1,713 research outputs found
Stretching the Inflaton Potential with Kinetic Energy
Inflation near a maximum of the potential is studied when non-local
derivative operators are included in the inflaton Lagrangian. Such terms can
impose additional sources of friction on the field. For an arbitrary spacetime
geometry, these effects can be quantified in terms of a local field theory with
a potential whose curvature around the turning point is strongly suppressed.
This implies that a prolonged phase of slow-roll inflation can be achieved with
potentials that are otherwise too steep to drive quasi-exponential expansion.
We illustrate this mechanism within the context of p-adic string theory.Comment: 4 page
Interacting dark energy, holographic principle and coincidence problem
The interacting and holographic dark energy models involve two important
quantities. One is the characteristic size of the holographic bound and the
other is the coupling term of the interaction between dark energy and dark
matter. Rather than fixing either of them, we present a detailed study of
theoretical relationships among these quantities and cosmological parameters as
well as observational constraints in a very general formalism. In particular,
we argue that the ratio of dark matter to dark energy density depends on the
choice of these two quantities, thus providing a mechanism to change the
evolution history of the ratio from that in standard cosmology such that the
coincidence problem may be solved. We investigate this problem in detail and
construct explicit models to demonstrate that it may be alleviated provided
that the interacting term and the characteristic size of holographic bound are
appropriately specified. Furthermore, these models are well fitted with the
current observation at least in the low red-shift region.Comment: 20 pages, 3 figure
Effects of Random Link Removal on the Photonic Band Gaps of Honeycomb Networks
We explore the effects of random link removal on the photonic band gaps of
honeycomb networks. Missing or incomplete links are expected to be common in
practical realizations of this class of connected network structures due to
unavoidable flaws in the fabrication process. We focus on the collapse of the
photonic band gap due to the defects induced by the link removal. We show that
the photonic band gap is quite robust against this type of random decimation
and survives even when almost 58% of the network links are removed
Complete Set of Homogeneous Isotropic Analytic Solutions in Scalar-Tensor Cosmology with Radiation and Curvature
We study a model of a scalar field minimally coupled to gravity, with a
specific potential energy for the scalar field, and include curvature and
radiation as two additional parameters. Our goal is to obtain analytically the
complete set of configurations of a homogeneous and isotropic universe as a
function of time. This leads to a geodesically complete description of the
universe, including the passage through the cosmological singularities, at the
classical level. We give all the solutions analytically without any
restrictions on the parameter space of the model or initial values of the
fields. We find that for generic solutions the universe goes through a singular
(zero-size) bounce by entering a period of antigravity at each big crunch and
exiting from it at the following big bang. This happens cyclically again and
again without violating the null energy condition. There is a special subset of
geodesically complete non-generic solutions which perform zero-size bounces
without ever entering the antigravity regime in all cycles. For these, initial
values of the fields are synchronized and quantized but the parameters of the
model are not restricted. There is also a subset of spatial curvature-induced
solutions that have finite-size bounces in the gravity regime and never enter
the antigravity phase. These exist only within a small continuous domain of
parameter space without fine tuning initial conditions. To obtain these
results, we identified 25 regions of a 6-parameter space in which the complete
set of analytic solutions are explicitly obtained.Comment: 38 pages, 29 figure
Cosmological scaling solutions of minimally coupled scalar fields in three dimensions
We examine Friedmann-Robertson-Walker models in three spacetime dimensions.
The matter content of the models is composed of a perfect fluid, with a
-law equation of state, and a homogeneous scalar field minimally
coupled to gravity with a self-interacting potential whose energy density
red-shifts as , where a denotes the scale factor. Cosmological
solutions are presented for different range of values of and .
The potential required to agree with the above red-shift for the scalar field
energy density is also calculated.Comment: LaTeX2e, 11 pages, 4 figures. To be published in Classical and
Quantum Gravit
Halo Properties in Cosmological Simulations of Self-Interacting Cold Dark Matter
We present a comparison of halo properties in cosmological simulations of
collisionless cold dark matter (CDM) and self-interacting dark matter (SIDM)
for a range of dark matter cross sections. We find, in agreement with various
authors, that CDM yields cuspy halos that are too centrally concentrated as
compared to observations. Conversely, SIDM simulations using a Monte Carlo
N-body technique produce halos with significantly reduced central densities and
flatter cores with increasing cross section. We introduce a concentration
parameter based on enclosed mass that we expect will be straightforward to
determine observationally, unlike that of Navarro, Frenk & White, and provide
predictions for SIDM and CDM. SIDM also produces more spherical halos than CDM,
providing possibly the strongest observational test of SIDM. We discuss our
findings in relation to various relevant observations as well as SIDM
simulations of other groups. Taking proper account of simulation limitations,
we find that a dark matter cross section per unit mass of sigma_DM ~=
10^{-23}-10^{-24} cm^2/GeV is consistent with all current observational
constraints.Comment: 14 pages, submitted to Ap
Penrose Quantum Antiferromagnet
The Penrose tiling is a perfectly ordered two dimensional structure with
fivefold symmetry and scale invariance under site decimation. Quantum spin
models on such a system can be expected to differ significantly from more
conventional structures as a result of its special symmetries. In one
dimension, for example, aperiodicity can result in distinctive quantum
entanglement properties. In this work, we study ground state properties of the
spin-1/2 Heisenberg antiferromagnet on the Penrose tiling, a model that could
also be pertinent for certain three dimensional antiferromagnetic
quasicrystals. We show, using spin wave theory and quantum Monte Carlo
simulation, that the local staggered magnetizations strongly depend on the
local coordination number z and are minimized on some sites of five-fold
symmetry. We present a simple explanation for this behavior in terms of
Heisenberg stars. Finally we show how best to represent this complex
inhomogeneous ground state, using the "perpendicular space" representation of
the tiling.Comment: 4 pages, 5 figure
A Dynamical Solution to the Problem of a Small Cosmological Constant and Late-time Cosmic Acceleration
Increasing evidence suggests that most of the energy density of the universe
consists of a dark energy component with negative pressure, a ``cosmological
constant" that causes the cosmic expansion to accelerate. In this paper, we
address the puzzle of why this component comes to dominate the universe only
recently rather than at some much earlier epoch. We present a class of theories
based on an evolving scalar field where the explanation is based entirely on
internal dynamical properties of the solutions. In the theories we consider,
the dynamics causes the scalar field to lock automatically into a negative
pressure state at the onset of matter-domination such that the present epoch is
the earliest possible time, consistent with nucleosynthesis restrictions, when
it can start to dominate.Comment: 5 pages, 3 figure
Self interacting Brans Dicke cosmology and Quintessence
Recent cosmological observations reveal that we are living in a flat
accelerated expanding universe. In this work we have investigated the nature of
the potential compatible with the power law expansion of the universe in a self
interacting Brans Dicke cosmology with a perfect fluid background and have
analyzed whether this potential supports the accelerated expansion. It is found
that positive power law potential is relevant in this scenario and can drive
accelerated expansion for negative Brans Dicke coupling parameter . The
evolution of the density perturbation is also analyzed in this scenerio and is
seen that the model allows growing modes for negative .Comment: 8pages, 5 figures, PRD style, some changes are made, figures added,
reference added. To be published in Int. J. Mod. Phys.
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