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In a study published on Nature Astronomy in September 2017, Researchers Paolo Pani from the Sapienza Department of Physics and Vitor Cardoso from the Superior Technical Institute of Lisbon discussed how, thanks to new theoretical models, gravitational astronomy can provide information on the nature of so-called "compact objects," a term used in Astrophysics to describe large masses concentrated in very small dimensions, such as black holes.

Gravitational waves contain information on the nature of compact objects, just like sound waves produced by musical instruments depend on the shapes, sizes and materials of the instruments that produced them. Their detection, less than two years ago, served as a fundamental confirmation of Einstein's General Relativity Theory and paved the way for this new chapter in physics.

According to Einstein's theory, at the end of its life cycle, a “massive star” collapses under its own weight and forms a black hole, an object that distorts space-time so drastically that not even light is able to escape from its horizon of events, the area beyond which phenomena can no longer be observed. New theoretical models suggest that the presence of gravitational echoes, if detected by LIGO and Virgo interferometers, may indicate that black holes are not exactly as hitherto hypothesized. Indeed, several quantum gravity models predict that the horizon of events does not develop and the gravitational collapse terminates with the creation of a "compact exotic object" rather than a real black hole. 

"These hypotheses will finally be denied or confirmed by the observations with the LIGO and Virgo gravitational interferometers, once they reach their maximum levels of precision," explains Physicist Paolo Pani. "If gravitational echoes are not observed, it will be a further confirmation of Einstein's theory in a regime that has never been explored so far, but if revealed, it would open up a completely new road."

This new line of research will investigate what could happen if, instead of the formation of a black hole, another compact exotic object came into being, or, in other words, what would be observed if there were no horizon of events.

Professor Paolo Pani has obtained a Starting Grant from the European ERC Programme for Project DarkGRA, which addresses these issues.

"Our aim with DarkGRA is to study new phenomena that affect the most extreme gravitational springs: black holes and neutron stars," explains Professor Pani. "These objects are true cosmic labs that help us to investigate the limits of Einstein's General Theory of Relativity, the nature of black holes' horizon of events and the properties of the dark matter that pervades our universe".

Tests for the Existence of Black Holes through Gravitational Wave Echoes - Vitor Cardoso & Paolo Pani - Nature Astronomy 1, 586-591 (2017) DOI: 10.1038 / s41550-017-0225-y