Echoes of gravitational waves: confirmation or disproof of black holes?

Paolo Pani, Department of Physics
2017-05-09
Physical Sciences and Engineering

In a paper recently published in “Nature Astronomy”, researchers Paolo Pani (Sapienza University of Rome) and Vitor Cardoso (Instituto Superior Técnico, Lisbon) argue that, by using novel theoretical models, gravitational-wave astronomy can provide unique information about the nature of so-called “compact objects”, namely astrophysical masses concentrated in very small and dense regions, such as the black holes that populate our universe.

The gravitational waves detected by the LIGO and Virgo interferometers contain the fingerprint of the compact objects that produced them, just like the sound waves produced by an instrument depend on the properties of the latter: its shape, size, material, etc. Two years ago, the first detection of gravitational waves has opened a new era for physics and astronomy and provided a new crucial confirmation of Einstein’s theory of general relativity.

According to Einstein’s theory of gravity, a massive star at the end of its life cycle should collapse under its own gravity and form a “black hole”, an object that distorts the spacetime in such a drastic way that not even light is able to escape its “event horizon”, the region beyond which it is impossible to observe any phenomenon from outside.

New theoretical models suggest that the putative existence of “gravitational echoes” – if detected by the LIGO and Virgo instruments – would demonstrate that black holes are different from what expected so far: several models of quantum gravity predict that the event horizon should not form at all, and that the outcome of the gravitational collapse is an exotic compact object, not a black hole.

“This fascinating hypothesis can be finally ruled out or confirmed through the new observations provided by LIGO and Virgo, especially when these instruments will reach their design sensitivity” - theoretical physicists Paolo Pani says - “If gravitational-wave echoes are not detected in the next few years, this would be yet another outstanding confirmation of Einstein’s theory in a regime that has never been explored before. However, any evidence of echoes would open a completely new path in the field”.

This new area of research is devoted to investigate what would happen if, instead of a black hole, a different exotic compact object new era for physics and astronomy and provided a new crucial confirmation of Einstein’s theory of general relativity. According to Einstein’s theory of gravity, a massive star at the end of its life cycle should collapse under its own gravity and form a “black hole”, an object that distorts the spacetime in such a drastic way that not even light is able to escape its “event horizon”, the region beyond which it is impossible to observe any phenomenon from outside.

New theoretical models suggest that the putative existence of “gravitational echoes” – if detected by the LIGO and Virgo instruments – would demonstrate that black holes are different from what expected so far: several models of quantum gravity predict that the event horizon should not form at all, and that the outcome of the gravitational collapse is an exotic compact object, not a black hole.

“This fascinating hypothesis can be finally ruled out or confirmed through the new observations provided by LIGO and Virgo, especially when these instruments will reach their design sensitivity”, theoretical physicists Paolo Pani says - “If gravitational-wave echoes are not detected in the next few years, this would be yet another outstanding confirmation of Einstein’s theory in a regime that has never been explored before. However, any evidence of echoes would open a completely new path in the field.”

This new area of research is devoted to investigate what would happen if, instead of a black hole, a different exotic compact object forms after the merger. In other words, what would LIGO or Virgo detect if there is no event horizon?This is the main goal of the DarkGRA project (http://darkgra.weebly.com), that was recently funded by the European Research Council (ERC) through a Starting Grant, contributing to make Sapienza the Italian institution hosting more ERC grants in 2017.

“The DarkGRA team will study some new phenomena that occurs in the most extreme gravitational sources, such as black holes and neutron stars” Pani explains. “These objects can be used as cosmic laboratories to study the limits of general relativity, the nature of the event horizon of a black holes, and the mysterious properties of the dark matter in our universe”.

 

INFO

Team Leader
Paolo Pani
Dip. di Fisica
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