K. Saifullah

Classification of Cylindrically Symmetric Static Spacetimes According to Their Ricci Collineations

A complete classification of cylindrically symmetric static Lorentzian manifolds according to their Ricci collineations (RCs) is provided. The Lie algebras of RCs for the non-degenerate Ricci tensor have dimensions 3 to 10, excluding 8 and 9. For the degenerate tensor the algebra is mostly but not always infinite dimensional; there are cases of 10-, 5-, 4- and 3-dimensional algebras. The RCs are compared with the Killing vectors (KVs) and homothetic motions (HMs). The (non-linear) constraints corresponding to the Lie algebras are solved to construct examples which include some exact solutions admitting proper RCs. Their physical interpretation is given. The classification of plane symmetric static spacetimes emerges as a special case of this classification when the cylinder is unfolded.

Quantum corrections to the entropy of Einstein–Maxwell dilaton–axion black holes

We study corrections to the entropy of Einstein–Maxwell dilaton–axion black holes beyond semiclassical approximations. We consider the entropy of the black hole as a state variable and derive these corrections using the exactness criteria of the first law of thermodynamics. We note that from this general frame-work the entropy corrections for “simpler” black holes like Schwarzschild, Reissner–Nordström and anti-de Sitter–Schwarzschild black holes follow easily. This procedure gives us the modified area law as well.

Comments on matter collineations of plane symmetric, cylindrically symmetric, and spherically symmetric space–times

Comments are made on some recently published papers on matter collineations of plane symmetric, cylindrically symmetric and spherically symmetric spacetimes.

LIE SYMMETRIES OF THE ENERGY–MOMENTUM TENSOR FOR PLANE SYMMETRIC STATIC SPACETIMES

Matter collineations (MCs) are the vector fields along which the energy–momentum tensor remains invariant under Lie transport. Invariance of the metric, the Ricci and the Riemann tensors have been studied extensively and the vectors along which these tensors remain invariant are called Killing vectors (KVs), Ricci collineations (RCs) and curvature collineations (CCs), respectively. In this paper, plane symmetric static spacetimes have been studied for their MCs. Explicit form of MCs together with the Lie algebra admitted by them has been presented. Examples of spacetimes have been constructed for which MCs have been compared with their RCs and KVs. The comparison shows that neither of the sets of RCs and MCs contains the other, in general.

GUP-corrected thermodynamics of accelerating and rotating black holes

When quantum gravity effects, that are based on generalized uncertainty principle with a minimal measurable length, are incorporated into black hole physics the Klein–Gordon and Dirac equations get modified. Using these modified equations we investigate tunneling of scalar particles and fermions from event and acceleration horizons of accelerating and rotating black holes and obtain the modified Hawking temperature with quantum gravity effects. We see that Hawking temperature depends on black hole parameters as well as the quantum numbers of emitted fermions. The quantum corrections slow down black hole evaporation and leave a black hole remnant. This contradicts complete evaporation of a black hole which is presaged by the standard temperature formula for black holes. The modified Hawking temperatures presented here, in appropriate limits, are consistent with the previous results in the literature.

ON THE DEFINITION OF MATTER COLLINEATIONS

It is shown that when the stress-energy tensor of a spacetime is diagonal and is written in the mixed form, its collineations admit infinite dimensional Lie algebras except possibly in the case when the tensor depends on all the spacetime coordinates. The result can be extended for more general second rank tensors.