57 research outputs found
Universality in the distribution of caustics in the expanding Universe
We numerically investigate the long--time evolution of density perturbations
after the first appearance of caustics in an expanding cosmological model with
one--dimensional `single--wave' initial conditions. Focussing on the
time--intervals of caustic appearances and the spatial distribution of caustics
at subsequent times, we find that the time--intervals of caustic appearances
approach a constant, i.e., their time--subsequent ratio converges to 1; it is
also found that the spatial distribution of caustics at a given time features
some universality rules, e.g., the ratio between the position of the nearest
caustic from the center and that of the second nearest caustic from the center
approaches a constant. Furthermore we find some rules for the mass distribution
for each caustic. Using these universality constants we are in the position to
predict the spatial distribution of caustics at an arbitrary time in order to
give an estimate for the power spectral index in the fully--developed
non--dissipative turbulent (`virialized') regime.Comment: 23 pages, 19 figure
Blue spectra and induced formation of primordial black holes
We investigate the statistical properties of primordial black hole (PBH)
formation in the very early Universe. We show that the high level of
inhomogeneity of the early Universe leads to the formation of the first
generation PBHs. %The existence of these PBHs This causes later the appearance
of a dust-like phase of the cosmological expansion. We discuss here a new
mechanism for the second generation of PBH formation during the dust-like
phase. This mechanism is based on the coagulation process. We demonstrate that
the blue power spectrum of initial adiabatic perturbations after inflation
leads to overproduction of primordial black holes with gg if the power index is .Comment: 16 pages, 2 figure
Smart windows: cation internal and anion external activation for electrochromic films of nickel hydroxide
Two methods - internal and external - were used for improving the electrochemical activity and electrochromic properties of Ni(OH)2 films. In the case of internal activation, Al3+ ions were added to the electrolyte composition during the electrochemical precipitation process. In the case of external activation, [Fe(CN)6]4â ions were used in the electrolyte during the cycling process. In all the experiments there were used Ni(OH)2 films synthesized by the electrochemical template method with polyvinyl alcohol (PVA) addition to the electrolyte composition. It was shown that the addition of [Fe(CN)6]4â ions into the electrolyte for cycling leads to significant improvement of electrochemical and electrochromic (colorization-bleaching) properties. At the same time, the addition of Al3+ ions into the electrolyte for Ni(OH)2 films precipitation leads to drastic worsening of their properties. Also, possible mechanisms of Al3+ and [Fe(CN)6]4â ions influence were considered in this work
Artificial Intelligence
Contains research objectives and reports on eight research projects.Computation Center, M.I.T
Perturbation Spectra in the Warm Inflationary Scenario
We investigate the phenomenology of warm inflation and present generic
results about the evolution of the inflaton and radiation fields. The general
conditions required for warm inflation to take place are derived and discussed.
A comprehensive approach is presented for the generation of thermally induced
adiabatic and isocurvature perturbations and the amplitude of their spectra
calculated. In addition we derive the ratio of tensor-to-scalar perturbations
and find the spectral indices for adiabatic, isocurvature and tensor
perturbations formed in the warm inflationary era. These results are presented
in a simplified and compact approach that is generally applicable. Our results
are illustrated by inflation models with polynomial and exponential potentials.
We compare our analytic results against numerical models and find excellent
agreement. Finally, presently available data is used to put constraints on warm
inflation and we discuss how near--future observations may distinguish the warm
inflationary scenario from standard supercooled inflation. The main observable
difference is the different scalar-to- tensor ratio, and that the consistency
relation between this and the slope of tensor perturbations does not hold for
warm inflation.Comment: 12 pages, Latex, Accepted by Phys Rev D., minor change
The comparison of velocity distribution between Adhesion approximation and the Euler-Jeans-Newton model
For the evolution of density fluctuation in nonlinear cosmological dynamics,
adhesion approximation (AA) is proposed as a phenomenological model, which is
especially useful for describing nonlinear evolution. However, the origin of
the artificial viscosity in AA is not clarified. Recently, Buchert and
Dom\'{\i}nguez report if the velocity dispersion of the dust fluid is regarded
as isotropic, it works on a principle similar to viscosity or effective
pressure, and they consider isotropic velocity dispersion as the origin of the
artificial viscosity in AA. They name their model the Euler-Jeans-Newton (EJN)
model. In this paper, we focus on the velocity distribution in AA and the EJN
model and examine the time evolution in both models. We find the behavior of AA
differs from that of the EJN model, i.e., although the peculiar velocity in the
EJN model oscillates, that in AA is monotonically decelerated due to viscosity
without oscillation. Therefore it is hard to regard viscosity in AA as
effective pressure in the EJN model.Comment: 11 pages, 5 figures; accepted for publication in Phys.Rev.
Inflation: flow, fixed points and observables to arbitrary order in slow roll
I generalize the inflationary flow equations of Hoffman and Turner to
arbitrary order in slow roll. This makes it possible to study the predictions
of slow roll inflation in the full observable parameter space of tensor/scalar
ratio , spectral index , and running . It also becomes
possible to identify exact fixed points in the parameter flow. I numerically
evaluate the flow equations to fifth order in slow roll for a set of randomly
chosen initial conditions and find that the models cluster strongly in the
observable parameter space, indicating a ``generic'' set of predictions for
slow roll inflation. I comment briefly on the the interesting proposed
correspondence between flow in inflationary parameter space and renormalization
group flow in a boundary conformal field theory.Comment: 16 pages, 7 figures. LaTeX. V4: Fixed important error in numerical
constant in the second-order slow roll expressions for the observables r, n,
and dn/dlog(k). See footnote after Eq. (48). New figures, minor changes to
conclusions. Supersedes version published in Phys. Rev.
Antiprotons from primordial black holes
Primordial black holes (PBHs) have motivated many studies since it was shown
that they should evaporate and produce all kinds of particles. Recent
experimental measurements of cosmic rays with great accuracy, theoretical
investigations on the possible formation mechanisms and detailed evaporation
processes have revived the interest in such astrophysical objects. This article
aims at using the latest developments on antiproton propagation models together
with new data from BESS, CAPRICE and AMS experiments to constrain the local
amount of PBH dark matter. Depending on the diffusion halo parameters and on
the details of emission mechanism, we derive an average upper limit of the
order of rho < 1.7E-33 g cm^-3.Comment: Accepted by A&A . Revision : clarification, results unchange
Sensory Communication
Contains table of contents for Section 2, an introduction and reports on twelve research projects.National Institutes of Health Grant 5 R01 DC00117National Institutes of Health Contract 2 P01 DC00361National Institutes of Health Grant 5 R01 DC00126National Institutes of Health Grant R01-DC00270U.S. Air Force - Office of Scientific Research Contract AFOSR-90-0200National Institutes of Health Grant R29-DC00625U.S. Navy - Office of Naval Research Grant N00014-88-K-0604U.S. Navy - Office of Naval Research Grant N00014-91-J-1454U.S. Navy - Office of Naval Research Grant N00014-92-J-1814U.S. Navy - Naval Training Systems Center Contract N61339-93-M-1213U.S. Navy - Naval Training Systems Center Contract N61339-93-C-0055U.S. Navy - Naval Training Systems Center Contract N61339-93-C-0083U.S. Navy - Office of Naval Research Grant N00014-92-J-4005U.S. Navy - Office of Naval Research Grant N00014-93-1-119
Study of the Mn2+ ions influence in the deposition electrolyte on the electrochromic properties of obtained Ni(OH)2 films
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