10,609 research outputs found
Unified Brane Gravity: Cosmological Dark Matter from Scale Dependent Newton Constant
We analyze, within the framework of unified brane gravity, the weak-field
perturbations caused by the presence of matter on a 3-brane. Although deviating
from the Randall-Sundrum approach, the masslessness of the graviton is still
preserved. In particular, the four-dimensional Newton force law is recovered,
but serendipitously, the corresponding Newton constant is shown to be
necessarily lower than the one which governs FRW cosmology. This has the
potential to puzzle out cosmological dark matter. A subsequent conjecture
concerning galactic dark matter follows.Comment: 6 pages, to be published in Phys. Rev.
Observational biases in Lagrangian reconstructions of cosmic velocity fields
Lagrangian reconstruction of large-scale peculiar velocity fields can be
strongly affected by observational biases. We develop a thorough analysis of
these systematic effects by relying on specially selected mock catalogues. For
the purpose of this paper, we use the MAK reconstruction method, although any
other Lagrangian reconstruction method should be sensitive to the same
problems. We extensively study the uncertainty in the mass-to-light assignment
due to luminosity incompleteness, and the poorly-determined relation between
mass and luminosity. The impact of redshift distortion corrections is analyzed
in the context of MAK and we check the importance of edge and finite-volume
effects on the reconstructed velocities. Using three mock catalogues with
different average densities, we also study the effect of cosmic variance. In
particular, one of them presents the same global features as found in
observational catalogues that extend to 80 Mpc/h scales. We give recipes,
checked using the aforementioned mock catalogues, to handle these particular
observational effects, after having introduced them into the mock catalogues so
as to quantitatively mimic the most densely sampled currently available galaxy
catalogue of the nearby universe. Once biases have been taken care of, the
typical resulting error in reconstructed velocities is typically about a
quarter of the overall velocity dispersion, and without significant bias. We
finally model our reconstruction errors to propose an improved Bayesian
approach to measure Omega_m in an unbiased way by comparing the reconstructed
velocities to the measured ones in distance space, even though they may be
plagued by large errors. We show that, in the context of observational data, a
nearly unbiased estimator of Omega_m may be built using MAK reconstruction.Comment: 29 pages, 21 figures, 6 tables, Accepted by MNRAS on 2007 October 2.
Received 2007 September 30; in original form 2007 July 2
Fruit flies and intellectual disability
Mental retardation-known more commonly nowadays as intellectual disability-is a severe neurological condition affecting up to 3% of the general population. As a result of the analysis of familial cases and recent advances in clinical genetic testing, great strides have been made in our understanding of the genetic etiologies of mental retardation. Nonetheless, no treatment is currently clinically available to patients suffering from intellectual disability. Several animal models have been used in the study of memory and cognition. Established paradigms in Drosophila have recently captured cognitive defects in fly mutants for orthologs of genes involved in human intellectual disability. We review here three protocols designed to understand the molecular genetic basis of learning and memory in Drosophila and the genes identified so far with relation to mental retardation. In addition, we explore the mental retardation genes for which evidence of neuronal dysfunction other than memory has been established in Drosophila. Finally, we summarize the findings in Drosophila for mental retardation genes for which no neuronal information is yet available. All in all, this review illustrates the impressive overlap between genes identified in human mental retardation and genes involved in physiological learning and memory
Nonlocal Gravity: Modified Poisson's Equation
The recent nonlocal generalization of Einstein's theory of gravitation
reduces in the Newtonian regime to a nonlocal and nonlinear modification of
Poisson's equation of Newtonian gravity. The nonlocally modified Poisson
equation implies that nonlocality can simulate dark matter. Observational data
regarding dark matter provide limited information about the functional form of
the reciprocal kernel, from which the original nonlocal kernel of the theory
must be determined. We study this inverse problem of nonlocal gravity in the
linear domain, where the applicability of the Fourier transform method is
critically examined and the conditions for the existence of the nonlocal kernel
are discussed. This approach is illustrated via simple explicit examples for
which the kernels are numerically evaluated. We then turn to a general
discussion of the modified Poisson equation and present a formal solution of
this equation via a successive approximation scheme. The treatment is
specialized to the gravitational potential of a point mass, where in the linear
regime we recover the Tohline-Kuhn approach to modified gravity.Comment: 27 pages, 4 figures; v2: minor improvements, accepted for publication
in J. Math. Phy
The Correlation Between Galaxy HI Linewidths and K' Luminosities
The relationship between galaxy luminosities and rotation rates is studied
with total luminosities in the K' band. Extinction problems are essentially
eliminated at this band centered at 2.1 micron. A template luminosity-linewidth
relation is derived based on 65 galaxies drawn from two magnitude-limited
cluster samples. The zero-point is determined using 4 galaxies with accurately
known distances. The calibration is applied to give the distance to the Pisces
Cluster (60 Mpc) at a redshift in the CMB frame of 4771 km/s. The resultant
value of the Hubble Constant is 81 km/s/Mpc. The largest sources of uncertainty
arises from the small number of zero-point calibrators at this time at K' and
present application to only one cluster.Comment: 13 pages including 5 figures and 2 tables. Accepted for publication
in Astrophysical Journa
Light Propagation in Inhomogeneous Universes. IV. Strong Lensing and Environmental Effects
We study the gravitational lensing of high-redshift sources in a LCDM
universe. We have performed a series of ray-tracing experiments, and selected a
subsample of cases of strong lensing (multiple images, arcs, and Einstein
rings). For each case, we identify a massive galaxy that is primarily
responsible for lensing, and studied how the various density inhomogeneities
along the line of sight (other galaxies, background matter) affect the
properties of the image. The matter located near the lensing galaxy, and
physically associated with it, has a small effect. The background matter
increases the magnification by a few percents at most, while nearby galaxies
can increase it by up to about 10 percent. The effect on the image separation
is even smaller. The only significant effect results from the random alignment
of physically unassociated galaxies, which can increase the magnification by
factors of several, create additional images, and turn arcs into rings. We
conclude that the effect of environment on strong lensing in negligible in
general, and might be important only in rare cases. We show that our conclusion
does not depend on the radial density profile of the galaxies responsible for
lensing.Comment: 23 pages, 7 figures (one in color). Accepted for publication in The
Astrophysical Journal. Minor typos correcte
The spacetime structure of MOND with Tully-Fisher relation and Lorentz invariance violation
It is believed that the modification of Newtonian dynamics (MOND) is possible
alternate for dark matter hypothesis. Although Bekenstein's TeVeS supplies a
relativistic version of MOND, one may still wish a more concise covariant
formulism of MOND. In this paper, within covariant geometrical framwork, we
present another version of MOND. We show the spacetime structure of MOND with
properties of Tully-Fisher relation and Lorentz invariance violation.Comment: 6 pages. arXiv admin note: substantial text overlap with
arXiv:1111.1383 and arXiv:1108.344
VV124 (UGC4879): A new transitional dwarf galaxy in the periphery of the Local Group
We present the first resolved-star photometry of VV124 (UGC4879) and find
that this is the most isolated dwarf galaxy in the periphery of the Local
Group. Based on imaging and spectroscopic follow up observations with the 6m
BTA telescope, we resolve VV124 into 1560 stars down to the limiting magnitude
levels of V~25.6 and I~23.9. The young blue stellar populations and emission
gas are found near the core, but noticeably displaced from the center of the
galaxy as traced by dominant evolved red stars. The mean radial velocity
derived from the spectra of two Blue Supergiant stars, an HII region and
unresolved continuum sources is -80+/-10 km/s. The evolved ``red tangle''
stellar populations, which contains the red giant branch (RGB), are identified
at large galactocentric radii. We use the I-band luminosity function to
determine the distance based on the Tip of RGB method, 1.1+/-0.1 Mpc. This is
~10 times closer than the values usually assumed in the literature, and we
provide revised distance dependent parameters. From the mean (V-I) color of the
RGB, we estimate the mean metallicity as [Fe/H]~-1.37 dex. Despite of its
isolated location, the properties of VV124 are clearly not those of a galaxy in
formation, but rather similar to a transitional dIrr/dSph type.Comment: 4 pages, submitted to MNRAS Letter
Introducing stable microgrooves in to fluid-leaving surface of plastic compressed collagen by embossing
Micropatterning of polymers is a technique used in tissue engineering to create additional features on the surface usually to manipulate cell behaviour. However, methods used for fabrication of such features on the surface of synthetic polymers are not always suitable for natural polymers. Here we describe a method of embossing developed for plastic compressed collagen constructs. Plastic compression (PC) of collagen is a technique that allows cell-independent fabrication of dense, tissue-like collagen constructs without compromising viability of resident cells (1). Method of PC creates potential anisotropy of the opposite surfaces â stiffer fluid leaving (closest to the blotting elements) and more elastic non-fluid leaving surface. We hypothesized that embossing into these surfaces will give more stable features on the fluid-leaving surface (FLS) as opposed to non fluid-leaving surface (NFLS). Objectives were to develop methods of embossing onto both surfaces, assess these features and to determine stability of the embossed pattern over time with and without cells. Slow-dissolving phosphate-based glass fibers were used as the embossing template in this study as fiber diameter and spacing is easily controlled
Correlates of sport participation in adults with long-standing illness or disability
Background Little is known about why people with a long-standing illness/disability are less likely to participate in sport than others. This study aimed to identify for the first time sport participation levels and their correlates among Northern Ireland (NI) adults who report a long-standing illness/disability.
Method Using data collected in the Continuous Household Survey, an annual survey of a random sample of the NI population, during 2007â2011, we examined responses for the total sample, those with a long-term illness/disability and those with no long-term health issues. We conducted univariate binary regression analysis for the whole sample and for those with a long-standing illness or disability, using sport participation as the dependent variable, and then carried significant variables into a multivariate analysis.
Results The sample included 13â
683 adults; 3550 (26%) reported a long-term illness or disability. Multivariate analysis showed that, for the total sample and for those with a long-standing illness or disability, sport participation correlated positively with being male, aged <56â
years, having a household car/van, health being âfairly goodâ/âgoodâ in the previous year, doing work and living in an urban location. Also, for those with a long-standing illness or disability, being single and less socioeconomically deprived correlated positively with sport participation.
Conclusions The findings suggest that more focused efforts may promote sport participation for people with a long-standing illness or disability who are female, older, not working, living rurally, married/cohabiting, socioeconomically deprived and report having had poor health in the past year. Our findings should inform public health policy and help in developing initiatives to support sport participation and reduce health inequalities
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