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Search Results: 1 - 10 of 339981 matches for " David J. Bacon "
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Impact of local structure on the cosmic radio dipole
Matthias Rubart,David Bacon,Dominik J. Schwarz
Physics , 2014, DOI: 10.1051/0004-6361/201423583
Abstract: We investigate the contribution that a local over- or under-density can have on linear cosmic dipole estimations. We focus here on radio surveys, such as the NRAO VLA Sky Survey (NVSS), and forthcoming surveys such as those with the LOw Frequency ARray (LOFAR), the Australian Square Kilometre Array Pathfinder (ASKAP) and the Square Kilometre Array (SKA). The NVSS has already been used to estimate the cosmic radio dipole; it was shown recently that this radio dipole amplitude is larger than expected from a purely kinematic effect, assuming the velocity inferred from the dipole of the cosmic microwave background. We show here that a significant contribution to this excess could come from a local void or similar structure. In contrast to the kinetic contribution to the radio dipole, the structure dipole depends on the flux threshold of the survey and the wave band, which opens the chance to distinguish the two contributions.
Weak lensing predictions for modified gravities at non-linear scales
Emma Beynon,David J. Bacon,Kazuya Koyama
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.16117.x
Abstract: We present a set of predictions for weak lensing correlation functions in the context of modified gravity models, including a prescription for the impact of the nonlinear power spectrum regime in these models. We consider the DGP and f(R) models, together with dark energy models with the same expansion history. We use the requirement that gravity is close to GR on small scales to estimate the non-linear power for these models. We then calculate weak lensing statistics, showing their behaviour as a function of scale and redshift, and present predictions for measurement accuracy with future lensing surveys, taking into account cosmic variance and galaxy shape noise. We demonstrate the improved discriminatory power of weak lensing for testing modified gravities once the nonlinear power spectrum contribution has been included. We also examine the ability of future lensing surveys to constrain a parameterisation of the non-linear power spectrum, including sensitivity to the growth factor.
Image Simulation with Shapelets
Richard Massey,Alexandre Refregier,Christopher J. Conselice,David J. Bacon
Physics , 2003, DOI: 10.1111/j.1365-2966.2004.07353.x
Abstract: We present a method to simulate deep sky images, including realistic galaxy morphologies and telescope characteristics. To achieve a wide diversity of simulated galaxy morphologies, we first use the shapelets formalism to parametrize the shapes of all objects in the Hubble Deep Fields. We measure this distribution of real galaxy morphologies in shapelet parameter space, then resample it to generate a new population of objects. These simulated galaxies can contain spiral arms, bars, discs, arbitrary radial profiles and even dust lanes or knots. To create a final image, we also model observational effects, including noise, pixellisation, astrometric distortions and a Point-Spread Function. We demonstrate that they are realistic by showing that simulated and real data have consistent distributions of morphology diagnostics: including galaxy size, ellipticity, concentration and asymmetry statistics. Sample images are made available on the world wide web. These simulations are useful to develop and calibrate precision image analysis techniques for photometry, astrometry, and shape measurement. They can also be used to assess the sensitivity of future telescopes and surveys for applications such as supernova searches, microlensing, proper motions, and weak gravitational lensing.
Weak Lensing Measurements in Simulations of Radio Images
Prina Patel,Filipe B. Abdalla,David J. Bacon,Barnaby Rowe,Oleg Smirnov,Rob J. Beswick
Physics , 2013, DOI: 10.1093/mnras/stu1588
Abstract: We present a study of weak lensing shear measurements for simulated galaxy images at radio wavelengths. We construct a simulation pipeline into which we can input galaxy images of known ellipticity, and with which we then simulate observations with eMERLIN and the international LOFAR array. The simulations include the effects of the CLEAN algorithm, uv sampling, observing angle, and visibility noise, and produce realistic restored images of the galaxies. We apply a shapelet-based shear measurement method to these images and test our ability to recover the true source ellipticities. We model and deconvolve the effective PSF, and find suitable parameters for CLEAN and shapelet decomposition of galaxies. We demonstrate that ellipticities can be measured faithfully in these radio simulations, with no evidence of an additive bias and a modest (10%) multiplicative bias on the ellipticity measurements. Our simulation pipeline can be used to test shear measurement procedures and systematics for the next generation of radio telescopes.
Genetic diversity and population structure of genes encoding vaccine candidate antigens of Plasmodium vivax
Stella M Chenet, Lorena L Tapia, Ananias A Escalante, Salomon Durand, Carmen Lucas, David J Bacon
Malaria Journal , 2012, DOI: 10.1186/1475-2875-11-68
Abstract: Genetic variability was assessed in important polymorphic regions of various vaccine candidate antigens in P. vivax among 106 isolates from the Amazon Region of Loreto, Peru. In addition, genetic diversity determined in Peruvian isolates was compared to population studies from various geographical locations worldwide.The structured diversity found in P. vivax populations did not show a geographic pattern and haplotypes from all gene candidates were distributed worldwide. In addition, evidence of balancing selection was found in polymorphic regions of the trap, dbp and ama-1 genes.It is important to have a good representation of the haplotypes circulating worldwide when implementing a vaccine, regardless of the geographic region of deployment since selective pressure plays an important role in structuring antigen diversity.Malaria is one of the major global public health problems that affect most tropical regions of the world. Even though Plasmodium falciparum is the most virulent, it is estimated that Plasmodium vivax produces around 80 to 300 million clinical cases per year [1]. Furthermore, there have been several reports of severe P. vivax malaria cases in the last few years [2-4]. In 2008, 560,221 malaria cases were reported in the Americas [5]; 74.2% of them caused by P. vivax and 25.7% by P. falciparum [1]. About 90% of these malaria cases originated in the Amazon basin shared by Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Venezuela, Suriname and Peru [5], whereas the other 10% was contributed by non-Amazon regions.Developing a vaccine for P. vivax represents a major challenge especially considering the lack of in vitro cultures. Thus, current efforts focus on orthologs of P. falciparum. Over the past four decades, experiments performed in animals and human subjects have led to the development of several Plasmodium vaccine candidates. Antigenic surface proteins such as the Circumsporozoite protein (CSP), Thrombospondin related anonymous protein
Genetic diversity of vaccine candidate antigens in Plasmodium falciparum isolates from the Amazon basin of Peru
Stella M Chenet, OraLee H Branch, Ananias A Escalante, Carmen M Lucas, David J Bacon
Malaria Journal , 2008, DOI: 10.1186/1475-2875-7-93
Abstract: DNA sequencing analysis was completed on 139 isolates of P. falciparum collected from endemic areas of the Amazon basin in Loreto, Peru from years 1998 to 2006. Genetic diversity was determined in immunological important regions in circumsporozoite protein (CSP), merozoite surface protein-1 (MSP-1), apical membrane antigen-1 (AMA-1), liver stage antigen-1 (LSA-1) and thrombospondin-related anonymous protein (TRAP). Alleles identified by DNA sequencing were aligned with the vaccine strain 3D7 and DNA polymorphism analysis and FST study-year pairwise comparisons were done using the DnaSP software. Multilocus analysis (MLA) was performed and average of expected heterozygosity was calculated for each loci and haplotype over time.Three different alleles for CSP, seven for MSP-1 Block 2, one for MSP-1 Block 17, three for AMA-1 and for LSA-1 each and one for TRAP were identified. There were 24 different haplotypes in 125 infections with complete locus typing for each gene.Characterization of the genetic diversity in Plasmodium isolates from the Amazon Region of Peru showed that P. falciparum T and B cell epitopes in these antigens have polymorphisms more similar to India than to Africa. These findings are helpful in the formulation of a vaccine considering restricted repertoire populations.Vaccine design for Plasmodium falciparum is hindered by polymorphisms in certain vaccine candidate loci [1,2]. Highly polymorphic regions have been observed in P. falciparum antigenic surface proteins, such as the circumsporozoite protein (CSP), the merozoite surface protein 1 (MSP-1), the apical membrane antigen 1 (AMA-1), the liver stage antigen (LSA-1) and the thrombospondin-related anonymous protein (TRAP) [3].One of the best characterized and widely accepted by many as a potential vaccine candidate for P. falciparum is CSP [4,5]. CSP is a 58-kDa protein and is the major antigen on the surface of malaria sporozoites [6,7]. The CSP protein can be subdivided into two non-repetitive reg
Testing Einstein Gravity with Cosmic Growth and Expansion
Gong-Bo Zhao,Hong Li,Eric V. Linder,Kazuya Koyama,David J. Bacon,Xinmin Zhang
Physics , 2011, DOI: 10.1103/PhysRevD.85.123546
Abstract: We test Einstein gravity using cosmological observations of both expansion and structure growth, including the latest data from supernovae (Union2.1), CMB (WMAP7), weak lensing (CFHTLS) and peculiar velocity of galaxies (WiggleZ). We fit modified gravity parameters of the generalized Poisson equations simultaneously with the effective equation of state for the background evolution, exploring the covariances and model dependence. The results show that general relativity is a good fit to the combined data. Using a Pad{\'e} approximant form for the gravity deviations accurately captures the time and scale dependence for theories like $f(R)$ and DGP gravity, and weights high and low redshift probes fairly. For current observations, cosmic growth and expansion can be fit simultaneously with little degradation in accuracy, while removing the possibility of bias from holding one aspect fixed.
Weak lensing predictions for coupled dark energy cosmologies at non-linear scales
Emma Beynon,Marco Baldi,David J. Bacon,Kazuya Koyama,Cristiano Sabiu
Physics , 2011, DOI: 10.1111/j.1365-2966.2012.20864.x
Abstract: We present non-linear weak lensing predictions for coupled dark energy models using the CoDECS simulations. We calculate the shear correlation function and error covariance expected for these models, for forthcoming ground-based (such as DES) and space-based (Euclid) weak lensing surveys. We obtain predictions for the discriminatory power of a ground-based survey similar to DES and a space-based survey such as Euclid in distinguishing between $\Lambda$CDM and coupled dark energy models; we show that using the non-linear lensing signal we could discriminate between $\Lambda$CDM and exponential constant coupling models with $\beta_0\geq0.1$ at $4\sigma$ confidence level with a DES-like survey, and $\beta_0\geq0.05$ at $5\sigma$ confidence level with Euclid. We also demonstrate that estimating the coupled dark energy models' non-linear power spectrum, using the $\Lambda$CDM Halofit fitting formula, results in biases in the shear correlation function that exceed the survey errors.
Cosmology with Doppler Lensing
David J. Bacon,Sambatra Andrianomena,Chris Clarkson,Krzysztof Bolejko,Roy Maartens
Physics , 2014, DOI: 10.1093/mnras/stu1270
Abstract: Doppler lensing is the apparent change in object size and magnitude due to peculiar velocities. Objects falling into an overdensity appear larger on its near side, and smaller on its far side, than typical objects at the same redshifts. This effect dominates over the usual gravitational lensing magnification at low redshift. Doppler lensing is a promising new probe of cosmology, and we explore in detail how to utilize the effect with forthcoming surveys. We present cosmological simulations of the Doppler and gravitational lensing effects based on the Millennium simulation. We show that Doppler lensing can be detected around stacked voids or unvirialised over-densities. New power spectra and correlation functions are proposed which are designed to be sensitive to Doppler lensing. We consider the impact of gravitational lensing and intrinsic size correlations on these quantities. We compute the correlation functions and forecast the errors for realistic forthcoming surveys, providing predictions for constraints on cosmological parameters. Finally, we demonstrate how we can make 3-D potential maps of large volumes of the Universe using Doppler lensing.
Density mapping with weak lensing and phase information
Rafal M. Szepietowski,David J. Bacon,Joerg P. Dietrich,Michael Busha,Risa Wechsler,Peter Melchior
Physics , 2013, DOI: 10.1093/mnras/stu380
Abstract: The available probes of the large scale structure in the Universe have distinct properties: galaxies are a high resolution but biased tracer of mass, while weak lensing avoids such biases but, due to low signal-to-noise ratio, has poor resolution. We investigate reconstructing the projected density field using the complementarity of weak lensing and galaxy positions. We propose a maximum-probability reconstruction of the 2D lensing convergence with a likelihood term for shear data and a prior on the Fourier phases constructed from the galaxy positions. By considering only the phases of the galaxy field, we evade the unknown value of the bias and allow it to be calibrated by lensing on a mode-by-mode basis. By applying this method to a realistic simulated galaxy shear catalogue, we find that a weak prior on phases provides a good quality reconstruction down to scales beyond l=1000, far into the noise domain of the lensing signal alone.
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