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]. 

The Unbearable Baldness of Black Holes

One of the most awe-inspiring properties of black holes is their absolute simplicity, or as John Wheeler famously put it, "black holes have no hair". As their progenitor collapses, its memory is forever lost, and all that remains is a quiescient, bald black hole. In a new article in Physical Review Letters, a team of scientists (that only by chance includes me...) has shown that black holes can nevertheless "grow hair" in the presence of matter, connecting them to the rest of the host galaxy.

Black holes are almost xeroxed copies of one another, differing at most in mass and rotation. These objects are described by a solution discovered by Roy Kerr in 1963. Remarkably, Kerr black holes are ubiquitous in almost any other theory of gravity, to the extend that the "Kerr hypothesis" is the current paradigm in astrophysics. 

First time I saw this picture was in one of Stephen Hawking's popular-science books, probably 'Brief History of Time'. It is supposed to describe the 'baldness' that this post refers too, am I the only one finding it a bit pathetic? :)

We have shown that in simple and attractive extensions of Einstein's theory (known as scalar-tensor gravity) black holes may not be described by the Kerr metric, as was previously thought. The crucial ingredient is the matter surrounding astrophysical black holes, typically in the form of accretion disks. The presence of matter triggers an instability that forces the bald Kerr black hole to develop a new charge -- a "scalar hair" -- connecting it to the matter around it and possibly to the entire galaxy. 

This hair growth is accompanied by a peculiar emission of gravitational waves, potentially by upcoming laser interferometers, which may test the Kerr hypothesis and probe the very foundations of gravity.

Read what real outreach journalists wrote on this on:

NewScientist

Huffington Post

Portuguese newspaper Público

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 (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!]