1,574 research outputs found
Physical Layer Security of Large Reflecting Surface Aided Communications with Phase Errors
The physical layer security (PLS) performance of a wireless communication
link through a large reflecting surface (LRS) with phase errors is analyzed.
Leveraging recent results that express the \ac{LRS}-based composite channel as
an equivalent scalar fading channel, we show that the eavesdropper's link is
Rayleigh distributed and independent of the legitimate link. The different
scaling laws of the legitimate and eavesdroppers signal-to-noise ratios with
the number of reflecting elements, and the reasonably good performance even in
the case of coarse phase quantization, show the great potential of LRS-aided
communications to enhance PLS in practical wireless set-ups.Comment: This work has been submitted to the IEEE for publication. Copyright
may be transferred without notice, after which this version may no longer be
accessibl
String Tension and the Generation of the Conformal Anomaly
The origin of the string conformal anomaly is studied in detail. We use a
reformulated string Lagrangian which allows to consider the string tension
as a small perturbation. The expansion parameter is the worldsheet
speed of light c, which is proportional to . We examine carefully the
interplay between a null (tensionless) string and a tensionful string which
includes orders and higher. The conformal algebra generated by the
constraints is considered. At the quantum level the normal ordering provides a
central charge proportional to . Thus it is clear that quantum null
strings respect conformal invariance and it is the string tension which
generates the conformal anomaly.Comment: More references are included. Final version, to appear in Phys.Rev.D.
6 pages, LaTex, no figure
String propagation in four-dimensional dyonic black hole background
We study string propagation in an exact, four-dimensional dyonic black hole
background. The general solutions describing string configurations are obtained
by solving the string equations of motion and constraints. By using the
covariant formalism, we also investigate the propagation of physical
perturbations along the string in the given curved background.Comment: 19 pages, Tex (macro phyzzx is needed
On the Effect of Correlation on the Capacity of Backscatter Communication Systems
We analyse the effect of correlation between the forward and backward links
on the capacity of backscatter communication systems. To that aim, we obtain an
analytical expression for the average capacity under a correlated Rayleigh
product fading channel, as well as closed-form asymptotic expressions for the
high and low signal-to-noise ratio (SNR) regimes. Our results show that
correlation is indeed detrimental for a fixed target SNR; contrarily to the
common belief, we also see that correlation can be actually beneficial in some
instances when a fixed power budget is considered.Comment: This work has been submitted for publication. Copyright may be
transferred without notice, after which this version may no longer be
accessibl
Advances in String Theory in Curved Backgrounds: A Synthesis Report
A synthetic report of the advances in the study of classical and quantum string dynamics in curved backgrounds is provided, namely: the new feature of multistring solutions; the effect of a cosmological constant and of spacial curvature on classical and quantum strings; classical splitting of fundamental strings;the general string evolution in constant curvature spacetimes;the conformal invariant effects;strings on plane waves, shock waves and spacetime singularities and its spectrum. New developments in string gravity and string cosmology are reported: string driven cosmology and its predictions;the primordial gravitation wave background; non-singular string cosmologies from exact conformal field theories;QFT, string temperature and the string phase of de Sitter space; the string phase of black holes;new dual relation between QFT regimes and string regimes and the 'QFT/String Tango'; new coherent string states and minimal uncertainty principle in string theor
Constraints on dark matter particles from theory, galaxy observations and N-body simulations
Mass bounds on dark matter (DM) candidates are obtained for particles
decoupling in or out of equilibrium with {\bf arbitrary} isotropic and
homogeneous distribution functions. A coarse grained Liouville invariant
primordial phase space density is introduced. Combining its
value with recent photometric and kinematic data on dwarf spheroidal satellite
galaxies in the Milky Way (dShps), the DM density today and -body
simulations, yields upper and lower bounds on the mass, primordial phase space
densities and velocity dispersion of the DM candidates. The mass of the DM
particles is bound in the few keV range. If chemical freeze out occurs before
thermal decoupling, light bosonic particles can Bose-condense. Such
Bose-Einstein {\it condensate} is studied as a dark matter candidate. Depending
on the relation between the critical()and decoupling()temperatures, a
BEC light relic could act as CDM but the decoupling scale must be {\it higher}
than the electroweak scale. The condensate tightens the upper bound on the
particle's mass. Non-equilibrium scenarios that describe particle production
and partial thermalization, sterile neutrinos produced out of equilibrium and
other DM models are analyzed in detail obtaining bounds on their mass,
primordial phase space density and velocity dispersion. Light thermal relics
with and sterile neutrinos lead to a
primordial phase space density compatible with {\bf cored} dShps and disfavor
cusped satellites. Light Bose condensed DM candidates yield phase space
densities consistent with {\bf cores} and if also with cusps.
Phase space density bounds from N-body simulations suggest a potential tension
for WIMPS with .Comment: 27 pages 8 figures. Version to appear in Phys. Rev.
Quantum corrections to the inflaton potential and the power spectra from superhorizon modes and trace anomalies
We obtain the effective inflaton potential during slow roll inflation by
including the one loop quantum corrections to the energy momentum tensor from
scalar curvature and tensor perturbations as well as quantum fluctuations from
light scalars and light Dirac fermions generically coupled to the inflaton.
During slow roll inflation there is a clean and unambiguous separation between
superhorizon and subhorizon contributions to the energy momentum tensor. The
superhorizon part is determined by the curvature perturbations and scalar field
fluctuations: both feature infrared enhancements as the inverse of a
combination of slow roll parameters which measure the departure from scale
invariance in each case.Fermions and gravitons do not exhibit infrared
divergences. The subhorizon part is completely specified by the trace anomaly
of the fields with different spins and is solely determined by the space-time
geometry. The one-loop quantum corrections to the amplitude of curvature and
tensor perturbations are obtained to leading order in slow-roll and in the
(H/M_PL)^2 expansion. This study provides a complete assessment of the
backreaction problem up to one loop including bosonic and fermionic degrees of
freedom. The result validates the effective field theory description of
inflation and confirms the robustness of the inflationary paradigm to quantum
fluctuations. Quantum corrections to the power spectra are expressed in terms
of the CMB observables:n_s, r and dn_s/dln k. Trace anomalies (especially the
graviton part) dominate these quantum corrections in a definite direction: they
enhance the scalar curvature fluctuations and reduce the tensor fluctuations.Comment: 18 pages, no figure
Strings Near a Rindler Or Black Hole Horizon
Orbifold techniques are used to study bosonic, type II and heterotic strings
in Rindler space at integer multiples N of the Rindler temperature, and near a
black hole horizon at integer multiples of the Hawking temperature, extending
earlier results of Dabholkar. It is argued that a Hagedorn transition occurs
nears the horizon for all N>1.Comment: 13 pages, harvmac, (references added
Quantum slow-roll and quantum fast-roll inflationary initial conditions: CMB quadrupole suppression and further effects on the low CMB multipoles
Quantum fast-roll initial conditions for the inflaton which are different
from the classical fast-roll conditions and from the quantum slow-roll
conditions can lead to inflation that last long enough. These quantum fast-roll
initial conditions for the inflaton allow for kinetic energies of the same
order of the potential energies and nonperturbative inflaton modes with nonzero
wavenumbers. Their evolution starts with a transitory epoch where the redshift
due to the expansion succeeds to assemble the quantum excited modes of the
inflaton in a homogeneous (zero mode) condensate, and the large value of the
Hubble parameter succeeds to overdamp the fast-roll of the redshifted inflaton
modes. After this transitory stage the effective classical slow-roll epoch is
reached. Most of the efolds are produced during the slow-roll epoch and we
recover the classical slow-roll results for the scalar and tensor metric
perturbations plus corrections. These corrections are important, both for
scalar and for tensor perturbations, if scales which are horizon-size today
exited the horizon by the end of the transitory stage and as a consequence the
lower CMB multipoles get suppressed (fast-roll) or enhanced (precondensate).
These two types of corrections can compete and combine in a scale dependent
manner. They arise as natural consequences of the quantum nonperturbative
inflaton dynamics, and provide a consistent and contrastable model for the
origin of the suppression of the quadrupole and for other departures of the low
CMB multipoles from the slow-roll inflation-LambdaCMB model which are to be
contrasted to the TE and EE multipoles and to the forthcoming and future CMB
data.Comment: LaTeX, 14 pages, 3 figure
CMB quadrupole suppression: II. The early fast roll stage
Within the effective field theory of inflation, an initialization of the
classical dynamics of the inflaton with approximate equipartition between the
kinetic and potential energy of the inflaton leads to a brief fast roll stage
that precedes the slow roll regime. The fast roll stage leads to an attractive
potential in the wave equations for the mode functions of curvature and tensor
perturbations. The evolution of the inflationary perturbations is equivalent to
the scattering by this potential and a useful dictionary between the scattering
data and observables is established.Implementing methods from scattering theory
we prove that this attractive potential leads to a suppression of the
quadrupole moment for CMB and B-mode angular power spectra. The scale of the
potential is determined by the Hubble parameter during slow roll. Within the
effective field theory of inflation at the grand unification (GUT) energy scale
we find that if inflation lasts a total number of efolds N_{tot} ~ 59, there is
a 10-20% suppression of the CMB quadrupole and about 2-4% suppression of the
tensor quadrupole. The suppression of higher multipoles is smaller, falling off
as 1/l^2. The suppression is much smaller for N_{tot} > 59, therefore if the
observable suppression originates in the fast roll stage, there is the upper
bound N_{tot} ~ 59.Comment: Some comments and references adde
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