Dark Energy, Inflation and Extra Dimensions

We consider how accelerated expansion, whether due to inflation or dark energy, imposes strong constraints on fundamental theories obtained by compactification from higher dimensions. For theories that obey the null energy condition (NEC), we find that inflationary cosmology is impossible for a wide range of compactifications; and a dark energy phase consistent with observations is only possible if both Newton's gravitational constant and the dark energy equation-of-state vary with time. If the theory violates the NEC, inflation and dark energy are only possible if the NEC-violating elements are inhomogeneously distributed in thecompact dimensions and vary with time in precise synchrony with the matter and energy density in the non-compact dimensions. Although our proofs are derived assuming general relativity applies in both four and higher dimensions and certain forms of metrics, we argue that similar constraints must apply for more general compactifications.Comment: 26 pages, 1 figure. v2: reference added, typos correcte

Influences of Host Density, Temperature, and Parasite Age on the Reproductive Potential of \u3ci\u3eBathyplectes Curculionis\u3c/i\u3e (Hymenoptera: Ichneumonidae), an Endoparasite of the Alfalfa Weevil (Coleoptera: Curculionidae)

Alfalfa weevil larvae were exposed to Bathyplectes curculionis (Thomson) to determine the effect of host density, temperature, and parasite age on the reproductive potential of curculionis. Percent parasitism was found to be inversely proportional to host density and most of the parasites distributed their eggs randomly regardless of host density. The number of eggs deposited was largely independent of temperature. Peak egg laying was reached in three days from which point the parasite\u27s capabilities diminished with increasing age. The longevity of ovipositing females was shorter than females that were not exposed to larvae

Exploring Bicycle and Public Transit Use by Low-Income Latino Immigrants: A Mixed-Methods Study in the San Francisco Bay Area

Latin American immigrants will continue to make up a large share of transit ridership, bicycling and walking in the United States for the foreseeable future, but there is relatively little research about them. This mixed-methods study compares the travel patterns of low-income immigrants living in the San Francisco Bay Area with that of other groups and investigates the barriers and constraints faced by low-income immigrants when taking transit and bicycling. Much of the previous work on immigrant travel has relied on national surveys and qualitative analysis, which underrepresent disadvantaged population groups and slower modes of travel, or are unable to speak to broader patterns in the population. We conducted interviews with 14 low-income immigrants and a paper-based intercept survey of 2,078 adults. Interviewees revealed five major barriers that made public transit use difficult for them, including safety, transit fare affordability, discrimination, system legibility, and reliability. Although crime was the most prominent issue in interviews, the survey results suggest transit cost is the most pressing concern for low-income immigrants. Low-income immigrants were less likely than those with higher-incomes to have access to a motor vehicle, and were less likely than higher-income immigrants or the U.S.-born of any income to have access to a bicycle or bus pass. Finally, although most barriers to public transit use were the same regardless of nativity or household income, low-income immigrants were much less willing to take public transit when they had the option to drive and less willing to bicycle for any purpose. The prevalence of concerns about transit affordability, crime, and reliability suggest transit agencies should consider income-based fare reductions, coordinated crime prevention with local law enforcement, and improved scheduling

On the Energetics of the HCO+^+ + C →\to CH+^+ + CO Reaction and Some Astrochemical Implications

We explore the energetics of the titular reaction, which current astrochemical databases consider open at typical dense molecular (i.e., dark) cloud conditions. As is common for reactions involving the transfer of light particles, we assume that there are no intersystem crossings of the potential energy surfaces involved. In the absence of any such crossings, we find that this reaction is endoergic and will be suppressed at dark cloud temperatures. Updating accordingly a generic astrochemical model for dark clouds changes the predicted gas-phase abundances of 224 species by greater than a factor of 2. Of these species, 43 have been observed in the interstellar medium. Our findings demonstrate the astrochemical importance of determining the role of intersystem crossings, if any, in the titular reaction.Comment: Accepted for publication in ApJ; 14 pages, 2 figures, and 1 tabl

Exchange-controlled single-electron-spin rotations in quantum dots

We show theoretically that arbitrary coherent rotations can be performed quickly (with a gating time ~1 ns) and with high fidelity on the spin of a single confined electron using control of exchange only, without the need for spin-orbit coupling or ac fields. We expect that implementations of this scheme would achieve gate error rates on the order of \eta ~ 10^{-3} in GaAs quantum dots, within reach of several known error-correction protocolsComment: 4+ pages, 3 figures; v2: Streamlined presentation, final version published in PRB (Rapid Comm.

Axion constraints in non-standard thermal histories

It is usually assumed that dark matter is produced during the radiation dominated era. There is, however, no direct evidence for radiation domination prior to big-bang nucleosynthesis. Two non-standard thermal histories are considered. In one, the low-temperature-reheating scenario, radiation domination begins as late as 1 MeV, and is preceded by significant entropy generation. Thermal axion relic abundances are then suppressed, and cosmological limits to axions are loosened. For reheating temperatures less than 35 MeV, the large-scale structure limit to the axion mass is lifted. The remaining constraint from the total density of matter is significantly relaxed. Constraints are also relaxed for higher reheating temperatures. In a kination scenario, a more modest change to cosmological axion constraints is obtained. Future possible constraints to axions and low-temperature reheating from the helium abundance and next-generation large-scale-structure surveys are discussed.Comment: 10 pages, 7 figures, revised to match version published in Phys. Rev. D. Fig. 7 and Eq. (20) modifie

Precise Model for Small-Body Thermal Radiation Pressure Acting on Spacecraft

A precise representation of small-body surface thermal radiation pressure effects acting on orbiting spacecraft is discussed. The proposed framework takes advantage of a general Fourier series expansion to compute small-body surface thermal radiation pressure. Fourier series expansion has been used before for the precise representation of solar radiation pressure effects on spacecraft orbiting small bodies. This framework takes into account the geometric relationship of orbiting spacecraft with the small-body surface, surface thermal parameters of the small body, and the shape and surface properties of spacecraft allowing for the computation of thermal radiation pressure, which may also be used for the generation of precise orbit determination solutions. After presenting the general model, an example application of the model for the OSIRIS-REx spacecraft in orbit about Asteroid (101955) Bennu is provided. Simulation studies were used to evaluate the effect of mismodeling of thermal radiation pressure on the spacecraft and study the use of the proposed method for generating precise orbit determination solutions