29,259 research outputs found
The Swampland, Quintessence and the Vacuum Energy
It has recently been conjectured that string theory does not admit de Sitter
vacua, and that quintessence explains the current epoch of accelerated cosmic
expansion. A proposed, key prediction of this scenario is time-varying
couplings in the dark sector, induced by the evolving quintessence field. We
note that cosmological models with varying couplings suffer from severe
problems with quantum corrections, beyond those shared by all quintessence
models. The vacuum energy depends on all masses and couplings of the theory,
and even small variations of parameters can lead to overwhelmingly large
corrections to the effective potential. We find that quintessence models with
varying parameters can be realised in consistent quantum theories by either: 1)
enforcing exceptional levels of fine-tuning; 2) realising some unknown
mechanism that cancels all undesirable contributions to the effective potential
with unprecedented accuracy; or 3) ensuring that the quintessence field couples
exclusively to very light states, and does not backreact on heavy fields.Comment: 4
Using HINODE/Extreme-Ultraviolet Imaging Spectrometer to confirm a seismologically inferred coronal temperature
The Extreme-Ultraviolet Imaging Spectrometer on board the HINODE satellite is used to examine the loop system described in Marsh et al. (2009) by applying spectroscopic diagnostic methods. A simple isothermal mapping algorithm is applied to determine where the assumption of isothermal plasma may be valid, and the emission measure locii technique is used to determine the temperature profile along the base of the loop system. It is found that, along the base, the loop has a uniform temperature profile with a mean temperature of 0.89 +- 0.09 MK which is in agreement with the temperature determined seismologically in Marsh et al. (2009), using observations interpreted as the slow magnetoacoustic mode. The results further strengthen the slow mode interpretation, propagation at a uniform sound speed, and the analysis method applied in Marsh et al. (2009). It is found that it is not possible to discriminate between the slow mode phase speed and the sound speed within the precision of the present observations
ECONOMIC FACTORS DETERMINING CHANGES IN DRESSED WEIGHTS OF LIVE CATTLE AND HOGS
Livestock dressed weights have experienced significant trends and volatility which affect wholesale production of red meats. An econometric model was used to estimate the impact of relative prices and technology on cattle and hog average dressed weights. For fed steers and heifers, the economic incentives affecting placement weights and weight added in feedlots were considered. Results indicate quarterly dressed weights of steers and heifers respond to contemporaneous profitability ratios and to lagged feeder prices, the effects being highly inelastic. Cow dressed weights also responded while hog dressed weights did not respond to profitability ratios. Technology changes may have accounted for about 83% of dressed weight growth for steers and about 62% for hogs from 1980-97.Livestock Production/Industries,
Demand and Supply Factors in the Feed Grain Market: Effects on Corn Producers
Demand and Price Analysis,
Evaluation of Beef Promotion Expenditures Under the U.S. Beef Checkoff Program
Agricultural and Food Policy,
Feed Grain Volatility and Effects on Feeder Cattle Producers
Production Economics,
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