Highlights of Classical and Quantum Gravity

Good news today, the paper Tidal acceleration of black holes and superradiance by Vitor (Cardoso) and myself was selected by the Editorial Board of Classical and Quantum Gravity (CQG) to be one of the journal’s Highlights of 2012-2013!

Besides being now free to download, the Highlights will be promoted in a number of campaigns over the next year  as a representation of some of the most interesting and high-quality work in gravitational physics.

But what makes the paper really worth a reading is the first paragraph, where we managed to refer to both Italian novelist Italo Calvino and mighty rock band Pink Floyd... that's quite an impressive achievement for a scientific paper!

If you wish to know what Pink Floyd's masterpiece "The Dark Side of the Moon" has to
 do with black holes, read the (not-very-technical) paper here.

Stephen Hawking's big ideas... made simple - animation

Credits: http://www.theguardian.com/science

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

Backreation on "Black holes and the Planck length"

“  According to Special Relativity, an object in motion relative to you appears shortened. The faster it is, the shorter it appears. This is effect known as Lorentz-contraction. According to General Relativity, an object that has a sufficiently high mass-density in a small volume collapses to a black hole. Does this mean that if a particle moves fast enough relative to you it turns into a black hole? No, it doesn't. But it's a confusion I've come across frequently. (Read on http://backreaction.blogspot.com/)

                 ”

NuSTAR on the spin of supermassive BHs

Some month ago, in this post, we reported about the launch of the Nuclear Spectroscopic Timing Array (NuSTAR).

Today NASA scheduled a media teleconference to announce one of the first scientific achievements of this mission: the first precise measurement of the spin of a supermassive black hole. 

The discovery will appear in this week Nature's issue.

It turns out that NGC 1365 hosts a rapidly spinning supermassive black hole at its center, and this black hole rotates very close to its theoretical limit imposed by Einstein's General Relativity.

Curiosity: 

one usually refers to these black holes as "extremal" because their spin is close to its maximum value. In "God-given" natural units, the angular momentum of an extremal black hole is J=M^2. 

Indeed, what is usually called the "Kerr limit" is J/M^2 = 1.  This is by no means an "extremal" value!! [For example, Earth also spins around its axis and it has J/M^2~ 10^9 in natural units!]

This doesn't mean that these black monsters in the sky don't spin fast, quite the opposite: because their mass is HUGE, the angular momentum of an extremal black hole of about 10^6 solar masses [roughly the mass of the black hole at the center of the NGC 1365 galaxy]  is as large as 10^13 times that of Earth... a pretty huge and fast spinning top!

PS:

The first author of the paper, Guido Risaliti is an Italian astrophysicists who works here at the Harvard-Smithsonian CfA and at Italian Institute for Astrophysics in Arcetri.

V Black Holes Workshop in Lisbon

Five years ago, Portuguese scientists working in black hole physics started gathering right before Xmas time in a short and informal workshop. The first edition of the Black Hole Workshop was held in Porto and was a sort of reunion of people working worldwide and coming back home for holidays.

Visit the webpage of the V Black Holes Workshop

Over the years, the workshop extended to nonPortuguese people (I started attending it since the second edition) and it developed to what is now a regular international workshop. This year, we in CENTRA have the pleasure to host the fifth edition, which will be held in Lisbon next Monday and Tuesday. We'll have over 60 participants, about 30 talks and a social dinner at the delicious Restaurante dos Passarinhos!

PS:

If you are wondering what the workshop logo is, it's an artistic view (courtesy of Ana Sousa) based on a very typical decoration in Rossio's square, downtown Lisbon.

An over-massive black hole in the compact lenticular galaxy NGC 1277

A new survey recently reported in Nature found a supermassive black hole (mass~17 billions of solar masses) at the center of a relatively "light" galaxy. This wouldn't be a surprise if the mass of the black hole wasn't more than half the mass of the buldge of the hosting galaxy.

Figure 3: The correlation between black-hole mass and near-infrared bulge luminosity.
The black line shows the mass–luminosity relation for galaxies with a directly measured black-hole mass.
NGC 1277 is a significant positive outlier.
[taken from Nature]

Poster at the XX SIGRAV Conference

On the 22nd of October I will attend the XX SIGRAV Conference (SIGRAV stands for Italian Society for General Relativity and Gravitation) in Naples and I will present this poster:

here is the pdf

which is done with the baposter package for LaTeX, a great tool to create fancy posters (i mean, fancier than this one). While in Naples, I'll surely write a brief summary on the conference, so stay tuned.

Light David defeats the supermassive Goliath [on NewScientist]

read the coverage

There must be something wrong in science if the NewScientist published this weird coverage on massive photons around black holes.

First, even my grandma knows that "black holes tend not to exist" [cit.], so that putting bounds on the mass of the photon using observations of supermassive black holes, as the author apparently do, doesn't sound very likely.
Second, I know personally most of the authors and at least one of them is dangerously close to be a crackpot, to be polite.

Thus, i strongly suggest you not to read the original paper, or to read it just for fun, and never ever trust those authors!

Btw, if you are not disgusted enough, here you can find another view on the problem.

Everything you always wanted to know about GR tests in Astrophysics but were afraid to ask

While I was looking for a refreshing reading on GR tests in Astrophysics, I stumbled upon this very nice review by Dimitrios Psaltis.
He's one of the most renowned experts on GR tests with compact objects (the one always invited at conferences to give review talks on the topic, if you know what I mean), and this paper gives a very nice overview of what constraints can be put on GR and alternative theories with astrophysical observations. It dates 2008 but nonetheless there are only a handful of things that need to be updated. For example, there will be no IXO (that was dropped by NASA, then became Athena, and was officially dropped by ESA yesterday, shame on them. Ok, I stop complaining about it. No I don't).

Anyways, if you are interested on this topic, you can read the review. And you should.

It starts by explaining in layman's terms (ok, maybe in young PhD-understandable terms) why it is important to test GR over alternative theories, and what are the differences between them.
An important thing that is pointed out is that our most successful tests of GR in Astrophysics have only shown that it works very well in the limit of very weak gravitational fields (low curvature of the spacetime). Even double neutron star observations like the one on this fantastic double pulsar, that follows GR predictions with an uncertainty of 0.05%, probe a spacetime whose curvature is not very different from the one probed in solar system tests. Wikipedia classifies, incorrectly, those tests as "Strong field tests". (But keep in mind that even weak fields can be used to test alternative theories, as a former post in this blog showed).

Is there a way to probe strong gravitational fields? Yes. We make observations of accreting black holes and neutron stars (the most compact objects present in the Universe), for example, and we measure (possibly relativistic) oscillations, broadened spectral lines, orbital decays and more that can be used to measure the physical properties of the compact object, model-dependently, and test the predictions of GR.

The constraints that we can put today on GR and alternatives are weak, but still significant. Some sporadic observations of particularly interesting sources might improve greatly our knowledge even with current instruments. For example, the recent discovery of a very heavy neutron star permitted to put unprecedented constraints on the equation of state of these objects, and hence to have better estimates on the gravitational pull at their surface. The importance for nuclear physicists of the constraints on neutron star equations of state is even greater, but we'll talk about it in the future.

Let's not talk about canceled missions (I cannot even pronounce her name without dropping a tear, poor Greek goddess) that would have had the features to give nice measures where now we struggle with upper limits, it's too painful. But let us hope that smaller but powerful missions like LOFT have better chance (If you are interested, we host a science meeting in Toulouse in September). Stay tuned.