Friday, March 17, 2017

Can exotic compact objects exist?

Spring is coming, restoring from hibernation, sorry for the long silence on this blog!

This week we have put a new paper out on the arXiv: Exotic Compact Objects and How to Quench their Ergoregion Instability, which is essentially the outcome of my student Elisa Maggio's master thesis, so I thought that summarizing our results was a good excuse to restart this blog.

Exotic compact objects (ECOs) is just a catchy name for several models that have been proposed as alternative to black holes. Black holes are the natural outcome of the gravitational collapse in classical general relativity, but a growing number of theoretical physicists is uneasy with some potential drawbacks associated with them, in particular with the consequences of forming an event horizon as the final state of the stellar collapse.

According to these arguments, the classical picture of black holes would be at clash with quantum mechanics, even for astrophysical objects for which, in principle, quantum corrections are expected to be small. Some of the potential problems include the famous information loss paradox, the recent firewall proposal, the huge entropy of a black hole, and the singularity that lurks in its interior, where general relativity has to break down. There is no general consensus on whether or not these are serious problems that should be addressed in the context of classical gravity, but a growing community of theoreticians is looking for alternatives.

In brief, models of ECOs aim at reproducing all properties of compact dark objects that we observe (namely, a radius very close to the Schwarzschild radius and an arbitrarily large mass) and the theoretical properties (stability, formation as the end state of physical astrophysical processes,...) that characterize the black hole dynamics but without having an event horizon nor a singularity. So far, none of the ECO models on the market succeeded in reproducing all these properties, which is already a strong result in support of the black hole picture. But we know that theoreticians are stubborn and they keep trying.

At the theoretical level, a possible problem with ECOs is the so-called ergoregion instability, an instability that develops in compact objects which are spinning fast and that, unlike a Kerr black hole, do not have a horizon. In this paper, we showed two things:

a) The instability is rather generic and quite strong when the ECO has a surface that does not absorb any radiation (like a perfectly reflecting mirror). Although apparently unrealistic, some quantum-gravity models predict precisely this situation, and might be ruled out by this effect.

b) If the ECO can absorb a fraction of the radiation in its interior (for example by converting it in heat and thermalizing the perturbation), then it is possible to quench the instability. Whether this makes the model viable or not depends on two crucial issues, that we don't address in the paper: a) Are there ECO models that absorb enough radiation to quench the instability? b) What's the final state of the instability?

It is likely that the answer to the latter question is that an unstable ECO would simply slow down as a result of the instability, until it's not unstable anymore. In such case, one should compare theoretical models with observations of highly spinning black hole candidates, to see whether the instability is incompatible with observations.

A lot of work remains to be done but this is surely an exciting time for this kind of studies. As I write this, I'm at Frankfurt airport on my way back after having attended this workshop. I have found great interest among the participants related to the possibility of ruling out or detecting ECOs using GW observations (see also this other recent paper), which was something unimaginable just a few years ago, before the gravitational-wave revolution.

Anyway, truth is, i wrote this to have the opportunity also to share some funny pics from Elisa's graduation last December :-)

Elisa going through the wormhole, she entered as an undergrad
and emerged from the throat as a grad student ready for her PhD
From the left: Leonardo, Elisa, and Paolo celebrating Elisa's defense

Thursday, December 1, 2016

Meanwhile, at the Department of Physics @ Sapienza....

Random snapshots taken at the Department of Physics these days...love this place!

1) Perci & the Ciclofficina
Roberto Perciballi used to work as the priceless "factotum" of the Physics Department. He was in charge of essentially any repair, construction, moving, removal in the institute. Besides being a celebrity among students, he was also so on the stage as the frontman and singer of Bloody Riot, one of the first italian hardcore-punk bands. "Perci" suddenly died last March, leaving behind two daughters and the sorrow of many fans and friends.

The members of the student association "Ciclofficina" (named after Perciballi earlier this year) paid homage to him with this emotional and spectacular painting, showing a younger (and possibly punker) Roberto Perciballi. The painting features the end of a hallway (below on the right) that is offered by the Department to the student association. The poles have been installed after the recent earthquake in Central Italy which, although mild in Rome, also left some damage to the building.





2) Fidel & Majorana
These days, many all over the world are paying homage to the controversial figure of Fidel Castro. In fact, the whole Sapienza campus is upholstered with the poster below. However, how many can show off Fidel right next to Ettore Majorana's statue in front of the classroom named after him (below on the right)?




3) < Yes | No > & the Italian Constitutional Referendum
Tomorrow, Full Professors of Theoretical Physics Giovanni Bachelet and Giorgio Parisi will lead a discussion on the hottest topic in Italian politics nowadays: the Constitutional Referendum to be held next Sunday.



Prof. Bachelet is a former member of the Parliament and supporter of the constitutional reform. Prof. Parisi is the founder of the recent initiative "Salviamo la Ricerca" (see his recent letter to the Nature) and will defend his criticisms on the reform. I am looking forward to seeing how the physics community of our Department will react and will definitely attend the debate.

Thursday, November 24, 2016

Wednesday, October 12, 2016

International Physicists Network is born!

Great news! A new network of young physics researchers is born!
International Physicists Network (IPN) is a new italian no-profit organisation with the aim of building a national and international network of young physics researchers, and with the goal to create fruitful collaborations within different physics areas.



IPN has been founded by 11 researchers and professionals with a general background but multidisciplinary specialisation in physics: Davide Pietrobon (president), Marina Migliaccio (vice-president), Giordano Cattani (treasurer), Emanuela Pusceddu (secretary), Fabio Agostini, Claudia Antolini, Marco Di Stefano, Giacomo Fragione, Lorenzo Pagnanini, Matteo Serra and Francesco Stellato.
The main projects of IPN are the organisation of the Young Researcher Meeting, an yearly multidisciplinary conference aimed at students and young researcher in physics, with the goal to foster discussions and exchange of ideas in an informal atmosphere, and the Dandelion project (in collaboration with Find your doctor), a platform which allows to complement the traditional top-down approach to the job-market with an innovative bottom-up mechanism.


Tuesday, September 27, 2016

Laura and Nicola are Masters!

This week, my first two master students, Nicola Franchini and Laura Sberna, have defended their theses, and obtained a master degree (MSc in Astrophysics and in Physics, respectively) at Sapienza with maximum marks!

Nicola's thesis is entitled "Constraining Black Holes with Light Bosonic Hair Using Quasi-periodic Oscillations", whereas Laura's thesis is entitled "Early-Universe Cosmology in Einstein-scalar-Gauss-Bonnet Gravity"


Little time to celebrate for them: Laura is moving to the Perimeter Institute to start a PhD with Neil Turok, whereas Nicola is already at the University of Nottingham and just started a PhD with Thomas Sotiriou.
Kudos to them and good luck!


Celebrating Laura's "laurea" in front of the Physics Building "Edificio Marconi" at Sapienza. 
Elisa (on the left) is next on the line to finish her thesis.


Saturday, September 10, 2016

NASA to rejoin forces with ESA to launch gravitational-wave space detector!

News was in the air since the LIGO discovery, but the 11th LISA symposium in Zürich made it official: after its dropout in 2011, NASA is ready to rejoin ESA in the effort to build the first gravitational-wave observatory on space. The effect of this dropout was not only to change the mission's name (from the original "LISA" to "eLISA") but also to drastically reduce its budget, resulting in a smaller apparatus and reduced science case.


The recent GW discovery has boosted this field and, after the announcement in February, it was immediately clear that the plan was to anticipate eLISA flight (initially tentatively scheduled around 2034). Now, it seems that eLISA could fly some years earlier (around 2030) and, most importantly, it will probably be build in its original LISA design, thanks to the joint effort of ESA and NASA.

eLISA is a triangular laser interferometer that will search for gravitational waves in space



After the LIGO/Virgo discovery, after the amazing success of the LISA Pathfinder, this is yet another great news for gravitational physics and science in general. Year 2016 will definitely be remembered as marking the birth of a new discipline.





Thursday, September 1, 2016

Lectures on Gravitational Waves

This and next week I will be giving a series of lectures on "Gravitational Waves" at the Escola de Astrofísica e Gravitação (EAG8) at IST (Lisbon) and at the international school Invisible16 at SISSA (Trieste), respectively. The lectures at SISSA will be also recorded.

Some material related to the lectures (references, problems and notebooks) can be found here (this is still work in progress, since I am still preparing the problems and the Mathematica notebooks...)


The poster of EAG8

The poster of Invisible16