145 research outputs found

### Acoustic analogs of two-dimensional black holes

We present a general method for constructing acoustic analogs of the black
hole solutions of two-dimensional (2D) dilaton gravity. Because by dimensional
reduction every spherically symmetric, four-dimensional (4D) black hole admits
a 2D description, the method can be also used to construct analogue models of
4D black holes. We also show that after fixing the gauge degrees of freedom the
2D gravitational dynamics is equivalent to an one-dimensional fluid dynamics.
This enables us to find a natural definition of mass $M$, temperature $T$ and
entropy $S$ of the acoustic black hole. In particular the first principle of
thermodynamics $dM=TdS$ becomes a consequence of the fluid dynamics equations.
We also discuss the general solutions of the fluid dynamics and two particular
cases, the 2D Anti-de sitter black hole and the 4D Schwarzschild black hole.Comment: some references adde

### An Einstein-like theory of gravity with a non-newtonian weak-field limit

We propose a model describing Einstein gravity coupled to a scalar field with
an exponential potential. We show that the weak-field limit of the model has
static solutions given by a gravitational potential behaving for large
distances as \ln r . The Newtonian term GM/r appears only as subleading. Our
model can be used to give a phenomenological explanation of the rotation curves
of the galaxies without postulating the presence of dark matter. This can be
achieved only by giving up at galactic scales Einstein equivalence principle.Comment: Final version, accepted for publication on General Relativity and
Gravitatio

### The dualities of 3D dilaton gravity

We investigate Brans-Dicke dilaton gravity theories in 2+1 dimensions. We
show that the reduced field equations for solutions with diagonal metric and
depending only on one spacetime coordinate have a continuous O(2) symmetry.
Using this symmetry we derive general static and cosmological solutions of the
theory. The action of the discrete group O(2,Z) on the space of the solutions
is discussed. Three-dimensional string effective theory and three-dimensional
general relativity are discussed in detail. In particular, we find that the
previously discovered black string solution is dual to a spacetime with a
conical singularity.Comment: 15 pages, LaTex file, no figure

### Induced gravity and entanglement entropy of 2D black holes

Using the fact that 2D Newton constant is wholly induced by a conformal field
theory, we derive a formula for the entanglement entropy of the anti-de Sitter
black hole in two spacetime dimensions. The leading term in the large black
hole mass expansion of our formula reproduces exactly the Bekenstein-Hawking
entropy S_{BH}, whereas the subleading term behaves as ln S_{BH}. This
subleading term has the universal form typical for the entanglement entropy of
physical systems described by effective conformal fields theories (e.g.
one-dimensional statistical models at the critical point).Comment: 6 page

### How is the Presence of Horizons and Localized Matter Encoded in the Entanglement Entropy?

Motivated by the new theoretical paradigm that views spacetime geometry as
emerging from the entanglement of a pre-geometric theory, we investigate the
issue of the signature of the presence of horizons and localized matter on the
entanglement entropy (EE) S_E for the case of three-dimensional AdS (AdS_3)
gravity. We use the holographically dual two-dimensional CFT on the torus and
the related modular symmetry in order to treat bulk black holes and conical
singularities (sourced by pointlike masses not shielded by horizons) on the
same footing. In the regime where boundary tori can be approximated by
cylinders we are able to give universal expressions for the EE of black holes
and conical singularities. We argue that the presence of horizons/localized
matter in the bulk is encoded in the EE in terms of (i) enhancement/reduction
of the entanglement of the AdS_3 vacuum, (ii) scaling as area/volume of the
leading term of the perturbative expansion of S_E, (iii) exponential/periodic
behaviour of S_E, (iv) presence of unaccessible regions in the
noncompact/compact dimension of the boundary cylinder. In particular, we show
that the reduction effect of matter on the entanglement of the vacuum found by
Verlinde for the de Sitter vacuum extends to the AdS_3 vacuumComment: 16 pages, no figure

### Asymptotically flat black holes sourced by a massless scalar field

We derive exact, asymptotically flat black hole solutions of Einstein-scalar
gravity sourced by a non trivial scalar field with $1/r$ asymptotic behaviour.
They are determined using an ansatz for the scalar field profile and working
out, together with the metric functions, the corresponding form of the scalar
self-interaction potential. Near to the singularity the black hole behaves as
the Janis-Newmann-Winicour-Wyman solution. We also work out a consistent
thermodynamical description of our black hole solutions. For large mass our
hairy black holes have the same thermodynamical behaviour of the Schwarzschild
black hole, whereas for small masses they differ substantially from the latter.Comment: 12 pages, no figures. Refs. [27-33] added. Some remarks about
previous derivation and stability of solution (5.4) adde

### Holography of charged dilatonic black branes at finite temperature

We investigate bulk and holographic features of finite-temperature black
brane solutions of 4D anti-de Sitter Einstein-Maxwell-dilaton-gravity (EMDG).
We construct, numerically, black branes endowed with non trivial scalar hairs
for broad classes of EMDG. We consider both exponential and power-law forms for
the coupling functions, as well as several charge configurations: purely
electric, purely magnetic and dyonic solutions. At finite temperature the field
theory holographically dual to these black brane solutions has a rich and
interesting phenomenology reminiscent of electron motion in metals: phase
transitions triggered by nonvanishing VEV of scalar operators, non-monotonic
behavior of the electric conductivities as function of the frequency and of the
temperature, Hall effect and sharp synchrotron resonances of the conductivity
in presence of a magnetic field. Conversely, in the zero temperature limit the
conductivities for these models show a universal behavior. The optical
conductivity has a power-law behavior as a function of the frequency, whereas
the DC conductivity is suppressed at small temperatures.Comment: 29 pages 14 figures, typos corrected, reference adde

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