CORDIS - Forschungsergebnisse der EU

Matter and strong-field gravity: New frontiers in Einstein’s theory

Periodic Reporting for period 4 - MaGRaTh (Matter and strong-field gravity: New frontiers in Einstein’s theory)

Berichtszeitraum: 2020-06-01 bis 2021-11-30

This project aims at understanding the physics of strong gravitational fields, such as those near black holes or neutron stars. How does matter behave close to black holes? This problem is important for a number of reasons. For example, it allows one to understand how dark matter behaves close to black holes. It allows one to quantity the evidence for black holes (what evidence do we have that dark, massive objects really are black holes?) If these objects really are black holes, are they described by Einstein's theory?

These are fundamental issues in our quest to understand the cosmos. The landmark (direct) detection of gravitational waves by LIGO now allows for unprecedented tests of gravity. This project will make precise predictions of exactly what one can test.
The MaGraTh team published over 70 papers in international scientific journals, including 10 in Physical Review Letters and two in Nature (most of them cover of the journal and/or Editor's Suggestions). The team organized 15 conferences and schools and delivered several outreach talks in highschools and elsewhere, as well as appearing regularly in the media (newspapers and television). We were invited to over 50 conferences and workshops as invited plenary speakers.

Among the main scientific results, we have

Shown that Strong Cosmic Censorship does not hold generically. Our acceptance of General Relativity as a theory describing our universe
hinges on the ability of the theory to hide potential singularities. This is known as the Cosmic Censorship
Conjecture, and is an unproven assumption. In a PRL Editor’s Suggestion, we have recently shown that Strong
Cosmic Censorship does not hold generically. This work has triggered a flurry of activity, of which we have
seen only the first developments.

V. Cardoso, J. L. Costa, K. Destounis, P. Hintz and A. Jansen, Quasinormal modes and Strong Cosmic Censorship,
Phys. Rev. Lett.120 no.3 031103 (2018)

2. Shown that LIGO historical detection is strong evidence for objects with photspheres, but is not in itself a proof of the existence of black holes.

V. Cardoso, E. Franzin
and P. Pani, Phys. Rev. Lett. 116 (2016) 171101; arXiv:1602.07309 [gr-qc]. This work was selected by the Editors of PRL to feature as Editor's Suggestion, Synopsis and PRL cover
article for further reading, for its novelty, groundbreaking and interdisciplinary character.

V. Cardoso, E. Franzin, A. Maselli, P. Pani and G. Raposo,
Phys. Rev. D 95, 084014 (2017); arXiv:1701.01116 [gr-qc].
Editor's Suggestion for outstanding quality)

V.Cardoso S. Hopper, C. F. B. Macedo, C. Palenzuela and P.Pani ``Gravitational-wave signatures of exotic compact objects and of quantum corrections at the horizon scale,''
Phys. Rev. D 94, no. 8, 084031 (2016)
[arXiv:1608.08637 [gr-qc]].

V. Cardoso and P. Pani, Testing the nature of dark compact objects: a status report, Living Rev. Rel.22 (2019)

These works introduced the concept of echoes in gravitational-wave science. Echoes are currently being searched for in LIGO data and detection strategies are being implemented in its pipeline. This work was followed by invited review articles to Nature Astronomy anf Living Reviews in Relativity.

3. Shown that future gravitational wave detectors, such as ESA's space-based LISA will be able to test Einstein's theory in the strongfield regime through the observation of the
vibrations of black holes (E. Berti, A. Sesana, E. Barausse, V. Cardoso and C. Belczynski, Phys. Rev. Lett. 117 (2016) 101102; arXiv:1605.09286 [gr-qc].)

4. Shown that rotating neutron stars, such as pulsars, can help in constraining dark matter models, through a mechanism known as superradiance. In parallel, we have shown how to demonstrate in the laboratory the existence of such an effect. The experiment is currently being implemented in Nottingham University (V. Cardoso, Antonin Coutant, Mauricio Richartz, Silke Weinfurtner, Phys. Rev. Lett. 117, 271101 (2016); arXiv:1607.01378 [gr-qc].)

5. Finally, in a follow-up to my ERC Starting Grant, we have cemented black hole physics as a potential tool to inform us on dark matter. This can happen either via electromagnetic observations of spinning objects, or through gravitational-wave detection of monochromatic waves from bosonic condensates:

R. Brito, S. Ghosh, E. Barausse, E. Berti, V. Cardoso, I. Dvorkin, A. Klein and P. Pani, Stochastic and resolvable gravitational waves from ultralight bosons,
Phys. Rev. Lett.119 no. 13, 131101 (2017)
[arXiv:1706.05097 [gr-qc]].

R. Brito, S. Ghosh, E. Barausse, E. Berti, V. Cardoso, I. Dvorkin, A. Klein and P. Pani, Gravitational wave searches for ultralight bosons with LIGO and LISA, Phys. Rev. D96, no. 6, 064050 (2017) doi:10.1103/PhysRevD.96.064050
[arXiv:1706.06311 [gr-qc]].

V. Cardoso, Ó. J. C. Dias, G. S. Hartnett, M. Middleton, P. Pani and J. E. Santos, Constraining the mass of dark photons and axion-like particles through black-hole superradiance,
JCAP 1803, no. 03, 043 (2018)
[arXiv:1801.01420 [gr-qc]].
Work 1 above changed the way we think about Einstein equations and is a breakthrough in the field in the last decades.

Work 2 is a first and significant step towards the understanding of compact objects, and towards quantifying the existence of horizons in the universe. Several scenarios
incorporating quantum gravity effects predict classical collapse to be halted before horizon formation. Our work provides a clear smoking gun for such effects.

Black holes, time warps and Einstein's theory appeals to the society at large, and our work has been disseminated wildly. It was covered by nearly all mainstream media (Le Monde, La Republica, New Scientist, etc), in radio and TV shows. Currently, an american TV program on black holes is being prepared which will include our own results: if objects are not really black holes, "echoes" should appear at late times in gravitational waves.

Works 3 - 5 are an extension of work where this team is pioneer: in the understanding of tests of General Relativity and of rotational superradiance effects (we published a book on the subject recently: "Superradiance" by Richard Brito, Vitor Cardoso & Paolo Pani, Lect.Notes Phys. 906 (2015).
Tidal disruption of a superradiant scalar condensate
Tidal effects on boson stars, allowing to distinguish them from black holes
How my institute looks like when seen distorted by a black hole
Gravitational-wave signal from black hole mimickers
Black hole binary immersed in a scalar