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Strong curvature corrections to General Relativity: consequences for astrophysics and for particle physics

Final Report Summary - ASTRONGR (Strong curvature corrections to General Relativity: consequences for astrophysics and for particle physics)

Einstein's theory of General Relativity will celebrate its 100th anniversary in 2015 and, after having passed many stringent tests, it is now accepted as the standard theory of gravity and one of mankind's greatest achievements. Nevertheless, most experiments can only probe regions of weakly-curved spacetime, while the strong-curvature regime remains essentially unexplored. In this regime the dynamics of black holes and neutron stars can sensibly differ from General Relativity, with dramatic and potentially observable effects.

The project aStronGR - "Strong curvature corrections to General Relativity: consequences for astrophysics and for particle physics" [PI: Dr. Paolo Pani, scientist in charge: Prof. Vitor Cardoso] aimed to address the following question:

"How can one probe the strong-curvature regime of gravity by means of astrophysical observations?"

To answer this question, we have developed new tools to study strong-field effects that take place near black holes and neutron stars.

Some of the major outcomes of the project are:

1) We developed a novel framework to study perturbations of slowly rotating black holes and proved, for the first time, the instability of spinning (Kerr) black holes triggered by a light vector field. Our results allow to put one of the most stringent constraints on the mass of the photon and of other dark matter particles. This study results in two papers published in Physical Review Letters and in Physical Review D, the former paper has been selected by the Editor as an American Physics Society highlight.

2) We applied the above framework to study perturbations of the Kerr-Newman black hole. This was a long-standing open problem in classical General Relativity. We provided the first stability analysis of this solution and found evidences for the isospectrality of its modes for slow rotation.

3) We have constructed a promising class of modified theories of gravity that can resolve some curvature singularities affecting General Relativity. Unfortunately, we have also discovered that this class of interesting theories have some other pathologies that cast serious doubts on their viability.

4) We have studied a novel phenomenon that can be used to derive stringent theoretical constraints on the dark-matter fraction in primordial black holes in the only mass range that remains phenomenologically allowed to date. Our results, in combination with previous constraints, allow to rule out primordial black holes as dominant dark-matter constituent.

5) We wrote an invited monograph on 'Advanced Methods in black-hole perturbation theory', which was published in the Numerical Relativity/High Energy Physics Special Issue of the International Journal of Modern Physics A.



Overall, the results of this project demonstrate that black holes and other compact objects have an extraordinary potential as "cosmic laboratories", where extensions of the Standard Model of particle physics, dark matter, and even the very foundations of Einstein's theory can be put to the test. We have selected various "smoking guns" for deviations from General Relativity, that will pave the way for novel studies in parallel with the advent of gravitational-wave astronomy. European Union has assumed a major leading role in the effort of detecting gravitational waves and testing Einstein theory within the next decade. In this context, our results will have an impact both for the fundamental physics community and also for the gravitational-wave community and they contribute to the European excellence in fundamental science.

As an outcome of this project 22 scientific papers were produced and they were published in major international journals, including 5 papers in the prestigious Physical Review Letters, the journal with the highest impact factor in the field, and other papers were selected by the Editorial Board as highlights of their journals.

All numerical codes developed and used during this project are publicly available at the group's webpage http://blackholes.ist.utl.pt/ and the original papers can be freely downloaded at the webpage http://www.arxiv.org.

The outcomes of the project were extensively covered by international media, including Nature News, Time, the Huffington Post, the American Physics Society magazine, NewScientist and various Portuguese newspapers. Together with these interviews, as part of the outreach initiatives the PI of the project created the outreach blog "The Gravity Room" [webpage http://thegravityroom.blogspot.pt/].

The results of the project were presented at the major international conferences of the field, including the 13th Marcel Grossmann Meetings, the GR20/Amaldi10 Conference, various workshops worldwide and numerous invited talk in world-renowned institutes, including the Perimeter Institute (Canada), the International School for Advanced Studies (SISSA, Italy), KEK (Japan), Rome U. Sapienza (Italy). As a part of the project, the PI has been a long-term Visiting Researcher at Harvard-Smithsonian Center for Astrophysics in Cambridge, MA (USA), one of the most prestigious institutes for astrophysics in the world.

In March 2013, the team organized the highly-successful "Strong Gravity Beyond GR" Workshop in Lisbon [webpage: http://blackholes.ist.utl.pt/stronggravity/] inviting almost 30 world-renowned experts of strong-field tests of General Relativity. The workshop has promoted new collaborations between the gravity group in Lisbon and various other institutions worldwide, including the Institute of Astrophysics in Paris, Harvard University, Nottingham University and Rome Sapienza.

As a part of the training activities, the PI has actively worked as a referee of all major international journals of the field, as a lecturer at the NR/HEP2 Spring School in Lisbon [webpage http://blackholes.ist.utl.pt/nrhep2/] as well as as co-advisor of master and Ph.D. students.