3,744 research outputs found
Local Lagrangian Approximations for the Evolution of the Density Distribution Function in Large-Scale Structure
We examine local Lagrangian approximations for the gravitational evolution of
the density distribution function. In these approximations, the final density
at a Lagrangian point q at a time t is taken to be a function only of t and of
the initial density at the same Lagrangian point. A general expression is given
for the evolved density distribution function for such approximations, and we
show that the vertex generating function for a local Lagrangian mapping applied
to an initially Gaussian density field bears a simple relation to the mapping
itself. Using this result, we design a local Lagrangian mapping which
reproduces nearly exactly the hierarchical amplitudes given by perturbation
theory for gravitational evolution. When extended to smoothed density fields
and applied to Gaussian initial conditions, this mapping produces a final
density distribution function in excellent agreement with full numerical
simulations of gravitational clustering. We also examine the application of
these local Lagrangian approximations to non-Gaussian initial conditions.Comment: LaTeX, 22 pages, and 11 postscript figure
Thawing quintessence with a nearly flat potential
The thawing quintessence model with a nearly flat potential provides a
natural mechanism to produce an equation of state parameter, w, close to -1
today. We examine the behavior of such models for the case in which the
potential satisfies the slow roll conditions: [(1/V)(dV/dphi)]^2 << 1 and
(1/V)(d^2 V/dphi^2) << 1, and we derive the analog of the slow-roll
approximation for the case in which both matter and a scalar field contribute
to the density. We show that in this limit, all such models converge to a
unique relation between 1+w, Omega_phi, and the initial value of
(1/V)(dV/dphi). We derive this relation, and use it to determine the
corresponding expression for w(a), which depends only on the present-day values
for w and Omega_phi. For a variety of potentials, our limiting expression for
w(a) is typically accurate to within delta w < 0.005 for w<-0.9. For redshift z
< 1, w(a) is well-fit by the Chevallier-Polarski-Linder parametrization, in
which w(a) is a linear function of a.Comment: 8 pages, 5 figures, discussion added, references updated, typos
corrected, to appear in Phys. Rev.
Anodal transcranial direct current stimulation does not induce analgesic effects on experimentally induced pain
Chronic pain is a disabling condition in which the adaptive link between pain intensity and tissue damage is lacking. Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that induces analgesic effects on experimentally induced pain when applied at the primary motor cortex (M1), and the dorsolateral prefrontal cortex (DLPFC). However, whether greater analgesic effects occur when tDCS is applied simultaneously at the M1 and the DLPFC is unknown, and is the primary aim of the current study. Nineteen healthy adult volunteers (12 male; Mage = 29.21, SD = 10.78, range 20 to 52) participated in a double blinded, crossover, sham controlled, randomised design. Dependent variables were self-reported pain ratings to punctuate pinprick stimuli, and the current level required of electrical stimulation to elicit moderate pain. These ratings were obtained pretest, posttest, and follow up of 20 min of anodal tDCS applied at the M1, DLPFC, M1 + DLPFC, or sham tDCS. Results indicate that pain to pinprick stimuli and the current level required to elicit moderate increased from pretest, posttest, and follow up. However, this was irrespective of the tDCS condition administered. Methodological inconsistencies pertaining to the administration of tDCS in the current study include lower current intensity and smaller electrode size as compared to past research. Thus, the tDCS stimulation parameters employed in the current study may have not been efficacious to inducing analgesic effects. Therefore, the current study highlights theoretical implications for future research to employ established tDCS parameters
User's Guide for ERB 7 SEFDT. Volume 1: User's Guide. Volume 2: Quality Control Report, Year 1
The Nimbus-7 ERB SEFDT Data User's Guide is presented. The guide consists of four subsections which describe: (1) the scope of the data User's Guide; (2) the background on Nimbus-7 Spacecraft and the ERB experiment; (3) the SEFDT data product and processing scenario; and (4) other related products and documents
Social dynamics in conferences: analyses of data from the Live Social Semantics application
Popularity and spread of online social networking in recent years has given a great momentum to the study of dynamics and patterns of social interactions. However, these studies have often been confined to the online world, neglecting its interdependencies with the offline world. This is mainly due to the lack of real data that spans across this divide. The Live Social Semantics application is a novel platform that dissolves this divide, by collecting and integrating data about people from (a) their online social networks and tagging activities from popular social networking sites, (b) their publications and co-authorship networks from semantic repositories, and (c) their real-world face-to-face contacts with other attendees collected via a network of wearable active sensors. This paper investigates the data collected by this application during its deployment at three major conferences, where it was used by more than 400 people. Our analyses show the robustness of the patterns of contacts at various conferences, and the influence of various personal properties (e.g. seniority, conference attendance) on social networking patterns
Limits on MeV Dark Matter from the Effective Number of Neutrinos
Thermal dark matter that couples more strongly to electrons and photons than
to neutrinos will heat the electron-photon plasma relative to the neutrino
background if it becomes nonrelativistic after the neutrinos decouple from the
thermal background. This results in a reduction in N_eff below the
standard-model value, a result strongly disfavored by current CMB observations.
Taking conservative lower bounds on N_eff and on the decoupling temperature of
the neutrinos, we derive a bound on the dark matter particle mass of m_\chi >
3-9 MeV, depending on the spin and statistics of the particle. For p-wave
annihilation, our limit on the dark matter particle mass is stronger than the
limit derived from distortions to the CMB fluctuation spectrum produced by
annihilations near the epoch of recombination.Comment: 5 pages, 1 figure, discussion added, references added and updated,
labels added to figure, to appear in Phys. Rev.
Phantom Dark Energy Models with a Nearly Flat Potential
We examine phantom dark energy models produced by a field with a negative
kinetic term and a potential that satisfies the slow roll conditions:
[(1/V)(dV/dphi)]^2 << 1 and (1/V)(d^2 V/dphi^2) << 1. Such models provide a
natural mechanism to produce an equation of state parameter, w, slightly less
than -1 at present. Using techniques previously applied to quintessence, we
show that in this limit, all such phantom models converge to a single
expression for w(a), which is a function only of the present-day values of
Omega_phi and w. This expression is identical to the corresponding behavior of
w(a) for quintessence models in the same limit. At redshifts z < 1, this
limiting behavior is well fit by the linear parametrization, w=w_0 + w_a(1-a),
with w_a \approx -1.5(1+w_0).Comment: 4 pages, 2 figures, discussion added, to appear in Phys. Rev.
Generalizing the generalized Chaplygin gas
The generalized Chaplygin gas is characterized by the equation of state p = -
A/rho^alpha, with alpha > -1 and w > -1. We generalize this model to allow for
the cases where alpha < -1 or w < -1. This generalization leads to three new
versions of the generalized Chaplygin gas: an early phantom model in which w <<
-1 at early times and asymptotically approaches w = -1 at late times, a late
phantom model with w \approx -1 at early times and w -> - \infty at late times,
and a transient model with w \approx -1 at early times and w -> 0 at late
times. We consider these three cases as models for dark energy alone and
examine constraints from type Ia supernovae and from the subhorizon growth of
density perturbations. The transient Chaplygin gas model provides a possible
mechanism to allow for a currently accelerating universe without a future
horizon, while some of the early phantom models produce w < -1 without either
past or future singularities.Comment: 8 pages, 9 figures, references added, to appear in Phys. Rev.
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