140 research outputs found
Astrophysical constraints on primordial black holes in Brans-Dicke theory
We consider cosmological evolution in Brans-Dicke theory with a population of
primordial black holes. Hawking radiation from the primordial black holes
impacts various astrophysical processes during the evolution of the Universe.
The accretion of radiation by the black holes in the radiation dominated era
may be effective in imparting them a longer lifetime. We present a detailed
study of how this affects various standard astrophysical constraints coming
from the evaporation of primordial black holes. We analyze constraints from the
present density of the Universe, the present photon spectrum, the distortion of
the cosmic microwave background spectrum and also from processes affecting
light element abundances after nucleosynthesis. We find that the constraints on
the initial primordial black hole mass fractions are tightened with increased
accretion efficiency.Comment: 15 page
Phase separation and stripe formation in the 2D t-J model: a comparison of numerical results
We make a critical analysis of numerical results for and against phase
separation and stripe formation in the t-J model. We argue that the frustrated
phase separation mechanism for stripe formation requires phase separation at
too high a doping for it to be consistent with existing numerical studies of
the t-J model. We compare variational energies for various methods, and
conclude that the most accurate calculations for large systems appear to be
from the density matrix renormalization group. These calculations imply that
the ground state of the doped t-J model is striped, not phase separated.Comment: This version includes a revised, more careful comparison of numerical
results between DMRG and Green's function Monte Carlo. In particular, for the
original posted version we were accidentally sent obsolete data by Hellberg
and Manousakis; their new results, which are what were used in their Physical
Review Letter, are more accurate because a better trial wavefunction was use
Synergistic warm inflation
We consider an alternative warm inflationary scenario in which scalar
fields coupled to a dissipative matter fluid cooperate to produce power--law
inflation. The scalar fields are driven by an exponential potential and the
bulk dissipative pressure coefficient is linear in the expansion rate. We find
that the entropy of the fluid attains its asymptotic value in a characteristic
time proportional to the square of the number of fields. This scenario remains
nearly isothermal along the inflationary stage. The perturbations in energy
density and entropy are studied in the long--wavelength regime and seen to grow
roughly as the square of the scale factor. They are shown to be compatible with
COBE measurements of the fluctuations in temperature of the CMB.Comment: 13 pages, Revtex 3 To be published in Physical Review
Phase Separation Based on U(1) Slave-boson Functional Integral Approach to the t-J Model
We investigate the phase diagram of phase separation for the hole-doped two
dimensional system of antiferromagnetically correlated electrons based on the
U(1) slave-boson functional integral approach to the t-J model. We show that
the phase separation occurs for all values of J/t, that is, whether or with J, the Heisenberg coupling constant and t, the hopping
strength. This is consistent with other numerical studies of hole-doped two
dimensional antiferromagnets. The phase separation in the physically
interesting J region, is examined by introducing
hole-hole (holon-holon) repulsive interaction. We find from this study that
with high repulsive interaction between holes the phase separation boundary
tends to remain robust in this low region, while in the high J region, J/t
> 0.4, the phase separation boundary tends to disappear.Comment: 4 pages, 2 figures, submitted to Phys. Rev.
Elementary Excitations in Dimerized and Frustrated Heisenberg Chains
We present a detailed numerical analysis of the low energy excitation
spectrum of a frustrated and dimerized spin Heisenberg chain. In
particular, we show that in the commensurate spin--Peierls phase the ratio of
the singlet and triplet excitation gap is a universal function which depends on
the frustration parameter only. We identify the conditions for which a second
elementary triplet branch in the excitation spectrum splits from the continuum.
We compare our results with predictions from the continuum limit field theory .
We discuss the relevance of our data in connection with recent experiments on
, , and .Comment: Corrections to the text + 1 new figure, will appear in PRB (august
98
Scalar perturbation spectra from warm inflation
We present a numerical integration of the cosmological scalar perturbation
equations in warm inflation. The initial conditions are provided by a
discussion of the thermal fluctuations of an inflaton field and thermal
radiation using a combination of thermal field theory and thermodynamics. The
perturbation equations include the effects of a damping coefficient
and a thermodynamic potential . We give an analytic expression for the
spectral index of scalar fluctuations in terms of a new slow-roll parameter
constructed from . A series of toy models, inspired by spontaneous
symmetry breaking and a known form of the damping coefficient, lead to a
spectrum with on large scales and on small scales.Comment: 12 pages, 5 figures, RevTeX 4, revised with extra figure
Detection Limits for Super-Hubble Suppression of Causal Fluctuations
We investigate to what extent future microwave background experiments might
be able to detect a suppression of fluctuation power on large scales in flat
and open universe models. Such suppression would arise if fluctuations are
generated by causal processes, and a measurement of a small suppression scale
would be problematic for inflation models, but consistent with many defect
models. More speculatively, a measurement of a suppression scale of the order
of the present Hubble radius could provide independent evidence for a
fine-tuned inflation model leading to a low-density universe. We find that,
depending on the primordial power spectrum, a suppression scale modestly larger
than the visible Horizon can be detected, but that the detectability drops very
rapidly with increasing scale. For models with two periods of inflation, there
is essentially no possibility of detecting a causal suppression scale.Comment: 8 pages, 4 figures, revtex, In Press Physical Review D 200
Multiwavelength studies of MHD waves in the solar chromosphere: An overview of recent results
The chromosphere is a thin layer of the solar atmosphere that bridges the
relatively cool photosphere and the intensely heated transition region and
corona. Compressible and incompressible waves propagating through the
chromosphere can supply significant amounts of energy to the interface region
and corona. In recent years an abundance of high-resolution observations from
state-of-the-art facilities have provided new and exciting ways of
disentangling the characteristics of oscillatory phenomena propagating through
the dynamic chromosphere. Coupled with rapid advancements in
magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly
investigate the role waves play in supplying energy to sustain chromospheric
and coronal heating. Here, we review the recent progress made in
characterising, categorising and interpreting oscillations manifesting in the
solar chromosphere, with an impetus placed on their intrinsic energetics.Comment: 48 pages, 25 figures, accepted into Space Science Review
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