402 research outputs found
Conditions for low-redshift positive apparent acceleration in smooth inhomogeneous models
It is known that a smooth LTB model cannot have a positive apparent central
acceleration. Using a local Taylor expansion method we study the low-redshift
conditions to obtain an apparent negative deceleration parameter
derived from the luminosity distance for a central observer in a LTB
space, confirming that central smoothness implies a positive central
deceleration. Since observational data is only available at redshift greater
than zero we find the critical values of the parameters defining a centrally
smooth LTB model which give a positive apparent acceleration at ,
providing a graphical representation of the conditions in the
plane, which are respectively the zero and first order
terms of the central Taylor expansion of . We finally derive a
coordinate independent expression for the apparent deceleration parameter based
on the expansion of the relevant functions in red-shift rather than in the
radial coordinate.
We calculate with two different methods to solve the null
geodesic equations, one based on a local central expansion of the solution in
terms of cosmic time and the other one using the exact analytical solution in
terms of generalized conformal time. %The expansion of the solution in terms of
cosmic time is quite useful also for other applications requiring foliation %of
space-time in space-like hyper-surfaces, such as spatial averaging, which is
much more difficult to study using the %analytical solution in terms of the
generalized conformal time coordinate.Comment: 18 pages, 3 figures, abstract, added section with coordinate
independent conditions, version accepted for publication in GR
Coulomb drag between two strange metals
We study the Coulomb drag between two strange-metal layers using the
Einstein-Maxwell-Dilaton model from holography. We show that the
low-temperature dependence of the drag resistivity is ,
which strongly deviates from the quadratic dependence of Fermi liquids. We also
present numerical results at room temperature, using typical parameters of the
cuprates, to provide an estimate of the magnitude of this effect for future
experiments. We find that the drag resistivity is enhanced by the plasmons
characteristic of the two-layer system.Comment: 5 pages, 3 figure
Godel-type space-time metrics
A simple group theoretic derivation is given of the family of space-time
metrics with isometry group SO(2,1) X SO(2) X R first described by Godel, of
which the Godel stationary cosmological solution is the member with a
perfect-fluid stress-energy tensor. Other members of the family are shown to be
interpretable as cosmological solutions with a electrically charged perfect
fluid and a magnetic field.Comment: Heavly rewritten respect to the orginal version, corrected some typos
due to files transfer in the last submitted versio
Vector and Tensor Contributions to the Luminosity Distance
We compute the vector and tensor contributions to the luminosity distance
fluctuations in first order perturbation theory and we expand them in spherical
harmonics. This work presents the formalism with a first application to a
stochastic background of primordial gravitational waves.Comment: 14 pages, 3 figure
Approximate Consistency Condition from Running Spectral Index in Slow-Roll Inflationary Models
Density perturbations generated from inflation almost always have a spectral
index n_s which runs (varies with the wavelength). We explore a running
spectral index scenario in which the scalar spectral index runs from blue (n_s
>1) on large length scales to red (n_s<1) on short length scales. Specifically,
we look for a correlation between the length scale at which n_s-1=0 and the
length scale at which tensor to scalar ratio T/S reaches a minimum for single
field slow roll inflationary models. By computing the distribution of length
scale ratios, we conclude that there indeed is a new approximate consistency
condition that is characteristic of running spectral index scenarios that run
from blue to red. Specifically, with strong running, we expect 96% of the slow
roll models to have the two length scales to be within a factor of 2, with the
length scale at which the tensor to scalar ratio reaching a minimum longer than
the wavelength at which n_s-1=0.Comment: 18 pages, 2 figure
Redshift spherical shell energy in isotropic Universes
We introduce the redshift spherical shell energy (RSSE), which can be used to
test in the redshift space the radial inhomogeneity of an isotropic universe,
providing additional constraints for LTB models, and a more general test of
cosmic homogeneity.Comment: 11 pages, 2 figures, Accepted by Physical Review D1
Evolution of Coordination in Social Networks: A Numerical Study
Coordination games are important to explain efficient and desirable social
behavior. Here we study these games by extensive numerical simulation on
networked social structures using an evolutionary approach. We show that local
network effects may promote selection of efficient equilibria in both pure and
general coordination games and may explain social polarization. These results
are put into perspective with respect to known theoretical results. The main
insight we obtain is that clustering, and especially community structure in
social networks has a positive role in promoting socially efficient outcomes.Comment: preprint submitted to IJMP
Spatial averaging and apparent acceleration in inhomogeneous spaces
As an alternative to dark energy that explains the observed acceleration of
the universe, it has been suggested that we may be at the center of an
inhomogeneous isotropic universe described by a Lemaitre-Tolman-Bondi (LTB)
solution of Einstein's field equations. To test this possibility, it is
necessary to solve the null geodesics. In this paper we first give a detailed
derivation of a fully analytical set of differential equations for the radial
null geodesics as functions of the redshift in LTB models. As an application we
use these equaions to show that a positive averaged acceleration obtained
in LTB models through spatial averaging can be incompatible with cosmological
observations. We provide examples of LTB models with positive which fail
to reproduce the observed luminosity distance . Since the apparent
cosmic acceleration is obtained from fitting the observed luminosity
distance to a FLRW model we conclude that in general a positive in LTB
models does not imply a positive .Comment: 16 pages, 12 figures. Explicit derivation of the fully analytical
null geodesic equations has been added. Published in GR
Testing homogeneity with galaxy number counts : light-cone metric and general low-redshift expansion for a central observer in a matter dominated isotropic universe without cosmological constant
As an alternative to dark energy it has been suggested that we may be at the
center of an inhomogeneous isotropic universe described by a
Lemaitre-Tolman-Bondi (LTB) solution of Einstein's field equations. In order to
test this hypothesis we calculate the general analytical formula to fifth order
for the redshift spherical shell mass. Using the same analytical method we
write the metric in the light-cone by introducing a gauge invariant quantity
which together with the luminosity distance completely
determine the light-cone geometry of a LTB model.Comment: 13 page
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