No Arabic abstract
Gravitational greybody factors are analytically computed for static, spherically symmetric black holes in d-dimensions, including black holes with charge and in the presence of a cosmological constant (where a proper definition of greybody factors for both asymptotically dS and AdS spacetimes is provided). This calculation includes both the low-energy case --where the frequency of the scattered wave is small and real-- and the asymptotic case --where the frequency of the scattered wave is very large along the imaginary axis-- addressing gravitational perturbations as described by the Ishibashi-Kodama master equations, and yielding full transmission and reflection scattering coefficients for all considered spacetime geometries. At low frequencies a general method is developed, which can be employed for all three types of spacetime asymptotics, and which is independent of the details of the black hole. For asymptotically dS black holes the greybody factor is different for even or odd spacetime dimension, and proportional to the ratio of the areas of the event and cosmological horizons. For asymptotically AdS black holes the greybody factor has a rich structure in which there are several critical frequencies where it equals either one (pure transmission) or zero (pure reflection, with these frequencies corresponding to the normal modes of pure AdS spacetime). At asymptotic frequencies the computation of the greybody factor uses a technique inspired by monodromy matching, and some universality is hidden in the transmission and reflection coefficients. For either charged or asymptotically dS black holes the greybody factors are given by non-trivial functions, while for asymptotically AdS black holes the greybody factor precisely equals one (corresponding to pure blackbody emission).
We study the absorption probability and Hawking radiation of the scalar field in the rotating black holes on codimension-2 branes. We find that finite brane tension modifies the standard results in Hawking radiation if compared with the case when brane tension is completely negligible. We observe that the rotation of the black hole brings richer physics. Nonzero angular momentum triggers the super-radiance which becomes stronger when the angular momentum increases. We also find that rotations along different angles influence the result in absorption probability and Hawking radiation. Compared with the black hole rotating orthogonal to the brane, in the background that black hole spins on the brane, its angular momentum brings less super-radiance effect and the brane tension increases the range of frequency to accommodate super-radiance. These information can help us know more about the rotating codimension-2 black holes.
We study the reflection coefficient, the transmission coefficient and the greybody factors for black holes with topologically non trivial transverse sections in 4 and d-dimensions, in the limit of low energy. Considering a massive scalar field in a topological massless black hole background, which is non minimally coupled to the curvature and assuming the horizon geometry with a negative constant curvature. Mainly, we show that there is range of modes which contribute to the absorption cross section in the zero-frequency limit, at difference of the result existing in the literature. Where, the mode with lowest angular momentum contribute to the absorption cross section. Also we show that the condition that the sum of the reflection coefficient and the transmission coefficient is equal to one is always satisfied for these kind of black holes.
Recently Maldacena and Strominger found that the calculation of greybody factors for $D=5$ black holes carrying three U(1) charges gives striking new evidence for their description as multiply wound effective strings. Here we show that a similar result holds for $D=4$ black holes with four $U(1)$ charges. In this case the effective string may be thought of as the triple intersection of the 5-branes in M-theory compactified on $T^7$.
We have shown that higher dimensional Reissner-Nordstrom-de Sitter black holes are gravitationally unstable for large values of the electric charge and cosmological constant in $D geq 7$ space-time dimensions. We have found the shape of the slightly perturbed black hole at the threshold point of instability. Why only $D=4, 5$ and 6 dimensional worlds are favorable as to the black stability remains unknown.
We present the general exact solutions for non-extremal rotating charged black holes in the Godel universe of five-dimensional minimal supergravity theory. They are uniquely characterized by four non-trivial parameters, namely the mass $m$, the charge $q$, the Kerr equal rotation parameter $a$, and the Godel parameter $j$. We calculate the conserved energy, angular momenta and charge for the solutions and show that they completely satisfy the first law of black hole thermodynamics. We also study the symmetry and separability of the Hamilton-Jacobi and the massive Klein-Gordon equations in these Einstein-Maxwell-Chern-Simons-Godel black hole backgrounds.