Cosimo Bambi

Natural extension of the generalized uncertainty principle

We discuss a gedanken experiment for the simultaneous measurement of the position and momentum of a particle in de Sitter spacetime. We propose an extension of the so-called generalized uncertainty principle (GUP) which implies the existence of a minimum observable momentum. The new GUP is directly connected to the nonzero cosmological constant, which becomes a necessary ingredient for a more complete picture of the quantum spacetime.

Rotating regular black holes

Abstract The formation of spacetime singularities is a quite common phenomenon in General Relativity and it is regulated by specific theorems. It is widely believed that spacetime singularities do not exist in Nature, but that they represent a limitation of the classical theory. While we do not yet have any solid theory of quantum gravity, toy models of black hole solutions without singularities have been proposed. So far, there are only non-rotating regular black holes in the literature. These metrics can be hardly tested by astrophysical observations, as the black hole spin plays a fundamental role in any astrophysical process. In this Letter, we apply the Newman–Janis algorithm to the Hayward and to the Bardeen black hole metrics. In both cases, we obtain a family of rotating solutions. Every solution corresponds to a different matter configuration. Each family has one solution with special properties, which can be written in Kerr-like form in Boyer–Lindquist coordinates. These special solutions are of Petrov type D, they are singularity free, but they violate the weak energy condition for a non-vanishing spin and their curvature invariants have different values at r = 0 depending on the way one approaches the origin. We propose a natural prescription to have rotating solutions with a minimal violation of the weak energy condition and without the questionable property of the curvature invariants at the origin.

Apparent shape of super-spinning black holes

We consider the possibility that astrophysical black holes (BHs) can violate the Kerr bound; i.e., they can have angular momentum greater than BH mass, J>M. We discuss implications on the BH apparent shape. Even if the bound is violated by a small amount, the shadow cast by the BH changes significantly (it is {approx} an order of magnitude smaller) from the case with J{<=}M and can be used as a clear observational signature in the search for super-spinning BHs. We discuss briefly recent observations in the mm range of the supermassive BH at the center of the Galaxy, speculating on the possibility that it might violate the Kerr bound.

Testing the space-time geometry around black hole candidates with the analysis of the broad K

Astrophysical black hole candidates are thought to be the Kerr black holes predicted by general relativity, but there is not yet clear evidence that the geometry of the space-time around these objects is really described by the Kerr metric. In order to confirm the Kerr black hole hypothesis, we have to observe strong gravity features and check that they are in agreement with the ones predicted by general relativity. In this paper, I study the broad

\alpha

\mathrm{K}\ensuremath{\alpha}

iron line

iron line, which is often seen in the x-ray spectrum of both stellar-mass and supermassive black hole candidates and whose shape is supposed to be strongly affected by the space-time geometry. As found in previous studies in the literature, there is a strong correlation between the spin parameter and the deformation parameter; that is, the line emitted around a Kerr black hole with a certain spin can be very similar to the one coming from the space-time around a non-Kerr object with a quite different spin. Despite that, the analysis of the broad

CONSTRAINING THE QUADRUPOLE MOMENT OF STELLAR-MASS BLACK HOLE CANDIDATES WITH THE CONTINUUM FITTING METHOD

\mathrm{K}\ensuremath{\alpha}

A Code to Compute the Emission of Thin Accretion Disks in Non-Kerr Spacetimes and Test the Nature of Black Hole Candidates

iron line is potentially more powerful than the continuum-fitting method, as it can put an interesting bound on possible deviations from the Kerr geometry independently of the value of the spin parameter and without additional measurements.

Terminating black holes in asymptotically free quantum gravity

Black holes in general relativity are known as Kerr black holes and are characterized solely by two parameters, the mass M and the spin J. All the higher multipole moments of the gravitational field are functions of these two parameters. For instance, the quadrupole moment is Q = –J 2/M, which implies that a measurement of M, J, and Q for black hole candidates would allow one to test whether these objects are really black holes as described by general relativity. While future gravitational-wave experiments will be able to test the Kerr nature of these objects with very high accuracy, in this paper we show that it is possible to put constraints on the quadrupole moment of stellar-mass black hole candidates by using presently available X-ray data of the thermal spectrum of their accretion disk.

K

Astrophysical black hole (BH) candidates are thought to be the Kerr BHs predicted by general relativity, but the actual nature of these objects has still to be proven. The analysis of the electromagnetic radiation emitted by a geometrically thin and optically thick accretion disk around a BH candidate can provide information about the geometry of the spacetime around the compact object and it can thus test the Kerr BH hypothesis. In this paper, I present a code based on a ray-tracing approach and capable of computing some basic properties of thin accretion disks in spacetimes with deviations from the Kerr background. The code can be used to fit current and future X-ray data of stellar-mass BH candidates and constrain possible deviations from the Kerr geometry in the spin parameter-deformation parameter plane.