacuum can be attributed to the presence of a fixed external field which specifies a preferred direction in spacetime. However, at the moment, there is not a single sign of such a non-abelian gauge field in neutrino experiments. They proposed that this scheme can be tested in the tritium decay, but again the indefinite mass mνα becomes an observable quantity. Also, given the current stringent bounds on Lorentz violations [8], it is unclear whether this scheme will survive... 
In this article, we first gave a detailed review on the current status of the understanding about the neutrino flavor states. At the end of the review, we were led to conclude that it is currently unclear how to construct a consistent and physically relevant Fock space of neutrino flavor states. We proceeded to prove that if one insists on second-quantizing the neutrino flavor fields and thereby constructing the flavor states, then they are approximately well-defined only when neutrinos are ultra-relativistic or the mass differences are negligible compared to energy...   
However, we showed that one can consistently describe weak interactions by only neutrino mass eigenstates. At the same time, we argued that the second quantization of neutrino flavor fields generally lacks physical relevance because their masses are indefinite. Thus, neutrino flavor states lose their physical significance and they should simply be interpreted as definitions to denote specific linear combinations of mass eigenstates involved in weak interactions. Under this interpretation, there is no physical motivation to construct the Fock space of neutrino flavor states from the first principles of quantum field theory. 
(Submitted on 16 Sep 2012 (v1), last revised 26 Nov 2012 (this version, v2))

mardi 25 août 2015

My quantum ostinato : the standard model can not stay out of the revolution of spacetime

This short post aims at two things:
  • to thank Jackson Clarke author of the very informative blog Syymmetries to put Transcyberphysix  in his recent list of "recommended (active) high energy physics news and blog links".
  • to suggest to any internaut arrived here through the former blog links to visit Quantum Ostinato another blog of mine which is currently more active than this one and might bring piece of information less covered by other blogs but relevant for people interested in high energy physics and astrophysics. 

lundi 29 juin 2015

Two scalars to rule the m(ass for almost) all (particles)? / Deux scalaires pour gouverner (presque) toutes les masses

The advanced art of massware in electroweak and QCD quantum vacua/ L'art subtil de la génération de masse dans les vides quantiques électrofaibles et chromodynamiques
This post is a follow-up to this one. / Ce billet fait écho à celui-ci vieux de plus d'un an.
In the standard model the masses of elementary particles arise from the Higgs field acting on the originally massless particles. When applied to the visible matter of the universe this explanation remains unsatisfactory as long as we consider the vacuum as an empty space. The QCD vacuum contains a condensate of up and down quarks. Condensate means that the q pairs are correlated via inter-quark forces mediated by gluon exchanges. As part of the vacuum structure the q pairs have to be in a scalar-isoscalar configuration. This suggests that the vacuum condensate may be described in terms of a scalar-isoscalar particle, |σ>=(|uu̅>+|dd̅>)/√2, providing the σ field. These two descriptions in terms of a vacuum condensate or a σ field are essentially equivalent and are the bases of the Nambu–Jona-Lasinio (NJL) model [28] and the linear σ model (LσM), [9] respectively. Furthermore, it is possible write down a bosonized version of the NJL model where the vacuum condensate is replaced by the vacuum expectation value of the σ field. 
In the QCD vacuum the largest part of the mass M of an originally massless quark, up (u) or down (d), is generated independent of the presence of the Higgs field and amounts to M = 326 MeV [1]. The Higgs field only adds a small additional part to the total constituent-quark mass leading to m u = 331 MeV and m d = 335 MeV for the up and down quark, respectively [1]. These constituent quarks are the building blocks of the nucleon in a similar way as the nucleons are in case of nuclei. Quantitatively, we obtain the experimental masses of the nucleons after including a binding energy of 19.6 MeV and 20.5 MeV per constituent quark for the proton and neutron, respectively, again in analogy to the nuclear case where the binding energies are 2.83 MeV per nucleon for 31 H and 2.57 MeV per nucleon for 3He. 
In the present work we extend our previous [1] investigation by exploring in more detail the rules according to which the effects of electroweak (EW) and strong-interaction symmetry breaking combine in order to generate the masses of hadrons. As a test of the concept, the mass of the π meson is precisely predicted on an absolute scale. In the strange-quark sector the Higgs boson is responsible for about 1/3 of the constituent quark mass, so that effects of the interplay of the two components of mass generation become essential. Progress is made by taking into account the predicted second σ meson, σ′(1344) = |ss̅> [7]. It is found that the coupling constant of the s-quark coupling to the σ′ meson is larger than the corresponding quantity of the u and d quarks coupling to the σ meson by a factor of √2. This leads to a considerable increase of the constituent quark masses in the strange-quark sector in comparison with the ones in the non-strange sector already in the chiral limit, i.e. without the effects of the Higgs boson. There is an additional sizable increase of the mass generation mediated by the Higgs boson due to a∼24 times stronger coupling of the s quark to the Higgs boson in comparison to the u and d quarks. In addition to the progress made in [1] as described above this paper contains a History of the subject from Schwinger’s seminal work of 1957 [10] to the discovery of the Brout-Englert-Higgs (BEH) mechanism, with emphasis on the Nobel prize awarded to Nambu in 2008. This is the reason why paper [1] has been published as a supplement of the Nobel lectures of Englert [11] and Higgs [12]...

The masses of constituent quarks are composed of the masses Mq predicted for the chiral limit and the mass of the respective current quark m0q provided by the Higgs boson (EW interaction) alone. For scalar mesons the sum of Mq and m0q leads to a zero-order approximation for the constituent-quark mass mq, but there are dynamical effects described by the NJL model which modify the simple relation mq=Mq+m0q  , except for the non-strange sector where this relation is a good approximation. Similar results are obtained for the octet baryons. A difference between the scalar mesons and the octet baryons is that that for scalar mesons binding energies do not play a rôle whereas they are of importance in case of octet baryons... 

(Submitted on 1 Jun 2015)

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