57,536 research outputs found
Insights into neutrino decoupling gleaned from considerations of the role of electron mass
We present calculations showing how electron rest mass influences entropy
flow, neutrino decoupling, and Big Bang Nucleosynthesis (BBN) in the early
universe. To elucidate this physics and especially the sensitivity of BBN and
related epochs to electron mass, we consider a parameter space of rest mass
values larger and smaller than the accepted vacuum value. Electromagnetic
equilibrium, coupled with the high entropy of the early universe, guarantees
that significant numbers of electron-positron pairs are present, and dominate
over the number of ionization electrons to temperatures much lower than the
vacuum electron rest mass. Scattering between the electrons-positrons and the
neutrinos largely controls the flow of entropy from the plasma into the
neutrino seas. Moreover, the number density of electron-positron-pair targets
can be exponentially sensitive to the effective in-medium electron mass. This
entropy flow influences the phasing of scale factor and temperature, the
charged current weak-interaction-determined neutron-to-proton ratio, and the
spectral distortions in the relic neutrino energy spectra. Our calculations
show the sensitivity of the physics of this epoch to three separate effects:
finite electron mass, finite-temperature quantum electrodynamic (QED) effects
on the plasma equation of state, and Boltzmann neutrino energy transport. The
ratio of neutrino to plasma component energy scales manifests in Cosmic
Microwave Background (CMB) observables, namely the baryon density and the
radiation energy density, along with the primordial helium and deuterium
abundances. Our results demonstrate how the treatment of in-medium electron
mass (i.e., QED effects) could translate into an important source of
uncertainty in extracting neutrino and beyond-standard-model physics limits
from future high-precision CMB data.Comment: 32 pages, 8 figures, 1 table. Version accepted by Nuclear Physics
Conservation Laws and Hamilton's Equations for Systems with Long-Range Interaction and Memory
Using the fact that extremum of variation of generalized action can lead to
the fractional dynamics in the case of systems with long-range interaction and
long-term memory function, we consider two different applications of the action
principle: generalized Noether's theorem and Hamiltonian type equations. In the
first case, we derive conservation laws in the form of continuity equations
that consist of fractional time-space derivatives. Among applications of these
results, we consider a chain of coupled oscillators with a power-wise memory
function and power-wise interaction between oscillators. In the second case, we
consider an example of fractional differential action 1-form and find the
corresponding Hamiltonian type equations from the closed condition of the form.Comment: 30 pages, LaTe
Bjorken flow from an AdS Schwarzschild black hole
We consider a large black hole in asymptotically AdS spacetime of arbitrary
dimension with a Minkowski boundary. By performing an appropriate slicing as we
approach the boundary, we obtain via holographic renormalization a gauge theory
fluid obeying Bjorken hydrodynamics in the limit of large longitudinal proper
time. The metric we obtain reproduces to leading order the metric recently
found as a direct solution of the Einstein equations in five dimensions. Our
results are also in agreement with recent exact results in three dimensions.Comment: 5 pages in two-column RevTeX; sharpened discussion to appear in PR
Optimal Bond Trading with Personal Taxes: Implications for Bond Prices and Estimated Tax Brackets and Yield Curves
The assumption that bondholders follow either a buy-and-hold or a continuous realization trading policy, rather than the optimal trading policy,is at variance with reality and, as we demonstrate, may seriously bias the estimation of the yield curve and the implied tax bracket of the marginal investor. Tax considerations which govern a bondholder's optimal trading policy include the following: realization of capital losses, short term if possible; deferment of the realization of capital gains, especially if they are short term; changing the holding period status from long term to short term by sale of the bond and repurchase, so that future capital losses may be realized short term; and raising the basis through sale of the bond and repurchase in order to deduct from ordinary income the amortized premium. Because of the interaction of these factors, no simple characterization of the optimal trading policy is possible. We can say, however, that it differs substantially from the buy-and-hold policy irrespective of whether the bondholder is a bank, a bond dealer, or an individual. We obtain these strong results even when we allow for transactions costs and explicitly consider numerous IRS regulations designed to curtail tax avoidance.
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