
Anderson, P. R., Balbinot, R., Fabbri, A., & Parentani, R. (2014). Graybody factor and infrared divergences in 1D BEC acoustic black holes. Phys. Rev. D, 90(10), 104044–6pp.
Abstract: It is shown that the graybody factor for a onedimensional elongated BoseEinstein condensate (BEC) acoustic black hole with one horizon does not vanish in the lowfrequency (omega > 0) limit. This implies that the analog Hawking radiation is dominated by the emission of an infinite number (1/omega) of soft phonons in contrast with the case of a Schwarzschild black hole where the graybody factor vanishes as omega > 0 and the spectrum is not dominated by lowenergy particles. The infrared behaviors of certain correlation functions are also discussed.



Anderson, P. R., Balbinot, R., Fabbri, A., & Parentani, R. (2013). Hawking radiation correlations in BoseEinstein condensates using quantum field theory in curved space. Phys. Rev. D, 87(12), 124018–18pp.
Abstract: The densitydensity correlation function is computed for the Bogoliubov pseudoparticles created in a BoseEinstein condensate undergoing a black hole flow. On the basis of the gravitational analogy, the method used relies only on quantum field theory in curved spacetime techniques. A comparison with the results obtained by ab initio full condensed matter calculations is given, confirming the validity of the approximation used, provided the profile of the flow varies smoothly on scales compared to the condensate healing length.



Anderson, P. R., Clark, R. D., Fabbri, A., & Good, M. R. R. (2019). Late time approach to Hawking radiation: Terms beyond leading order. Phys. Rev. D, 100(6), 061703–5pp.
Abstract: Black hole evaporation is studied using wave packets for the modes. These allow for approximate frequency and time resolution. The leading order late time behavior gives the wellknown Hawking radiation that is independent of how the black hole formed. The focus here is on the higher order terms and the rate at which they damp at late times. Some of these terms carry information about how the black hole formed. A general argument is given which shows that the damping is significantly slower (power law) than what might be naively expected from a stationary phase approximation (exponential). This result is verified by numerical calculations in the cases of 2D and 4D black holes that form from the collapse of a null shell.



Anderson, P. R., Fabbri, A., & Balbinot, R. (2015). Low frequency graybody factors and infrared divergences: Rigorous results. Phys. Rev. D, 91(6), 064061–18pp.
Abstract: Formal solutions to the mode equations for both spherically symmetric black holes and BoseEinstein condensate acoustic black holes are obtained by writing the spatial part of the mode equation as a linear Volterra integral equation of the second kind. The solutions work for a massless minimally coupled scalar field in the swave or zero angular momentum sector for a spherically symmetric black hole and in the longitudinal sector of a onedimensional BoseEinstein condensate acoustic black hole. These solutions are used to obtain in a rigorous way analytic expressions for the scattering coefficients and graybody factors in the zero frequency limit. They are also used to study the infrared behaviors of the symmetric twopoint function and two functions derived from it: the pointsplit stressenergy tensor for the massless minimally coupled scalar field in Schwarzschildde Sitter spacetime and the densitydensity correlation function for a BoseEinstein condensate acoustic black hole.



Anderson, P. R., Siahmazgi, S. G., Clark, R. D., & Fabbri, A. (2020). Method to compute the stressenergy tensor for a quantized scalar field when a black hole forms from the collapse of a null shell. Phys. Rev. D, 102(12), 125035–26pp.
Abstract: A method is given to compute the stressenergy tensor for a massless minimally coupled scalar field in a spacetime where a black hole forms from the collapse of a spherically symmetric null shell in four dimensions. Part of the method involves matching the modes for the in vacuum state to a complete set of modes in Schwarzschild spacetime. The other part involves subtracting from the unrenormalized expression for the stressenergy tensor when the field is in the in vacuum state, the corresponding expression when the field is in the Unruh state and adding to this the renormalized stressenergy tensor for the field in the Unruh state. The method is shown to work in the twodimensional case where the results are known.



Babichev, E., & Fabbri, A. (2014). A class of charged black hole solutions in massive (bi)gravity. J. High Energy Phys., 07(7), 016–10pp.
Abstract: We present a new class of solutions describing charged black holes in massive (bi)gravity. For a generic choice of the parameters of the massive gravity action, the solution is the ReissnerNordstromde Sitter metric written in the EddingtonFinkelstein coordinates for both metrics. We also study a special case of the parameters, for which the space of solutions contains an extra symmetry.



Babichev, E., & Fabbri, A. (2014). Stability analysis of black holes in massive gravity: A unified treatment. Phys. Rev. D, 89(8), 081502–5pp.
Abstract: We consider the analytic solutions of massive (bi) gravity which can be written in a simple form using advanced EddingtonFinkelstein coordinates. We analyze the stability of these solutions against radial perturbations. First we recover the previously obtained result on the instability of the bidiagonal biSchwarzschild solutions. In the nonbidiagonal case (which contains, in particular, the Schwarzschild solution with Minkowski fiducial metric), we show that generically there are physical spherically symmetric perturbations, but no unstable modes.



Babichev, E., & Fabbri, A. (2014). Rotating black holes in massive gravity. Phys. Rev. D, 90(8), 084019–7pp.
Abstract: We present a solution for rotating black holes in massive gravity. We first give a solution of massive gravity with one dynamical metric. Both metrics of this solution are expressed in the advanced EddingtonFinkelsteinlike coordinates: the physical metric has the original Kerr line element, while the fiducial metric is flat, but written in a rotating EddingtonFinkelstein form. For the bigravity theory we give an analogue of this solution: the two metrics have the original Kerr form, but, in general, different black hole masses. The generalization of the solution to include the electric charge is also given; it is an analogue of the KerrNewman solution in general relativity. We also discuss further possible ways to generalize the solutions.



Babichev, E., & Fabbri, A. (2013). Instability of black holes in massive gravity. Class. Quantum Gravity, 30(15), 152001–7pp.
Abstract: We show that linear perturbations around the simplest black hole solution of massive bigravity theories, the biSchwarzschild solution, exhibit an unstable mode featuring the GregoryLaflamme instability of higher dimensional black strings. The result is obtained for the massive gravity theory which is free from the BoulwareDeser ghost, as well as for its extension with two dynamical metrics. These results may indicate that static black holes in massive gravity do not exist. For the graviton mass of the order of the Hubble scale, however, the instability timescale is of order of the Hubble time.



Balbinot, R., Carusotto, I., Fabbri, A., & Recati, A. (2010). Testing Hawking Particle Creation By Black Holes Through Correlation Measurements. Int. J. Mod. Phys. D, 19(14), 2371–2377.
Abstract: Hawking's prediction of thermal radiation by black holes has been shown by Unruh to be expected also in condensed matter systems. We show here that in a black holelike configuration realized in a BEC this particlecreation does indeed take place and can be unambiguously identified via a characteristic pattern in the densitydensity correlations. This opens the concrete possibility of the experimental verification of this effect.

