286 research outputs found
Modified Gravity Makes Galaxies Brighter
We investigate the effect of modifed gravity with screening mechanisms, such
as the chameleon or symmetron models, upon the structure of main sequence
stars. We find that unscreened stars can be significantly more luminous and
ephemeral than their screened doppelgangers. By embedding these stars into
dwarf galaxies, which can be unscreened for values of the parameters not yet
ruled out observationally, we show that the cumulative effect of their
increased luminosity can enhance the total galactic luminosity. We estimate
this enhancement and find that it can be considerable given model parameters
that are still under experimental scrutiny. By looking for systematic offsets
between screened dwarf galaxies in clusters and unscreened galaxies in voids,
these effects could form the basis of an independent observational test that
can potentially lower the current experimental bounds on the model independent
parameters of these theories by and order of magnitude or more.Comment: 16 pages, six figure
Formation of Relativistic Axion Stars
Axions and axion-like particles are compelling candidates for the missing
dark matter of the universe. As they undergo gravitational collapse, they can
form compact objects such as axion stars or even black holes. In this paper, we
study the formation and distribution of such objects. First, we simulate the
formation of compact axion stars using numerical relativity with aspherical
initial conditions that could represent the final stages of axion dark matter
structure formation. We show that the final states of such collapse closely
follow the known relationship of initial mass and axion decay constant .
Second, we demonstrate with a toy model how this information can be used to
scan a model density field to predict the number densities and masses of such
compact objects. In addition to being detectable by the LIGO/VIRGO
gravitational wave interferometer network for axion mass of eV, we show using peak statistics that for , there
exists a "mass gap" between the masses of axion stars and black holes formed
from collapse
An engineering model of lower thalamo-cortico-basal ganglionic circuit function
Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003.Includes bibliographical references (leaves 59-60).An engineering model of lower thalamo-cortico-basal ganglionic circuit functionality was extended and tested. This model attempts to explain the circuitry of the basal ganglia, examine its functional properties, and integrate these properties into an understanding of the diseases of the basal ganglia, such as Parkinson's disease and Huntington's disease. Using this model, simulations of various movements were developed, specifically those of the following: 1) one-step, cruise movements, 2) asynchronous, cruise movements, and 3) sequential cruise movements. Results of these movements include simulated movements of both normal patients and patients with movement disorders.by Eugene J. Lim.M.Eng
Cosmological Information in Weak Lensing Peaks
Recent studies have shown that the number counts of convergence peaks
N(kappa) in weak lensing (WL) maps, expected from large forthcoming surveys,
can be a useful probe of cosmology. We follow up on this finding, and use a
suite of WL convergence maps, obtained from ray-tracing N-body simulations, to
study (i) the physical origin of WL peaks with different heights, and (ii)
whether the peaks contain information beyond the convergence power spectrum
P_ell. In agreement with earlier work, we find that high peaks (with amplitudes
>~ 3.5 sigma, where sigma is the r.m.s. of the convergence kappa) are typically
dominated by a single massive halo. In contrast, medium-height peaks (~0.5-1.5
sigma) cannot be attributed to a single collapsed dark matter halo, and are
instead created by the projection of multiple (typically, 4-8) halos along the
line of sight, and by random galaxy shape noise. Nevertheless, these peaks
dominate the sensitivity to the cosmological parameters w, sigma_8, and
Omega_m. We find that the peak height distribution and its dependence on
cosmology differ significantly from predictions in a Gaussian random field. We
directly compute the marginalized errors on w, sigma_8, and Omega_m from the
N(kappa) + P_ell combination, including redshift tomography with source
galaxies at z_s=1 and z_s=2. We find that the N(kappa) + P_ell combination has
approximately twice the cosmological sensitivity compared to P_ell alone. These
results demonstrate that N(kappa) contains non-Gaussian information
complementary to the power spectrum.Comment: 24 pages, 12 figures, 14 tables. Accepted for publication in PRD
(version before proofs
Higher derivative theories with constraints : Exorcising Ostrogradski's Ghost
We prove that the linear instability in a non-degenerate higher derivative
theory, the Ostrogradski instability, can only be removed by the addition of
constraints if the original theory's phase space is reduced.Comment: 17 pages, no figures, version published in JCA
Visualization of microscale particle focusing in diluted and whole blood using particle trajectory analysis
Inertial microfluidics has demonstrated the potential to provide a rich range of capabilities to manipulate biological fluids and particles to address various challenges in biomedical science and clinical medicine. Various microchannel geometries have been used to study the inertial focusing behavior of particles suspended in simple buffer solutions or in highly diluted blood. One aspect of inertial focusing that has not been studied is how particles suspended in whole or minimally diluted blood respond to inertial forces in microchannels. The utility of imaging techniques (i.e., high-speed bright-field imaging and long exposure fluorescence (streak) imaging) primarily used to observe particle focusing in microchannels is limited in complex fluids such as whole blood due to interference from the large numbers of red blood cells (RBCs). In this study, we used particle trajectory analysis (PTA) to observe the inertial focusing behavior of polystyrene beads, white blood cells, and PC-3 prostate cancer cells in physiological saline and blood. Identification of in-focus (fluorescently labeled) particles was achieved at mean particle velocities of up to 1.85 m s[superscript −1]. Quantitative measurements of in-focus particles were used to construct intensity maps of particle frequency in the channel cross-section and scatter plots of particle centroid coordinates vs. particle diameter. PC-3 cells spiked into whole blood (HCT = 45%) demonstrated a novel focusing mode not observed in physiological saline or diluted blood. PTA can be used as an experimental frame of reference for understanding the physical basis of inertial lift forces in whole blood and discover inertial focusing modes that can be used to enable particle separation in whole blood
Probing Cosmology with Weak Lensing Minkowski Functionals
In this paper, we show that Minkowski Functionals (MFs) of weak gravitational
lensing (WL) convergence maps contain significant non-Gaussian,
cosmology-dependent information. To do this, we use a large suite of
cosmological ray-tracing N-body simulations to create mock WL convergence maps,
and study the cosmological information content of MFs derived from these maps.
Our suite consists of 80 independent 512^3 N-body runs, covering seven
different cosmologies, varying three cosmological parameters Omega_m, w, and
sigma_8 one at a time, around a fiducial LambdaCDM model. In each cosmology, we
use ray-tracing to create a thousand pseudo-independent 12 deg^2 convergence
maps, and use these in a Monte Carlo procedure to estimate the joint confidence
contours on the above three parameters. We include redshift tomography at three
different source redshifts z_s=1, 1.5, 2, explore five different smoothing
scales theta_G=1, 2, 3, 5, 10 arcmin, and explicitly compare and combine the
MFs with the WL power spectrum. We find that the MFs capture a substantial
amount of information from non-Gaussian features of convergence maps, i.e.
beyond the power spectrum. The MFs are particularly well suited to break
degeneracies and to constrain the dark energy equation of state parameter w (by
a factor of ~ three better than from the power spectrum alone). The
non-Gaussian information derives partly from the one-point function of the
convergence (through V_0, the "area" MF), and partly through non-linear spatial
information (through combining different smoothing scales for V_0, and through
V_1 and V_2, the boundary length and genus MFs, respectively). In contrast to
the power spectrum, the best constraints from the MFs are obtained only when
multiple smoothing scales are combined.Comment: 19 pages, 9 figures, 5 table
Validation of a New Predictive Risk Model: Measuring the Impact of the Major Modifiable Risks of Death for Patients and Populations
Background: Modifiable risks account for a large fraction of disease and death, but clinicians and patients lack tools to identify high risk populations or compare the possible benefit of different interventions.
Methods: We used data on the distribution of exposure to 12 major behavioral and biometric risk factors inthe US population, mortality rates by cause, and estimates of the proportional hazards of risk factor exposure from published systematic reviews to develop a risk prediction model that estimates an adult\u27s 10 year mortality risk compared to a population with optimum risk factors. We compared predicted risk to observed mortality in 8,241 respondents in NHANES 1988-1994 and NHANES 1999-2004 with linked mortality data up to the end of 2006
Scale Invariance without Inflation?
We propose a new alternative mechanism to seed a scale invariant spectrum of
primordial density perturbations that does not rely on inflation. In our
scenario, a perfect fluid dominates the early stages of an expanding,
non-inflating universe. Because the speed of sound of the fluid decays,
perturbations are left frozen behind the sound horizon, with a spectral index
that depends on the fluid equation of state. We explore here a toy model that
realizes this idea. Although the model can explain an adiabatic, Gaussian,
scale invariant spectrum of primordial perturbations, it turns out that in its
simplest form it cannot account for the observed amplitude of the primordial
density perturbations.Comment: 6 two-column pages, 1 figure. Uses RevTeX4. v2: References added and
number of required e-folds refine
Can we detect Hot or Cold spots in the CMB with Minkowski Functionals?
In this paper, we investigate the utility of Minkowski Functionals as a probe
of cold/hot disk-like structures in the CMB. In order to construct an accurate
estimator, we resolve a long-standing issue with the use of Minkowski
Functionals as probes of the CMB sky -- namely that of systematic differences
("residuals") when numerical and analytical MF are compared. We show that such
residuals are in fact by-products of binning, and not caused by pixelation or
masking as originally thought. We then derive a map-independent estimator that
encodes the effects of binning, applicable to beyond our present work. Using
this residual-free estimator, we show that small disk-like effects (as claimed
by Vielva et al.) can be detected only when a large sample of such maps are
averaged over. In other words, our estimator is noise-dominated for small disk
sizes at WMAP resolution. To confirm our suspicion, we apply our estimator to
the WMAP7 data to obtain a null result.Comment: 15 pages, 13 figure
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