Written by Alexandru Dima **
Black holes are physicists’ favorite playground for tinkering around and trying to uncover the hidden mechanisms behind the fundamental forces of Nature. In fact, it is in these incredible compact objects where the most extreme and less understood regime of gravity is realized. The advent of gravitational wave astronomy has opened a novel and exciting window on astrophysical compact objects like black holes and neutron stars, making them the perfect laboratory where to probe our current understanding of gravitation. Despite the complex and rich phenomenology of these objects, black holes in General Relativity are fully characterized by only two quantities: their mass and their angular momentum. In Einstein’s theory of gravity, a mathematical theorem ensures that any additional structure –in jargon “hair”– on top of black holes cannot be sustained. Following the iconic analogy, black holes eventually shed their "hair" away through the emission of gravitational waves, similarly to the way people can eventually lose their hair. While Einstein’s black holes are destined to baldness, this is not necessarily true in alternative theories of gravity. Physicists have been investigating plenty of possible extensions of General Relativity in order to solve theoretical issues –linked to the ongoing search for a quantized theory of gravity– or as potential explanations of the puzzles that still await in the “dark side” of gravitational physics, like dark energy or dark matter. Even though “no-hair” theorems exist also for many alternative theories, the modified theoretical setup allows to circumvent them in some cases. In our work, we have considered a wide class of modified gravity theories that generally predict significant deviations from General Relativity in extreme regions such as the surroundings of black holes or neutron stars. Thanks to numerical simulations of “bald” rotating black holes, we have observed that in these theories they can spontaneously grow the simplest form of hair (a scalar field) when the space-time around them is sufficiently curved. We have discovered that the hair-growth mechanism is controlled by how fast the black holes are spinning: static or slowly rotating black holes are indistinguishable from what General Relativity predicts, while black holes that rotate sufficiently fast spontaneously transit to their “hairy” version. In particular, our result suggests that, depending on the rotational velocity of the objects involved, the gravitational waves produced as a consequence of the merger of black hole binary systems might be consistently different from what was previously expected. In the near future, observation of such an effect –or lack thereof– by gravitational-wave experiments would consequently allow us to falsify a wide class of alternative theories of gravity, or possibly discover novel hints of new physics beyond General Relativity.
This short outreach article is in reference to the following paper https://arxiv.org/abs/2006.03095
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