29221858
Time travel on a string
15289059
Black hole evaporation and higher-derivative gravity
We examine the role which higher-derivative gravity interactions may play in black hole evaporation. The thermodynamic properties of black holes in Lovelock gravity are described. In certain cases, the specific heat of a black hole becomes positive at a small mass. This results in an infinite lifetime for the black hole (and also allows it to achieve stable equilibrium with a thermal environment). Thus no conflict with unitary time evolution would arise in such theories.
7802314
Einstein equation with quantum corrections reduced to second order.
We consider the Einstein equation with first-order (semiclassical) quantum corrections. Although the quantum corrections contain up to fourth-order derivatives of the metric, the solutions which are physically relevant satisfy reduced equations which contain derivatives no higher than second order. We obtain the reduced equations for a range of stress-energy tensors. These reduced equations are suitable for a numerical solution, are expected to contain fewer numerical instabilities than the original fourth-order equations, and yield only physically relevant solutions. We give analytic and numerical solutions or reduced equations for particular examples, including Friedmann-Lema\^{\i}tre universes with a cosmological constant, a spherical body of constant density, and more general conformally flat metrics.
15433766
Black-hole thermodynamics in Lovelock gravity.
The thermodynamic properties of black holes in Lovelock gravity are examined. In particular, the case of the Einstein Lagrangian plus the four-dimensional Euler density is discussed in detail. In five dimensions, one finds that the specific heat of a black hole becomes positive at small mass, allowing the black hole to achieve stable equilibrium with its environment and giving it an infinite lifetime. This behavior is not universal, however, but it always occurs in 2k+1 dimensions for a Lovelock theory including the 2k-dimensional Euler density. For theories including six-derivative or higher-order interactions, black holes with degenerate zero-temperature horizons are also possible.
116957296
Higher derivative expansions and non-locality, with applications to gravity and the stability of flat space
7991045
Unitarity of interacting fields in curved spacetime.
On globally hyperbolic spacetimes, each foliation by spacelike hypersurfaces corresponds to a Hamiltonian description of field theory, and unitarity follows formally from the Hermiticity of the Hamiltonian. For a renormalizable theory, unitarity at each order in perturbation theory follows from the corresponding Hermiticity of each term in the time-ordered product of interaction Hamiltonians. For more general spacetimes, one can still use the path integral to obtain a generalized Lehmann-Symanzik-Zimmermann reduction formula for $S$-matrix elements and the corresponding perturbative expansion. Unitarity imposes an infinite set of identities on the scattering amplitudes, which are the generalizations of the flat-spacetime Cutkosky rules. We find these explicitly to $O({\ensuremath{\lambda}}^{3})$ in a $\ensuremath{\lambda}{\ensuremath{\phi}}^{4}$ theory, and show how to find the relations to any order. For globally hyperbolic spacetimes the unitarity identities are satisfied [at least to $O({\ensuremath{\lambda}}^{3})$] because of a single property of the configuration-space propagator that reflects the causal structure of the spacetime.
117296504
WORLD VOLUME REALIZATION OF AUTOMORPHISMS
The relation among spacetime supersymmetry algebras and the world volume approach to string theory is reviewed. The realization of some of the automorphism transformations of these superalgebras on the world volume theory is discussed. We distinguish among linear realizations and non-local ones. The consistency of the latter with duality in M/string theory is checked.
56350886
EXTREMAL BLACK HOLES, HOLOGRAPHY AND COARSE GRAINING
I review some of the concepts at the crossroads of gravitational thermodynamics, holography and quantum mechanics. First, the origin of gravitational thermodynamics due to coarse graining of quantum information is exemplified using the half-BPS sector of SYM and its LLM description in type IIB supergravity. The notion of black holes as effective geometries, its relation to the fuzzball programme and some of the puzzles raising for large black holes are discussed. Second, I review recent progress for extremal black holes, both microscopically, discussing a constituent model for stationary extremal non-BPS black holes, and semiclassically, discussing the extremal black hole/CFT conjecture. The latter is examined from the AdS3/CFT2 perspective. Third, I review the importance of the holographic principle to encode nonlocal gravity features allowing us to relate the gravitational physics of local observers with thermodynamics and the role causality plays in these arguments by identifying horizons (screens) as diathermic walls. I speculate with the emergence of an approximate CFT in the deep IR close to any horizon and its relation with an effective dynamical description of the degrees of freedom living on these holographic screens.
122026820
Field Theory on Curved Space Time and the TCP Theorem
A generalization is given to the Wightman axioms of field theory which permits to treat gravitational field as a Wightman field. This generalization permits to deduce TCP theorem for the gravitational field. The note ends with some critical remarks concerning the physical interpretation of the TCP theorem.
15838992
Killing spectroscopy of closed timelike curves
We analyse the existence of closed timelike curves in spacetimes which possess an isometry. In particular we check which discrete quotients of such spaces lead to closed timelike curves. As a by-product of our analysis, we prove that the notion of existence or non-existence of closed timelike curves is a T-duality invariant notion, whenever the direction along which we apply such transformations is everywhere spacelike. Our formalism is straightforwardly applied to supersymmetric theories. We provide some new examples in the context of D-branes and generalized pp-waves.
9475033
Constituent Model of Extremal non-BPS Black Holes
We interpret extremal non-BPS black holes in four dimensions as threshold bound states of four 1/2-BPS constituents. We verify the no-force condition for each of the primitive constituents in the probe approximation. Our computations are for a seed solution with D0 − D4 charges and equal B-fields, but symmetries extend the result to any U-dual frame. We make the constituent model for the D0 − D6 system explicit, and also discuss a duality frame where the constituents are D3 branes at angles. We demonstrate stability of the constituent model in the weak coupling description of the constituent D-branes. We discuss the relation between the BPS and non-BPS branches of configuration space.