6,479 research outputs found
Geometric Precipices in String Cosmology
We consider the effects of graviton multiplet fields on transitions between
string gas phases. Focusing on the dilaton field, we show that it may obstruct
transitions between different thermodynamic phases of the string gas, because
the sign of its dimensionally reduced, -duality invariant, part is conserved
when the energy density of the universe is positive. Thus, many interesting
solutions for which this sign is positive end up in a future curvature
singularity. Because of this, some of the thermodynamic phases of the usual
gravitating string gases behave like superselection sectors. For example, a
past-regular Hagedorn phase and an expanding FRW phase dominated by string
momentum modes cannot be smoothly connected in the framework of string
cosmology with positive sources. The singularity separates them like a
geometric precipice in the moduli space, preventing the dynamics of the theory
from bridging across. Sources which simultaneously violate the positivity of
energy and NEC could modify these conclusions. We provide a quantitative
measure of positivity of energy and NEC violations that would be necessary for
such transitions. These effects must dominate the universe at the moment of
transition, altering the standard gas pictures. At present, it is not known how
to construct such sources from first principles in string theory.Comment: 27 pages, 6 figures, References added, accepted in PR
Impact of solar photovoltaics on the low-voltage distribution network in New Zealand
Residential rooftop-mounted solar photovoltaic (PV) panels are being installed at an increasing rate, both in New Zealand and globally. There have been concerns over possible issues such as overvoltage and overcurrent. These PV systems are mostly connected at low voltage (LV). This study presents a case study of simulating the entire LV network from a single utility, comprising 10,558 11 kV–415 V transformers and their associated distribution feeders. These results are also presented by network type. Various solar PV penetration levels are added to the model and the power-flow results are presented. From these results, possible maximum limits of solar PV penetration are investigated and measures to alleviate overvoltage problems are simulated. The effect of using PV inverters with voltage regulation is simulated. Results show that some minor overvoltage problems can be expected in the future, particularly in urban areas. However, in most cases the overvoltage would not be much higher than the statutory limit of 1.06 p.u
Effective Field Theory Approach to String Gas Cosmology
We derive the 4D low energy effective field theory for a closed string gas on
a time dependent FRW background. We examine the solutions and find that
although the Brandenberger-Vafa mechanism at late times no longer leads to
radion stabilization, the radion rolls slowly enough that the scenario is still
of interest. In particular, we find a simple example of the string inspired
dark matter recently proposed by Gubser and Peebles.Comment: 19 pages, 2 figures, comments adde
Stabilization of Extra Dimensions at Tree Level
By considering the effects of string winding and momentum modes on a time
dependent background, we find a method by which six compact dimensions become
stabilized naturally at the self-dual radius while three dimensions grow large.Comment: 15 pages, 2 figures, minor typos correcte
Moduli Stabilization with Long Winding Strings
Stabilizing all of the modulus fields coming from compactifications of string
theory on internal manifolds is one of the outstanding challenges for string
cosmology. Here, in a simple example of toroidal compactification, we study the
dynamics of the moduli fields corresponding to the size and shape of the torus
along with the ambient flux and long strings winding both internal directions.
It is known that a string gas containing states with non-vanishing winding and
momentum number in one internal direction can stabilize the radius of this
internal circle to be at self-dual radius. We show that a gas of long strings
winding all internal directions can stabilize all moduli, except the dilaton
which is stabilized by hand, in this simple example.Comment: title changed, improved presentation; reference added. 18 pages, JHEP
styl
String Gas Cosmology
We present a critical review and summary of String Gas Cosmology. We include
a pedagogical derivation of the effective action starting from string theory,
emphasizing the necessary approximations that must be invoked. Working in the
effective theory, we demonstrate that at late-times it is not possible to
stabilize the extra dimensions by a gas of massive string winding modes. We
then consider additional string gases that contain so-called enhanced symmetry
states. These string gases are very heavy initially, but drive the moduli to
locations that minimize the energy and pressure of the gas. We consider both
classical and quantum gas dynamics, where in the former the validity of the
theory is questionable and some fine-tuning is required, but in the latter we
find a consistent and promising stabilization mechanism that is valid at
late-times. In addition, we find that string gases provide a framework to
explore dark matter, presenting alternatives to CDM as recently
considered by Gubser and Peebles. We also discuss quantum trapping with string
gases as a method for including dynamics on the string landscape.Comment: 55 pages, 1 figure, minor corrections, version to appear in Reviews
of Modern Physic
Effect of Three Initial Implant Programs with a Common Terminal Revalor®- 200 on Feedlot Performance and Carcass Traits of Weaned Steers
A commercial feedlot study utilizing 1,350 calf- fed steers (initial BW = 623 lb; ±23 lb) compared three initial implant strategies: Revalor®- IS (day 1), Revalor®- IS (day 1) and Revalor®- 200 (day 67), or Revalor®- XS (day 1). Each initial implant strategy was followed by a terminal Revalor®- 200 implant (day 133) to determine effects on performance and carcass traits. No differences in final body weight, intake, gain, or feed conversion were observed on either a live, or carcass adjusted basis. Th ere were also no differences in hot carcass weight, USDA quality grade, or USDA yield grade. Results from this study suggest initial implant strategy has minimal impact on feedlot and carcass performance when following with a terminal Revalor®- 200 implant
Inflation without Inflaton(s)
We propose a model for early universe cosmology without the need for
fundamental scalar fields. Cosmic acceleration and phenomenologically viable
reheating of the universe results from a series of energy transitions, where
during each transition vacuum energy is converted to thermal radiation. We show
that this `cascading universe' can lead to successful generation of adiabatic
density fluctuations and an observable gravity wave spectrum in some cases,
where in the simplest case it reproduces a spectrum similar to slow-roll models
of inflation. We also find the model provides a reasonable reheating
temperature after inflation ends. This type of model may also be relevant for
addressing the smallness of the vacuum energy today.Comment: 13 pages, 4 figures, published versio
Living on a dS brane: Effects of KK modes on inflation
We develop a formalism to study non-local higher-dimensional effects in
braneworld scenarios from a four-dimensional effective theory point of view and
check it against the well-known Garriga-Tanaka result in the appropriate limit.
We then use this formalism to study the spectrum of density perturbations
during inflation as seen from the lower-dimensional effective theory. In
particular, we find that the gravitational potential is greatly enhanced at
short wavelengths. The consequences to the curvature perturbations are
nonetheless very weak and will lead to no characteristic signatures on the
power spectrum.Comment: 21 pages, no figure
- …