Paper of the day: "Damn it! Why wasn't me to write this??"

One of my favorite songwriters, now retired Francesco Guccini, wasn't used to sing pieces written by other authors. One of the rare occasions in which he decided to do so is this one:

where he sings Roberto Vecchioni's "Luci a San Siro". Guccini's incipit starts by saying some like 

"The song I am going to sing is titled: - Damn it! Why wasn't me to write this song? 

... Well, the paper I am going to review today is titled"

"Damn it! Why wasn't me to write this paper?"

The paper I am referring to appeared some days ago on the arXiv,

it is written by Carlos Herdeiro and Eugen Radu from the University of Aveiro. I have to admit it, this paper is just beautiful. Seriously. Not only the result circumvents one of the classical theorems of General Relativity [the black hole no-hair theorem, see below] but, in doing so, it also connects elegantly two solutions which were previously thought to be very different. As if that was not enough, it is beautifully written in such a way that the overall feeling is the one that only great papers can give -- a feeling that only scientists have the privilege to appreciate [and possibly artists can do so too, while watching/listening to//performing other colleagues' pieces of arts as in the video above]. 

Paper of the week: Quanto è stabile il fotone?

In questo articolo, pubblicato su Physical Review Letters, viene discussa la stabilità del fotone, testando la possibilità che esso possa decadere. Per fare ciò chiaramente dovrebbe possedere una seppur piccola massa, il chè non è a priori vietato da alcuna legge fisica (a parte qualche piccolo "dettaglio" sulla rinormalizzabilità della teoria, che però può essere risolto).

Ma facciamo un po’ di chiarezza: innanzitutto ciò che potrebbe suonare strano è il concetto di massa del fotone, infatti tutti o quasi tutti sappiamo che il fotone è una particella priva di massa, tuttavia asserzioni di questo tipo in fisica vanno corroborate da risultati sperimentali.

Esistono dei limiti sperimentali alla massa del fotone che possono essere determinati in svariati modi.

Un fotone con una massa non nulla avrebbe degli effetti osservabili, per esempio la legge di Coulomb ne verrebbe modificata e il campo elettromagnetico avrebbe un ulteriore grado di libertà. Se la legge di Coulomb non fosse esattamente valida, allora potrebbe causare la presenza di un campo elettrico all'interno di un conduttore cavo sottoposto ad un campo elettrico esterno. In questo modo è quindi possibile testare la legge di Coulomb con altissima precisione [2]. Un risultato nullo di tale esperimento ha fissato un limite di \(m \lesssim 10^{-14} eV\) [3].

Higgs o non Higgs questo è il dilemma!!

In questo articolo uscito il 2 Agosto, sul sito Arxiv.org nella sezione hep-ph, gli autori Jhon Ellis e Dae Sung Hwang si domandano se l'eccesso che è stato recentemente riportato dagli esperimenti ATLAS e CMS, con una massa di circa 125 GeV e con caratteristiche simili a quelle attese per il bosone di Higgs, possa avere o meno spin zero così come richiesto dal Modello Standard.

Paper of the Day: A relativistic model for Strange Quark Star

Il paper del giorno della sezione General Relativity and Quantum Cosmology (gr-qc) di ArXiv, comparso in data 30 Maggio 2012, va a: A relativistic model for Strange Quark Star. Gli autori di questo articolo propongono un modello relativistico delle cosiddette “strange quark stars”, nell’ambito del modello a quark denominato “MIT Bag model”. Essi discutono varie caratteristiche fisiche del modello e mostrano che il modello soddisfa tutte le condizioni di regolarità. Ciò che gli autori si propongono di fare è di interpretare infatti le relazioni Massa-Raggio di alcune stelle di neutroni che non sono compatibili con gli usuali modelli standard per le “normali” stelle di neutroni. Nel caso in questione i candidati sono 4U 1820-30, Her X-1, SAX J 1808.4-3658(SS1) e SAX J 1808.4-3658(SS2). Essi trovano che per la “strange star” 4U 1820-30 il valore stimato della costante del Bag è molto vicino ai valori accettati. 

Paper of the day: hep-th

http://arxiv.org/abs/1205.3807

About 10 years ago, gravitational physics, cosmology  and high-energy physics  witnessed two orthogonal revolutions, both related to the cosmological constant, "Einstiein's biggest blunder".

On the one hand, in 1998 some observations of Type Ia supernovae were announced, which indicated that the Universe is accelerating in its expansion. The most natural (and somehow easiest) way to accommodate these observations, together with measurements of the cosmic microwave background (CMB) radiation, is to allow for a small and positive cosmological constant (by "small" here we mean 10^{-123} in Planck units!). Actually, there are many other problems (generically referred to as "The Cosmological Constant Problem") that follow from this ridiculous smallness. But this is topic for another post.

On the other hand, at about the same time, in late 1997, Juan Maldacena proposed one of the most revolutionary ideas in high-energy physics of all time. His (as well Witten's, t'Hooft, Sussking, Gubser's and many others) idea is now known as AdS/CFT correspondence, gauge/gravity duality, Maldacena's conjecture etc... This can easily be the topic of another post too (by the way, apparently it's also the topic of 8283 papers citing Maldacena's orginal work to date). What is important now for the remaining is "just" that the ingredients of the AdS/CFT duality are two (a priori completely different) entities: 1) a gravitational theory in Anti de Sitter spacetime in D dimensions and 2) a gauge theory without gravity living on the boundary of this spacetime, i.e. in D-1 dimension. 

Naively, Anti de Sitter space means negative cosmological constant, whereas a positive cosmological constant (as that suggested by observations) is related to a de Sitter metric. 

Nowadays, the accelerating expansion of the Universe and the AdS/CFT correspondence are two of the most active research fields in physics. That is to say, what Einstein considered as his greatest failure (i.e. having introduced the cosmological constant in his equations) turned out to be revolutionary both in cosmology and in theoretical physics, but for different reasons, because observations require a positive cosmological constant, while Maldacena's idea calls for a negative one. 

So, are cosmological observations and high-energy theoretical studies completely disjointed? 

There would be nothing drammatic in answering affermatively, since both research fields have their own right and proved to be of fundamental relevance.

However, what is amazing is that, as triple-H (not this one, but Hartle, Hawking and Herzog) showed in their paper, the answer to this question could be no! 

Indeed, quantum effects can produce a semiclassical cosmology describing an accelerating and expanding Universe, even if the fundamental quantum gravity theory is defined in an Anti de Sitter space! That is, we might well see an "effective" positive cosmological constant, which is quantum-generated from a theory where the "bare" cosmological constant is negative... I wish i could say more on this, if only i could understand the paper :) [i promise: studying it is on the ToDo list, in my partial defense, the authors do not need presentation...].

Paper of the Day: GSI anomaly and spin-rotation coupling

L’articolo vincitore del giorno è: "GSI anomaly and spin-rotation coupling" nel quale gli autori G. Lambiase, G. Papini e G. Scarpetta (curiosa la coincidenza delle tre G.) propongono di poter interpretare la modulazione sinusoidale, recentemente riscontrata nel rate di decadimento degli ioni idrogenoidi ¹⁴⁰Pr , ¹⁴²Pm e ¹²²I, come il risultato dell’accoppiamento della rotazione dello spin dell’elettrone e del nucleo. Il modello mostra inoltre che l’accoppiamento spin-spin dell’elettrone e del nucleo non contribuisce alla modulazione anomala se il moto è rettilineo ma esso si presenterà se il moto degli ioni è fermato bruscamente su una targhetta, così come è compiuto nell'esperimento “incriminato”. In realtà la scelta dell’articolo del giorno non va tanto alla qualità di questo articolo (che non mi permetto di giudicare e a cui, con onestà, ammetto di non avergli dedicato il tempo necessario) ma principalmente perché mi permette di introdurre appunto la cosiddetta anomalia GSI. 

"Paper of the day": IS IT A BUBBLE?

Recently I read a notice and after the relative paper titled “A Tentative Gamma-Ray Line from Dark Matter Annihilation at the Fermi Large Area Telescope”. The Fermi Large Area Telescope (also known with the name GLAST, Gamma-Ray Large Area Telescope) is an international mission designed with the aim to explore the sky at very high energies. It is a space telescope capable of detecting photons in the range from 30 MeV to 300 GeV. The Fermi space observatory is a NASA mission with a broad international collaboration (Italy, Japan, France, Sweden). After the activation in orbit, the mission has been dedicated to Enrico Fermi and nowadays it is known as the Fermi Gamma-Ray Telescope. The autor of the paper, independently from the Fermi official collaboration, claimed to observe a gamma-ray signature in the cosmic-ray coming from dark matter annihilation in the Universe (indeed the pair-annihilation of Galactic dark matter produce a monochromatic or internal bremsstrahlung gamma rays that rise from the continuous background). 

Paper of the Day (gr-qc)

http://arxiv.org/abs/1204.4524

During the weekend I though it could have been a good experiment having a small "journal club" on papers submitted daily on the arXiv (i'll write something on arXiv one of these days). I cannot promise this blog-review will be very regular, but i'll do my best for the gr-qc and hep-th sections. Other contributors are in charge for other sections.

Our idea is to pick selected papers and describe them very briefly and informally. This brings me to the next problem: selecting the papers.

ArXivwise, Monday is usually quite a boring day. Papers appearing on Mondays are those submitted during the weekend and this explains why there are (on average) less papers. Well, not today!

Today the choice was quite hard (and not only because Frank Wilczek's "A Long View of Particle Physics"). Anyway my choice goes to Kent Yagi's paper above, which (or probably also because) is closely related to some projects I'm recently involved to.

Yagi is interested in constraining "alternative theories of gravity", i.e. theories that modify General Relativity (GR) in some regime. Typically, alternative theories differ from GR in  the description of the dynamics of black holes, neutron stars and other compact objects, but they are conceived in such a way that they pass Solar System tests. I'll postpone the motivation to investigate these theories and my personal interest for them to a future post, for the time being let me just say that it is very hard to modify GR in a way that is compatible with current and past experiment.

In brief, the paper above discuss: (i) a way to put very stringent constraints (in fact, much more stringent than those coming from Solar System experiments) to a particular class of theories that modify GR in the strong-curvature regime and (ii) that, despite these strong constraints, these theories "could" explain an unexpected astrophysical result: apparently, the orbital separation of some binary system (i.e. the distance between two compact objects orbiting each other) decreases in time faster than what is predicted in GR. That the distance decreases is well-known: it is due to the emission of gravitational waves (GWs) from the system. The fact that the this decrease is faster than expected, points to some poor understanding of the astrophysical processes involved in the process or to a more copious emission of GWs than what expected in GR. A quite common feature of modified gravities is that they predict a more efficient GW emission but (iii) the results of this paper would be very difficult to obtain by considering a alternative theory other than what Yagi considers. This should come as a surprise, given the ridiculously large number of proposed alternatives but, on the other hand, it makes the results more interesting. 

As the author stresses all over the paper, the cause of this discrepancy is most probably of astrophysical origin. However, it's intriguing to observe that current observations do not rule possible deviations from Einstein's theory at astrophysical level and, most importantly, that these corrections may eventually play a role in explaining the dynamics of compact stars and black holes [i'm sure the astro-contributors won't agree with me here :)]

[Note added in proof: next posts will be shorter!]