Dirty black holes are more interesting
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, almost featureless, black hole. In a new article to appear in Physical Review Letters, our group questions whether this conclusion applies to realistic, astrophysical black holes.
Isolated, “clean” 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.
We have shown that in simple, well-motivated extensions of Einstein’s theory known as scalar-tensor theories, 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 in such “dirty” black holes forces the hairless Kerr black hole to develop a new charge — new “hair” — anchoring it to the surrounding matter and possibly to the entire galaxy”.
This hair growth is accompanied by a peculiar emission of gravitational waves, potentially detectable by upcoming laser interferometers, which may test the Kerr hypothesis and probe the very foundations of gravity.
See the coverage in New Scientist, Huffington Post, and Público .
Our group just uncovered a curious case of cannibalism among black holes. It’s well known that black hole encounters give birth to gravitational waves. For encounters at close to the speed of light, surprising results emerge: the black holes, regardless of their structure, eat half of their gravitational-wave progeny. These results may change the way we think about ultrarelativistic collisions and black hole production and are the outcome of a recent study to be published in the prestigious Physical Review Letters. More here, for the original article click here. See also the Ciência Hoje coverage (in portuguese).
Light David defeats Goliath
(or how to weigh small particles with supermassive black holes)
Supermassive black holes can be used to measure the mass of extremely light particles to unprecedented levels and rule out the existence of new exotic particles, perhaps constraining the nature of dark matter. These are the results of a recent study to be published in the prestigious Physical Review Letters. More here, for the original article click here. See also the New Scientist, Expresso, Phys. Org and APS Synopsis coverage.
On the radio
The gravity group of CENTRA was featured in portuguese radio TSF program “Mundo Novo”. You can listen to it here, where Jorge Rocha makes a clear description of our activities (in portuguese).
Floating orbits
Our recent paper on floating orbits was published in Physical Review Letters. The INAF website has a nice outreach article on our work. INAF is the Italian Institute of Astrophysics, and the article is in Italian (you have been warned!). Thanks to Marco Galliani for the write-up! . See the original article and the coverage by the INAF
Gravity without singularities
Our results show that a new theory of gravity avoids singularities in the collapse of matter and gives rise to dark-matter stars in a natural way. See the original article and the coverage by the INAF (Italian National Institute for Astrophysics) on an interview of Paolo Pani by Marco Galliani
Superhero move saves black holes
Our results show that Cosmic Censorship is safe, for now. These results have been covered by New Scientist and Buzz Blog.
DEISA Extreme Computing
We have been granted one and a half million CPU hours in the DEISA Extreme Computing Initiative, an European Consortium for challenging computational problems
Access to Supercomputer
We have been granted one million our CPU time in Magerit, part of the Spanish MareNostrum Consortium
No naked black holes
Our results on high-energy collisions of black holes, showing that head-on encounters radiate 14% of the center-of mass energy, were covered by Science News
Black holes and water droplets
An invited viewpoint for the prestigious American Physical Society, written by Vitor Cardoso. It explains recent breakthroughs in the experimental realization of spinning water droplets, and their relation to black hole physics.