190 research outputs found
Effective Geometry
We introduce the concept of effective geometry by studying several systems in
which it arises naturally. As an example of the power and conciseness of the
method, it is shown that a flowing dielectric medium with a linear response to
an external electric field can be used to generate an analog geometry that has
many of the formal properties of a \Sch black hole for light rays, in spite of
birefringence. The surface gravity of this analog black hole has a contribution
that depends only on the dielectric properties of the fluid (in addition to the
usual term dependent on the acceleration). This term may be give a hint to a
new mechanism to increase the temperature of Hawking radiation.Comment: 13 pages, RevTex4, Contribution to the Proceedings of the Xth
Brazilian School of Gravitation and Cosmology, to be published by AI
On the emergence of the CDM model from self-interacting Brans-Dicke theory in
We investigate whether a self-interacting Brans-Dicke theory in without
matter and with a time-dependent metric can describe, after dimensional
reduction to , the FLRW model with accelerated expansion and
non-relativistic matter. By rewriting the effective 4-dimensional theory as an
autonomous three-dimensional dynamical system and studying its critical points,
we show that the CDM cosmology cannot emerge from such a model. This
result suggests that a richer structure in may be needed to obtain the
accelerated expansion as well as the matter content of the 4-dimensional
universe.Comment: 7 pages, 7 figure
Evolution of Vacuum Bubbles Embeded in Inhomogeneous Spacetimes
We study the propagation of bubbles of new vacuum in a radially inhomogeneous
background filled with dust or radiation, and including a cosmological
constant, as a first step in the analysis of the influence of inhomogeneities
in the evolution of an inflating region. We also compare the cases with dust
and radiation backgrounds and show that the evolution of the bubble in
radiation environments is notably different from that in the corresponding dust
cases, both for homogeneous and inhomogeneous ambients, leading to appreciable
differences in the evolution of the proper radius of the bubble.Comment: 18 pages, 15 figures, accepted for publication in Journal of
Cosmology and Astroparticle Physics (new version with a few cosmetic changes
w.r.t. the published one
Accretion disks around black holes in modified strong gravity
Stellar-mass black holes offer what is perhaps the best scenario to test
theories of gravity in the strong-field regime. In particular, f(R) theories,
which have been widely discuss in a cosmological context, can be constrained
through realistic astrophysical models of phenomena around black holes. We aim
at building radiative models of thin accretion disks for both Schwarzschild and
Kerr black holes in f(R) gravity. We study particle motion in
f(R)-Schwarzschild and Kerr space-times. We present the spectral energy
distribution of the accretion disk around constant Ricci scalar f(R) black
holes, and constrain specific f(R) prescriptions using features of these
systems. A precise determination of both the spin and accretion rate onto black
holes along with X-ray observations of their thermal spectrum might allow to
identify deviations of gravity from General Relativity. We use recent data on
the high-mass X-ray binary Cygnus X-1 to restrict the values of the parameters
of a class of f(R) models.Comment: 16 pages, 20 figures, accepted for publication in Astronomy &
Astrophysic
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