## Quantum Black holes & The Quantum Universe

##### Ram Brustein

Interpreting the thermodynamic properties of quantum black holes and cosmological space-times with horizons and uncovering their underlying quantum statistical mechanics remains a challenge in spite of the intense efforts over the last 40 years. What does the black hole entropy measure, the degeneracy of microstates, entanglement entropy between the inside and outside of the horizon, or some intrinsic gravitational entropy? Is the quantum mechanics of space-times with causal boundaries unitary? If so, why do some of them look thermal and non-unitary in some approximation?

We have recently proposed that black holes and the universe have to be described by quantum stateswithin string theory and that the extreme approximation of treating spacetime as strictly classical geometric object is at the origin of many of the above mentioned issues. The idea was applied to study quantum black holes and we showed in a series of recent articles that indeed, when the the quantum nature of the black hole is taken into account the evolution is consistent with unitarity and other quantum mechanical constraints. More recently, we applied this idea to black hole mergers and proposed that gravitational waves detectors should see specific signatures of the quantum nature of black holes.