1,733 research outputs found
Electromagnetic waves around dilatonic stars and naked singularities
We study the propagation of classical electromagnetic waves on the simplest
four-dimensional spherically symmetric metric with a dilaton background field.
Solutions to the relevant equations are obtained perturbatively in a parameter
which measures the strength of the dilaton field (hence parameterizes the
departure from Schwarzschild geometry). The loss of energy from outgoing modes
is estimated as a back-scattering process against the dilaton background, which
would affect the luminosity of stars with a dilaton field. The radiation
emitted by a freely falling point-like source on such a background is also
studied by analytical and numerical methods.Comment: 9 pages, 1 figur
Bootstrapping Newton Gravity
A non-linear equation obtained by adding gravitational self-interaction terms
to the Poisson equation for Newtonian gravity is here employed in order to
analyse a static spherically sym- metric homogeneous compact source of given
proper mass and radius and the outer vacuum. The main feature of this picture
is that, although the freedom of shifting the potential by an ar- bitrary
constant is of course lost, the solutions remain qualitatively very close to
the Newtonian behaviour. We also notice that the negative gravitational
potential energy is smaller than the proper mass for sources with small
compactness, but for sources that should form black holes according to General
Relativity, the gravitational potential energy becomes of the same order of
magnitude of the proper mass, or even larger. Moreover, the pressure overcomes
the energy density for large values of the compactness, but it remains finite
for finite compactness, hence there exists no Buchdahl limit. This classical
description is meant to serve as the starting point for investigating quantum
features of (near) black hole configurations within the corpuscular picture of
gravity in future developments.Comment: 23 pages, 20 plots. New section and appendix about stability and the
pressure clarify comparison with GR. Conclusions rewritten to make
motivations cleare
The Poincare' coset models ISO(d-1,1)/R^n and T-duality
We generalize a family of Lagrangians with values in the Poincar\'e group
ISO(d-1,1), which contain the description of spinning strings in flat (d-1)+1
dimensions, by including symmetric terms in the world-sheet coordinates. Then,
by promoting a subgroup H=R^n, n less than or equal to d, which acts
invariantly from the left on the element of ISO(d-1,1), to a gauge symmetry of
the action, we obtain a family of sigma-models. They describe bosonic strings
moving in (generally) curved, and in some cases degenerate, space-times with an
axion field. Further, the space-times of the effective theory admit in general
T-dual geometries. We give explicit results for two non degenerate cases.Comment: LaTeX, 24 pages, no figure
Brane-world black holes and the scale of gravity
A particle in four dimensions should behave like a classical black hole if
the horizon radius is larger than the Compton wavelength or, equivalently, if
its degeneracy (measured by entropy in units of the Planck scale) is large. For
spherically symmetric black holes in 4 + d dimensions, both arguments again
lead to a mass threshold MC and degeneracy scale Mdeg of the order of the
fundamental scale of gravity MG. In the brane-world, deviations from the
Schwarzschild metric induced by bulk effects alter the horizon radius and
effective four-dimensional Euclidean action in such a way that MC \simeq Mdeg
might be either larger or smaller than MG. This opens up the possibility that
black holes exist with a mass smaller than MG and might be produced at the LHC
even if M>10 TeV, whereas effects due to bulk graviton exchanges remain
undetectable because suppressed by inverse powers of MG. Conversely, even if
black holes are not found at the LHC, it is still possible that MC>MG and MG
\simeq 1TeV.Comment: 4 pages, no figur
Exploring the bulk of tidal charged micro-black holes
We study the bulk corresponding to tidal charged brane-world black holes. We
employ a propagating algorithm which makes use of the three-dimensional
multipole expansion and analytically yields the metric elements as functions of
the five-dimensional coordinates and of the ADM mass, tidal charge and brane
tension. Since the projected brane equations cannot determine how the charge
depends on the mass, our main purpose is to select the combinations of these
parameters for which black holes of microscopic size possess a regular bulk.
Our results could in particular be relevant for a better understanding of
TeV-scale black holes.Comment: Latex, 15 pages, 1 table, 5 figures; Section 3.2 extended, typos
corrected, no change in conclusion
Theoretical survey of tidal-charged black holes at the LHC
We analyse a family of brane-world black holes which solve the effective
four-dimensional Einstein equations for a wide range of parameters related to
the unknown bulk/brane physics. We first constrain the parameters using known
experimental bounds and, for the allowed cases, perform a numerical analysis of
their time evolution, which includes accretion through the Earth. The study is
aimed at predicting the typical behavior one can expect if such black holes
were produced at the LHC. Most notably, we find that, under no circumstances,
would the black holes reach the (hazardous) regime of Bondi accretion.
Nonetheless, the possibility remains that black holes live long enough to
escape from the accelerator (and even from the Earth's gravitational field) and
result in missing energy from the detectors.Comment: RevTeX4, 12 pages, 4 figures, 5 tables, minor changes to match the
accepted version in JHE
e-EVN monitoring of M87
M87 is a privileged laboratory for a detailed study of the properties of jets, owing to its proximity (D=16.7 Mpc, 1 mas = 0.080 pc), its massive black hole (~6.0 x 10^9M) and its conspicuous emission at radio wavelengths and above. We started on November 2009 a monitoring program with the e-EVN at 5 GHz, in correspondence of the season of Very High Energy (VHE) observations. Indeed, two episodes of VHE activity have been reported in February and April 2010. We present here the main results of these multi-epoch observations: the inner jet and HST-1 are both detected and resolved in our datasets. We study the apparent velocity of HST-1, which seems to be increasing since 2005, and the flux density variability in the inner jet. All in all, the radio counterpart to this year’s VHE event seems to be different from the ones in 2005 and 2008, opening new scenario for the radio-high energy connection
Holography and trace anomaly: what is the fate of (brane-world) black holes?
The holographic principle relates (classical) gravitational waves in the bulk
to quantum fluctuations and the Weyl anomaly of a conformal field theory on the
boundary (the brane). One can thus argue that linear perturbations in the bulk
of static black holes located on the brane be related to the Hawking flux and
that (brane-world) black holes are therefore unstable. We try to gain some
information on such instability from established knowledge of the Hawking
radiation on the brane. In this context, the well-known trace anomaly is used
as a measure of both the validity of the holographic picture and of the
instability for several proposed static brane metrics. In light of the above
analysis, we finally consider a time-dependent metric as the (approximate)
representation of the late stage of evaporating black holes which is
characterized by decreasing Hawking temperature, in qualitative agreement with
what is required by energy conservation.Comment: 11 pages, 2 figures, a few comments and references added, accepted
for publication in Phys. Rev.
Minimum length effects in black hole physics
We review the main consequences of the possible existence of a minimum
measurable length, of the order of the Planck scale, on quantum effects
occurring in black hole physics. In particular, we focus on the ensuing minimum
mass for black holes and how modified dispersion relations affect the Hawking
decay, both in four space-time dimensions and in models with extra spatial
dimensions. In the latter case, we briefly discuss possible phenomenological
signatures.Comment: 29 pages, 12 figures. To be published in "Quantum Aspects of Black
Holes", ed. X. Calmet (Springer, 2014
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