Linear Perturbations in Brane Gas Cosmology

We consider the effect of string inhomogeneities on the time dependent background of Brane Gas Cosmology. We derive the equations governing the linear perturbations of the dilaton-gravity background in the presence of string matter sources. We focus on long wavelength fluctuations and find that there are no instabilities. Thus, the predictions of Brane Gas Cosmology are robust against the introduction of linear perturbations. In particular, we find that the stabilization of the extra dimensions (moduli) remains valid in the presence of dilaton and string perturbations.Comment: 17 pages, 1 figur

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

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

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, $T$-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

Albumin concentrations are primarily determined by the body cell mass and the systemic inflammatory response in cancer patients with weight loss

The association between hypoalbuminemia and poor prognosis in patients with cancer is well recognized. However, the factors that contribute to the fall in albumin concentrations are not well understood. In the present study, we examined the relationship between circulating albumin concentrations, weight loss, the body cell mass (measured using total body potassium), and the presence of an inflammatory response (measured using C- reactive protein) in male patients (n=40) with advanced lung or gastrointestinal cancer. Albumin concentrations were significantly correlated with the percent ideal body weight (r=0.390, p lt 0.05), extent of reported weight loss (r=-0.492, p lt 0.01), percent predicted total body potassium (adjusted for age, height, and weight, r=0.686, p lt 0.001), and logo C-reactive protein concentrations (r=-0.545, p lt 0.001). On multiple regression analysis, the percent predicted total body potassium and log(10) C-reactive protein concentrations accounted for 63% of the variation in albumin concentrations (r(2) = 0.626, p lt 0.001). The interrelationship between albumin, body cell mass, and the inflammatory response is consistent with the concept that the presence of an ongoing inflammatory response contributes to the progressive loss of these vital protein components of the body and the subsequent death of patients with advanced cancer

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

SOLVING SPLINE COLLOCATION APPROXIMATIONS TO NONLINEAR TWO-POINT BOUNDARY VALUE PROBLEMS BY A HOMOTOPY METHOD

The Chow-Yorke algorithm is a homotopy method that has been proved globally convergent for Brouwer fixed point problems, certain classes of zero linding and nonlinear programming problems, and two-point boundary value approximations based on shooting and finite differences. The method is numerically stable and has been successfully applied to a wide range of practical engineering problems. Here the Chow-Yorke algorithm is proved globally convergent for a class of spline collocation approxlmetions to nonlinear two-point boundary value problems. Several numerical implementations of the algorithm are briefly described. and computational results are presented for a fairly difficult hid dynamics boundary value problem

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 $\Lambda$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

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