136 research outputs found
Born-Infeld type Gravity
Generalizations of gravitational Born-Infeld type lagrangians are
investigated. Phenomenological constraints (reduction to Einstein-Hilbert
action for small curvature, spin two ghost freedom and absence of Coulomb like
Schwarschild singularity) select one effective lagrangian whose dynamics is
dictated by the tensors g_{\mu\nu} and R_{\mu\nu\rho\sigma}(not R_{\mu\nu} or
the scalar R).Comment: 7 pages, 3 figures, revte
Quantization of massive scalar fields over static black string backgrounds
The renormalized mean value of the corresponding components of the
Energy-Momentum tensor for massive scalar fields coupled to an arbitrary
gravitational field configuration having cylindrical symmetry are analytically
evaluated using the Schwinger-DeWitt approximation, up to second order in the
inverse mass value. The general results are employed to explicitly derive
compact analytical expressions for the Energy-Momentum tensor in the particular
background of the Black-String spacetime. In the case of the Black String
considered in this work, we proof that a violation of the weak energy condition
occur at the horizon of the space-time for values of the coupling constant,
that include as particular cases the most interesting of minimal and conformal
coupling.Comment: 4 page
Tunnelling, Temperature and Taub-NUT Black Holes
We investigate quantum tunnelling methods for calculating black hole
temperature, specifically the null geodesic method of Parikh and Wilczek and
the Hamilton-Jacobi Ansatz method of Angheben et al. We consider application of
these methods to a broad class of spacetimes with event horizons, inlcuding
Rindler and non-static spacetimes such as Kerr-Newman and Taub-NUT. We obtain a
general form for the temperature of Taub-NUT-Ads black holes that is
commensurate with other methods. We examine the limitations of these methods
for extremal black holes, taking the extremal Reissner-Nordstrom spacetime as a
case in point.Comment: 22 pages, 3 figures; added references, fixed figures, added comments
to extremal section, added footnot
The Theory of a Quantum Noncanonical Field in Curved Spacetimes
Much attention has been recently devoted to the possibility that quantum
gravity effects could lead to departures from Special Relativity in the form of
a deformed Poincar\`e algebra. These proposals go generically under the name of
Doubly or Deformed Special Relativity (DSR). In this article we further explore
a recently proposed class of quantum field theories, involving noncanonically
commuting complex scalar fields, which have been shown to entail a DSR-like
symmetry. An open issue for such theories is whether the DSR-like symmetry has
to be taken as a physically relevant symmetry, or if in fact the "true"
symmetries of the theory are just rotations and translations while boost
invariance has to be considered broken. We analyze here this issue by extending
the known results to curved spacetime under both of the previous assumptions.
We show that if the symmetry of the free theory is taken to be a DSR-like
realization of the Poincar\'e symmetry, then it is not possible to render such
a symmetry a gauge symmetry of the curved physical spacetime. However, it is
possible to introduce an auxiliary spacetime which allows to describe the
theory as a standard quantum field theory in curved spacetime. Alternatively,
taking the point of view that the noncanonical commutation of the fields
actually implies a breakdown of boost invariance, the physical spacetime
manifold has to be foliated in surfaces of simultaneity and the field theory
can be coupled to gravity by making use of the ADM prescription.Comment: 9 pages, no figure
Hawking Radiation on an Ion Ring in the Quantum Regime
This paper discusses a recent proposal for the simulation of acoustic black
holes with ions. The ions are rotating on a ring with an inhomogeneous, but
stationary velocity profile. Phonons cannot leave a region, in which the ion
velocity exceeds the group velocity of the phonons, as light cannot escape from
a black hole. The system is described by a discrete field theory with a
nonlinear dispersion relation. Hawking radiation is emitted by this acoustic
black hole, generating entanglement between the inside and the outside of the
black hole. We study schemes to detect the Hawking effect in this setup.Comment: 42 pages (one column), 17 figures, published revised versio
Inflationary spectra and partially decohered distributions
It is generally expected that decoherence processes will erase the quantum
properties of the inflationary primordial spectra. However, given the weakness
of gravitational interactions, one might end up with a distribution which is
only partially decohered. Below a certain critical change, we show that the
inflationary distribution retains quantum properties. We identify four of
these: a squeezed spread in some direction of phase space, non-vanishing
off-diagonal matrix elements, and two properties used in quantum optics called
non--representability and non-separability. The last two are necessary
conditions to violate Bell's inequalities. The critical value above which all
these properties are lost is associated to the `grain' of coherent states. The
corresponding value of the entropy is equal to half the maximal (thermal)
value. Moreover it coincides with the entropy of the effective distribution
obtained by neglecting the decaying modes. By considering backreaction effects,
we also provide an upper bound for this entropy at the onset of the adiabatic
era.Comment: 42 pages, 9 figures; 1 ref. adde
About Starobinsky inflation
It is believed that soon after the Planck era, space time should have a
semi-classical nature. According to this, the escape from General Relativity
theory is unavoidable. Two geometric counter-terms are needed to regularize the
divergences which come from the expected value. These counter-terms are
responsible for a higher derivative metric gravitation. Starobinsky idea was
that these higher derivatives could mimic a cosmological constant. In this work
it is considered numerical solutions for general Bianchi I anisotropic
space-times in this higher derivative theory. The approach is ``experimental''
in the sense that there is no attempt to an analytical investigation of the
results. It is shown that for zero cosmological constant , there are
sets of initial conditions which form basins of attraction that asymptote
Minkowski space. The complement of this set of initial conditions form basins
which are attracted to some singular solutions. It is also shown, for a
cosmological constant that there are basins of attraction to a
specific de Sitter solution. This result is consistent with Starobinsky's
initial idea. The complement of this set also forms basins that are attracted
to some type of singular solution. Because the singularity is characterized by
curvature scalars, it must be stressed that the basin structure obtained is a
topological invariant, i.e., coordinate independent.Comment: Version accepted for publication in PRD. More references added, a few
modifications and minor correction
Renormalizable acausal theories of classical gravity coupled with interacting quantum fields
We prove the renormalizability of various theories of classical gravity
coupled with interacting quantum fields. The models contain vertices with
dimensionality greater than four, a finite number of matter operators and a
finite or reduced number of independent couplings. An interesting class of
models is obtained from ordinary power-counting renormalizable theories,
letting the couplings depend on the scalar curvature R of spacetime. The
divergences are removed without introducing higher-derivative kinetic terms in
the gravitational sector. The metric tensor has a non-trivial running, even if
it is not quantized. The results are proved applying a certain map that
converts classical instabilities, due to higher derivatives, into classical
violations of causality, whose effects become observable at sufficiently high
energies. We study acausal Einstein-Yang-Mills theory with an R-dependent gauge
coupling in detail. We derive all-order formulas for the beta functions of the
dimensionality-six gravitational vertices induced by renormalization. Such beta
functions are related to the trace-anomaly coefficients of the matter
subsector.Comment: 36 pages; v2: CQG proof-corrected versio
Back Reaction of Hawking Radiation on Black Hole Geometry
We propose a model for the geometry of a dynamical spherical shell in which
the metric is asymptotically Schwarzschild, but deviates from Ricci-flatness in
a finite neighbourhood of the shell. Hence, the geometry corresponds to a
`hairy' black hole, with the hair originating on the shell. The metric is
regular for an infalling shell, but it bifurcates, leading to two disconnected
Schwarzschild-like spacetime geometries. The shell is interpreted as either
collapsing matter or as Hawking radiation, depending on whether or not the
shell is infalling or outgoing. In this model, the Hawking radiation results
from tunnelling between the two geometries. Using this model, the back reaction
correction from Hawking radiation is calculated.Comment: Latex file, 15 pages, 4 figures enclosed, uses eps
Renormalized Thermodynamic Entropy of Black Holes in Higher Dimensions
We study the ultraviolet divergent structures of the matter (scalar) field in
a higher D-dimensional Reissner-Nordstr\"{o}m black hole and compute the matter
field contribution to the Bekenstein-Hawking entropy by using the Pauli-Villars
regularization method. We find that the matter field contribution to the black
hole entropy does not, in general, yield the correct renormalization of the
gravitational coupling constants. In particular we show that the matter field
contribution in odd dimensions does not give the term proportional to the area
of the black hole event horizon.Comment: Final Revision Form as to be published in Physical Review D, ReVTeX,
No Figure
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