Quantum Black Holes in the Sky
  Black Holes are possibly the most enigmatic objects in our Universe. From
their detection in gravitational waves upon their mergers, to their snapshot
eating at the centres of galaxies, black hole astrophysics has undergone an
observational renaissance in the past 4 years. Nevertheless, they remain active
playgrounds for strong gravity and quantum effects, where novel aspects of the
elusive theory of quantum gravity may be hard at work. In this review article,
we provide an overview of the strong motivations for why "Quantum Black Holes"
may be radically different from their classical counterparts in Einstein's
General Relativity. We then discuss the observational signatures of quantum
black holes, focusing on gravitational wave echoes as smoking guns for quantum
horizons (or exotic compact objects), which have led to significant recent
excitement and activity. We review the theoretical underpinning of
gravitational wave echoes and critically examine the seemingly contradictory
observational claims regarding their (non-)existence. Finally, we discuss the
future theoretical and observational landscape for unraveling the "Quantum
Black Holes in the Sky".
