1,300 research outputs found
Constraints on a New Post-General Relativity Cosmological Parameter
A new cosmological variable is introduced which characterizes the degree of
departure from Einstein's General Relativity (GR) with a cosmological constant.
The new parameter, \varpi, is the cosmological analog of \gamma, the
parametrized post-Newtonian variable which measures the amount of spacetime
curvature per unit mass. In the cosmological context, \varpi measures the
difference between the Newtonian and longitudinal potentials in response to the
same matter sources, as occurs in certain scalar-tensor theories of gravity.
Equivalently, \varpi measures the scalar shear fluctuation in a dark energy
component. In the context of a "vanilla" LCDM background cosmology, a non-zero
\varpi signals a departure from GR or a fluctuating cosmological constant.
Using a phenomenological model for the time evolution \varpi=\varpi_0
\rho_{DE}/\rho_{M} which depends on the ratio of energy density in the
cosmological constant to the matter density at each epoch, it is shown that the
observed cosmic microwave background (CMB) temperature anisotropies limit the
overall normalization constant to be -0.4 < \varpi_0 < 0.1 at the 95%
confidence level. Existing measurements of the cross-correlations of the CMB
with large-scale structure further limit \varpi_0 > -0.2 at the 95% CL. In the
future, integrated Sachs-Wolfe and weak lensing measurements can more tightly
constrain \varpi_0, providing a valuable clue to the nature of dark energy and
the validity of GR.Comment: 9 pages, 7 figures; added reference
Expression of Retroviral Transduced Human CD18 in Murine Cells: An In Vitro Model of Gene Therapy for Leukocyte Adhesion Deficiency
Overview summary In developing human gene therapy clinical protocols, it is helpful to have an animal model that mimics the human disease to be treated. A natural animal model for leukocyte adhesion deficiency (LAD) does not exist. Krauss et al. have developed a clever strategy for producing a mouse model for LAD gene therapy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/63397/1/hum.1991.2.3-221.pd
HST Images Flash Ionization of Old Ejecta by the 2011 Eruption of Recurrent Nova T Pyxidis
T Pyxidis is the only recurrent nova surrounded by knots of material ejected
in previous outbursts. Following the eruption that began on 2011 April 14.29,
we obtained seven epochs (from 4 to 383 days after eruption) of Hubble Space
Telescope narrowband Ha images of T Pyx . The flash of radiation from the nova
event had no effect on the ejecta until at least 55 days after the eruption
began. Photoionization of hydrogen located north and south of the central star
was seen 132 days after the beginning of the eruption. That hydrogen recombined
in the following 51 days, allowing us to determine a hydrogen atom density of
at least 7e5 cm^-3 - at least an order of magnitude denser than the previously
detected, unresolved [NII] knots surrounding T Pyx. Material to the northwest
and southeast was photoionized between 132 and 183 days after the eruption
began. 99 days later that hydrogen had recombined. Both then (282 days after
outburst) and 101 days later, we detected almost no trace of hydrogen emission
around T Pyx. There is a large reservoir of previously unseen, cold diffuse
hydrogen overlapping the previously detected, [NII] - emitting knots of T Pyx
ejecta. The mass of this newly detected hydrogen is probably an order of
magnitude larger than that of the [NII] knots. We also determine that there is
no significant reservoir of undetected ejecta from the outer boundaries of the
previously detected ejecta out to about twice that distance, near the plane of
the sky. The lack of distant ejecta is consistent with the Schaefer et al
(2010) scenario for T Pyx, in which the star underwent its first eruption
within five years of 1866 after many millennia of quiescence, followed by the
six observed recurrent nova eruptions since 1890. This lack of distant ejecta
is not consistent with scenarios in which T Pyx has been erupting continuously
as a recurrent nova for many centuries or millennia.Comment: 27 pages, 10 figures, submitted to the Astrophysical Journa
Yahtzee: An Anonymized Group Level Matching Procedure
Researchers often face the problem of needing to protect the privacy of subjects while also needing to integrate data that contains personal information from diverse data sources. The advent of computational social science and the enormous amount of data about people that is being collected makes protecting the privacy of research subjects ever more important. However, strict privacy procedures can hinder the process of joining diverse sources of data that contain information about specific individual behaviors. In this paper we present a procedure to keep information about specific individuals from being leaked\u27\u27 or shared in either direction between two sources of data without need of a trusted third party. To achieve this goal, we randomly assign individuals to anonymous groups before combining the anonymized information between the two sources of data. We refer to this method as the Yahtzee procedure, and show that it performs as predicted by theoretical analysis when we apply it to data from Facebook and public voter records
SPIRITS 16tn in NGC 3556: A heavily obscured and low-luminosity supernova at 8.8 Mpc
We present the discovery by the SPitzer InfraRed Intensive Transients Survey
(SPIRITS) of a likely supernova (SN) in NGC 3556 at only 8.8 Mpc, which,
despite its proximity, was not detected by optical searches. A luminous
infrared (IR) transient at mag (Vega), SPIRITS 16tn is
coincident with a dust lane in the inclined, star-forming disk of the host.
Using IR, optical, and radio observations, we attempt to determine the nature
of this event. We estimate 8 - 9 mag of extinction, placing it
among the three most highly obscured IR-discovered SNe to date. The [4.5] light
curve declined at a rate of 0.013 mag day, and the color
grew redder from 0.7 to 1.0 mag by 184.7 days post discovery.
Optical/IR spectroscopy shows a red continuum, but no clearly discernible
features, preventing a definitive spectroscopic classification. Deep radio
observations constrain the radio luminosity of SPIRITS 16tn to erg s Hz between 3 - 15 GHz, excluding many
varieties of radio core-collapse SNe. A type Ia SN is ruled out by the observed
red IR color, and lack of features normally attributed to Fe-peak elements in
the optical and IR spectra. SPIRITS 16tn was fainter at [4.5] than typical
stripped-envelope SNe by 1 mag. Comparison of the spectral energy
distribution to SNe II suggests SPIRITS 16tn was both highly obscured, and
intrinsically dim, possibly akin to the low-luminosity SN 2005cs. We infer the
presence of an IR dust echo powered by a peak luminosity of the transient of erg s erg s,
consistent with the observed range for SNe II. This discovery illustrates the
power of IR surveys to overcome the compounding effects of visible extinction
and optically sub-luminous events in completing the inventory of nearby SNe.Comment: 25 pages, 10 figures, submitted to Ap
Response of a catalytic reaction to periodic variation of the CO pressure: Increased CO_2 production and dynamic phase transition
We present a kinetic Monte Carlo study of the dynamical response of a
Ziff-Gulari-Barshad model for CO oxidation with CO desorption to periodic
variation of the CO presure. We use a square-wave periodic pressure variation
with parameters that can be tuned to enhance the catalytic activity. We produce
evidence that, below a critical value of the desorption rate, the driven system
undergoes a dynamic phase transition between a CO_2 productive phase and a
nonproductive one at a critical value of the period of the pressure
oscillation. At the dynamic phase transition the period-averged CO_2 production
rate is significantly increased and can be used as a dynamic order parameter.
We perform a finite-size scaling analysis that indicates the existence of
power-law singularities for the order parameter and its fluctuations, yielding
estimated critical exponent ratios and . These exponent ratios, together with theoretical symmetry
arguments and numerical data for the fourth-order cumulant associated with the
transition, give reasonable support for the hypothesis that the observed
nonequilibrium dynamic phase transition is in the same universality class as
the two-dimensional equilibrium Ising model.Comment: 18 pages, 10 figures, accepted in Physical Review
The Physical Characteristics of a CO2 Seeping Fault: the implications of fracture permeability for carbon capture and storage integrity
To ensure the effective long-term storage of CO2 in candidate geological storage sites, evaluation of potential leakage pathways to the surface should be undertaken. Here we use a series of natural CO2 seeps along a fault in South Africa to assess the controls on CO2 leakage to the surface. Geological mapping and detailed photogrammetry reveals extensive fracturing along the mapped fault trace. Measurements of gas flux and CO2 concentration across the fracture corridor give maximum soil gas measurements of 27% CO2 concentration and a flux of 191 g mâ2 dâ1. These measurements along with observations of gas bubbles in streams and travertine cones attest to CO2 migration to the surface. Permeability measurements on the host rock units show that the tillite should act as an impermeable seal to upward CO2 migration. The combined permeability and fracture mapping data indicate that fracture permeability creates the likely pathway for CO2 migration through the low permeability tillite to the surface. Heterogeneity in fracture connectivity and intensity at a range of scales will create local higher permeability pathways along the fracture corridor, although these may seal with time due to fluid-rock interaction. The results have implications for the assessment and choice of geological CO2 storage sites, particularly in the assessment of sub-seismic fracture networks
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