850 research outputs found
Visible Branes with Negative Tension in Heterotic M-Theory
It is shown that there exist large classes of BPS vacua in heterotic M-theory
which have negative tension on the visible orbifold plane, positive tension on
the hidden plane and positive tension, physical five-branes in the bulk space.
Explicit examples of such vacua are presented. Furthermore, it is demonstrated
that the ratio, beta/|alpha|, of the bulk five-brane tension to the visible
plane tension can, for several large classes of such vacua, be made arbitrarily
small. Hence, it is straightforward to find vacua with the properties required
in the examples of the Ekpyrotic theory of cosmology - a visible brane with
negative tension and beta/|alpha| small. This contradicts recent claims in the
literature.Comment: 30 page
Langevin Analysis of Eternal Inflation
It has been widely claimed that inflation is generically eternal to the
future, even in models where the inflaton potential monotonically increases
away from its minimum. The idea is that quantum fluctuations allow the field to
jump uphill, thereby continually revitalizing the inflationary process in some
regions. In this paper we investigate a simple model of this process,
pertaining to inflation with a quartic potential, in which analytic progress
may be made. We calculate several quantities of interest, such as the expected
number of inflationary efolds, first without and then with various selection
effects. With no additional weighting, the stochastic noise has little impact
on the total number of inflationary efoldings even if the inflaton starts with
a Planckian energy density. A "rolling" volume factor, i.e. weighting in
proportion to the volume at that time, also leads to a monotonically decreasing
Hubble constant and hence no eternal inflation. We show how stronger selection
effects including a constraint on the initial and final states and weighting
with the final volume factor can lead to a picture similar to that usually
associated with eternal inflation.Comment: 22 pages, 2 figure
Coupled quintessence and curvature-assisted acceleration
Spatially homogeneous models with a scalar field non-minimally coupled to the
space-time curvature or to the ordinary matter content are analysed with
respect to late-time asymptotic behaviour, in particular to accelerated
expansion and isotropization. It is found that a direct coupling to the
curvature leads to asymptotic de Sitter expansion in arbitrary exponential
potentials, thus yielding a positive cosmological constant although none is
apparent in the potential. This holds true regardless of the steepness of the
potential or the smallness of the coupling constant. For matter-coupled scalar
fields, the asymptotics are obtained for a large class of positive potentials,
generalizing the well-known cosmic no-hair theorems for minimal coupling. In
this case it is observed that the direct coupling to matter does not impact the
late-time dynamics essentially.Comment: 17 pages, no figures. v2: typos correcte
Optical Absorption Characteristics of Silicon Nanowires for Photovoltaic Applications
Solar cells have generated a lot of interest as a potential source of clean
renewable energy for the future. However a big bottleneck in wide scale
deployment of these energy sources remain the low efficiency of these
conversion devices. Recently the use of nanostructures and the strategy of
quantum confinement have been as a general approach towards better charge
carrier generation and capture. In this article we have presented calculations
on the optical characteristics of nanowires made out of Silicon. Our
calculations show these nanowires form excellent optoelectronic materials and
may yield efficient photovoltaic devices
Dark-Energy Dynamics Required to Solve the Cosmic Coincidence
Dynamic dark energy (DDE) models are often designed to solve the cosmic
coincidence (why, just now, is the dark energy density , the same
order of magnitude as the matter density ?) by guaranteeing for significant fractions of the age of the universe. This
typically entails ad-hoc tracking or oscillatory behaviour in the model.
However, such behaviour is neither sufficient nor necessary to solve the
coincidence problem. What must be shown is that a significant fraction of
observers see . Precisely when, and for how long, must a
DDE model have in order to solve the coincidence? We
explore the coincidence problem in dynamic dark energy models using the
temporal distribution of terrestrial-planet-bound observers. We find that any
dark energy model fitting current observational constraints on and
the equation of state parameters and , does have for a large fraction of observers in the universe. This demotivates DDE
models specifically designed to solve the coincidence using long or repeated
periods of .Comment: 16 pages, 8 figures, Submitted to Phys. Rev.
Can black holes be torn up by phantom dark energy in cyclic cosmology?
Infinitely cyclic cosmology is often frustrated by the black hole problem. It
has been speculated that this obstacle in cyclic cosmology can be removed by
taking into account a peculiar cyclic model derived from loop quantum cosmology
or the braneworld scenario, in which phantom dark energy plays a crucial role.
In this peculiar cyclic model, the mechanism of solving the black hole problem
is through tearing up black holes by phantom. However, using the theory of
fluid accretion onto black holes, we show in this paper that there exists
another possibility: that black holes cannot be torn up by phantom in this
cyclic model. We discussed this possibility and showed that the masses of black
holes might first decrease and then increase, through phantom accretion onto
black holes in the expanding stage of the cyclic universe.Comment: 6 pages, 2 figures; discussions adde
A Tale of Two Tilings
What do you get when you cross a crystal with a quasicrystal? The surprising
answer stretches from Fibonacci to Kepler, who nearly 400 years ago showed how
the ancient tiles of Archimedes form periodic patterns.Comment: 3 pages, 1 figur
Sequestration of vacuum energy and the end of the universe
Recently, we proposed a mechanism for sequestering the standard model vacuum energy that predicts that the Universe will collapse. Here we present a simple mechanism for bringing about this collapse, employing a scalar field whose potential is linear and becomes negative, providing the negative energy density required to end the expansion. The slope of the potential is chosen to allow for the expansion to last until the current Hubble time, about 10 10 years, to accommodate our Universe. Crucially, this choice is technically natural due to a shift symmetry. Moreover, vacuum energy sequestering selects radiatively stable initial conditions for the collapse, which guarantee that immediately before the turnaround the Universe is dominated by the linear potential which drives an epoch of accelerated expansion for at least an e fold. Thus, a single, technically natural choice for the slope ensures that the collapse is imminent and is preceded by the current stage of cosmic acceleration, giving a new answer to the “why now?” problem
Primordial Black Hole Formation from Inflaton
Measurements of the distances to SNe Ia have produced strong evidence that
the Universe is really accelarating, implying the existence of a nearly uniform
component of dark energy with the simplest explanation as a cosmological
constant. In this paper a small changing cosmological term is proposed, which
is a function of a slow-rolling scalar field, by which the de Sitter primordial
black holes' properties, for both charged and uncharged cases, are carefully
examined and the relationship between the black hole formation and the energy
transfer of the inflaton within this cosmological term is eluciatedComment: 6 pages, Late
Robustness of slow contraction to cosmic initial conditions
We present numerical relativity simulations of cosmological scenarios in
which the universe is smoothed and flattened by undergoing a phase of slow
contraction and test their sensitivity to a wide range of initial conditions.
Our numerical scheme enables the variation of all freely specifiable physical
quantities that characterize the initial spatial hypersurface, such as the
initial shear and spatial curvature contributions as well as the initial field
and velocity distributions of the scalar that drives the cosmological
evolution. In particular, we include initial conditions that are far outside
the perturbative regime of the well-known attractor scaling solution. We
complement our numerical results by analytically performing a complete
dynamical systems analysis and show that the two approaches yield consistent
results.Comment: 41 pages, 18 figures; accepted for publication in JCA
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