56 research outputs found
On four-dimensional Einsteinian gravity, quasitopological gravity, cosmology and black holes
We show that the combination of cubic invariants defining five-dimensional
quasitopological gravity, when written in four dimensions, reduce to the
version of four-dimensional Einsteinian gravity recently proposed by Arciniega,
Edelstein & Jaime, that produces second order equations of motion in a FLRW
ansatz, with a purely geometrical inflationary period. We introduce a quartic
version of the four-dimensional Einsteinian theory with similar properties, and
study its consequences. In particular we found that there exists a region on
the space of parameters which allows for thermodynamically stable black holes,
as well as a well-defined cosmology with geometrically driven inflation. We
briefly discuss the cosmological inhomogeneities in this setup. We also provide
a combination of quintic invariants with those properties.Comment: Minor corrections, 14 pages, 1 figur
Homogeneous Black Strings in Einstein-Gauss-Bonnet with Horndeski hair and beyond
In this paper we construct new exact solutions in Einstein-Gauss-Bonnet and
Lovelock gravity, describing asymptotically flat black strings. The solutions
exist also under the inclusion of a cosmological term in the action, and are
supported by scalar fields with finite energy density, which are linear along
the extended direction and have kinetic terms constructed out from Lovelock
tensors. The divergenceless nature of the Lovelock tensors in the kinetic terms
ensures that the whole theory is second order. For spherically, hyperbolic and
planar symmetric spacetimes on the string, we obtain an effective Wheeler's
polynomial which determines the lapse function up to an algebraic equation. For
the sake of concreteness, we explicitly show the existence of a family of
asymptotically flat black strings in six dimensions, as well as asymptotically
AdS black string solutions and compute the temperature, mass
density and entropy density. We compute the latter by Wald's formula and show
that it receives a contribution from the non-minimal kinetic coupling of the
matter part, shifting the one-quarter factor coming from the Einstein term, on
top of the usual non areal contribution arising from the quadratic Gauss-Bonnet
term. Finally, for a special value of the couplings of the theory in six
dimensions, we construct strings that contain asymptotically AdS wormholes as
well as rotating solutions on the transverse section. By including more scalars
the strings can be extended to -branes, in arbitrary dimensions.Comment: 20 pages, 2 figures. V2: 20 pages, 2 figures. Explicit construction
of the asymptotically flat black strings in 6D EGB, included. To appear in
EJP
Ehlers Transformations as a Tool for Constructing Accelerating NUT Black Holes
This paper investigates the integrability properties of Einstein's theory of
gravity in the context of accelerating NUT spacetimes by utilizing Ernst's
description of stationary and axially symmetric electro-vacuum solutions. We
employ Ehlers transformations, Lie point symmetries of the Einstein field
equations, to efficiently endorse accelerating metrics with a nontrivial NUT
charge. Under this context, we begin by re-deriving the known C-metric NUT
spacetime described by Chng, Mann, and Stelea in a straightforward manner, and
in the new form of the solution introduced by Podolsk\'y and Vr\'atn\'y. Next,
we construct for the first time an accelerating NUT black hole dressed with a
conformally coupled scalar field. These solutions belong to the general class
of type I spacetimes, therefore cannot be obtained from any limit of the
Pleban\'ski-Demia\'nski family whatsoever and their integration needs to be
carried out independently. Including Maxwell fields is certainly permitted,
however, the use of Ehlers transformations is subtle and requires further
modifications. Ehlers transformations do not only partially rotate the mass
parameter such that its magnetic component appears, but also rotate the
corresponding gauge fields. Notwithstanding, the alignment of the
electromagnetic potentials can be successfully performed via a duality
transformation, hence providing a novel Reissner-Nordstr\"om-C-metric NUT black
hole that correctly reproduces the Reissner-Nordstr\"om-C-metric and
Reissner-Nordstr\"om-NUT configurations in the corresponding limiting cases. We
describe the main geometric features of these solutions and discuss possible
embeddings of our geometries in external electromagnetic and rotating
backgrounds.Comment: A misleading interpretation of the duality rotation performed in
Section V has been deleted. The interpretation of the solution has been
corrected and the mathematical result remains unchange
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