1,721,379 research outputs found
Neutrino masses, dark energy and the gravitational lensing of pre-galactic H i
No full textWe study the constraints which the next generation of radio telescopes could place on the mass and number of neutrino species by studying the gravitational lensing of high-redshift 21-cm emission in combination with wide-angle surveys of galaxy lensing. We use simple characterizations of reionization history and of proposed telescope designs to forecast the constraints and detectability threshold for neutrinos. It is found that the degeneracy between neutrino parameters and dark energy parameters is significantly reduced by incorporating 21-cm lensing. The combination of galaxy and 21-cm lensing could constrain the sum of the neutrino masses to within ∼0.04 eV and the number of species to within ∼0.1. This is an improvement of a factor of 2.6 in mass and 1.4 in number over a galaxy lensing survey alone. This includes marginalizing over an 11-parameter cosmological model with a two-parameter model for the dark energy equation of state. If the dark energy equation of state is held fixed at w ≡ p/ρ = -1, the constraints improve to ∼0.025 eV and 0.04. These forecasted errors depend critically on the fraction of sky that can be surveyed in redshifted 21-cm emission (25 per cent is assumed here) and the redshift of reionization (z = 7 is assumed here). It is also found that neutrinos with masses too small to be detected in the data could none the less cause a significant bias in the measured dark energy equation of state. © 2009 The Author. Journal compilation. © 2009 RAS
Gravitational lensing of pregalactic 21 cm radiation
No full textLow-frequency radio observations of neutral hydrogen during and before the epoch of cosmic reionization will provide hundreds of quasi-independent source planes, each of precisely known redshift, if a resolution of ∼ 1 arcminutes or better can be attained. These planes can be used to reconstruct the projected mass distribution of foreground material. A wide-area survey of 21 cm lensing would provide very sensitive constraints on cosmological parameters, in particular on dark energy. These are up to 20 times tighter than the constraints obtainable from comparably sized, very deep surveys of galaxy lensing although the best constraints come from combining data of the two types. Any radio telescope capable of mapping the 21 cm brightness temperature with good frequency resolution (∼ 0,05 MHz) over a band of width ≳ 10 MHz should be able to make mass maps of high quality. If the reionization epoch is at z ≲ 9 very large amounts of cosmological information will be accessible. The planned Square Kilometer Array (SKA) should be capable of mapping the mass with a resolution of a few arcminutes depending on the reionization history of the Universe and how successfully foreground sources can be subtracted. The Low-Frequency Array (LOFAR) will be able to measure an accurate matter power spectrum if the same conditions are met
Gravitational microlensing by clustered machos
No full textIt has been proposed that the MACHOs in our Galaxy could be clumped in globular cluster-like associations or RAMBOs (robust associations of massive baryonic objects) (Moore & Silk). Here we investigate the effect such clustering has on the microlensing of stars in the Large Magellanic Cloud. We find that the lensing in a 1 square degree field could be dominated by just a few clusters. As a result the lensing properties vary widely depending on the position and velocity of those clusters which happen to lie between us and the LMC. Moreover, we find a large variance in timescale distributions that suggests that the small-number statistics could easily be dominated by events in the tails of the unclustered distribution (e.g., by long periods). We compare our results with the MACHO collaboration data and find that a "standard" halo made entirely of MACHOs is not strongly disfavored if the clusters have masses of 106 M. For less massive clusters such a halo is not as likely. For 104 M clusters the microlensing statistics are essentially unchanged from the unclustered case. It may be possible to detect very massive clusters from the distribution of events in timescale and space. We provide some examples of timescale distributions. © 1996. The American Astronomical Society. All rights reserved
On breaking cosmic degeneracy
No full textIt has been argued that the power spectrum of the anisotropies in the cosmic microwave background may be effectively degenerate, namely, that the observable spectrum does not determine a unique set of cosmological parameters. We describe the physical origin of this degeneracy and show that at small angular scales it is broken by gravitational lensing: effectively degenerate spectra become distinguishable at l ∼ 3000 because lensing causes their damping tails to fall at different rates with increasing l. This effect also helps in distinguishing nearly degenerate power spectra such as those of mixed dark matter models. Forthcoming interferometer experiments should provide the means of measuring otherwise degenerate parameters at the 5%-25% level. © 1998. The American Astronomical Society. All rights reserved
Cosmological information in the gravitational lensing of pregalactic H I
No full textWe study the constraints which the next generation of radio telescopes could place on the nature of dark energy, dark matter and inflation by studying the gravitational lensing of high redshift 21 m emission, and we compare with the constraints obtainable from wide-angle surveys of galaxy lensing. If the reionization epoch is effectively at z ∼ 8 or later, very large amounts of cosmological information will be accessible to telescopes like Square Kilometer Array (SKA) and Low Frequency Array (LOFAR). We use simple characterizations of reionization history and of proposed telescope designs to investigate how well the two-dimensional convergence power spectrum, the three-dimensional matter power spectrum, the evolution of the linear growth function and the standard cosmological parameters can be measured from radio data. The power spectra can be measured accurately over a wide range of wavenumbers at z ∼ 2, and the evolution in the cosmic energy density can be probed from z ∼ 0.5 to 7. This results in a characterization of the shape of the power spectra (i.e. of the nature of dark matter and of inflationary structure generation) which is potentially more precise than that obtained from galaxy lensing surveys. On the other hand, the dark energy parameters in their conventional parametrization (ΩΛ, wo, wa) are somewhat less well constrained by feasible 21 cm lensing surveys than by an all-sky galaxy lensing survey. This is because dark energy is felt primarily at relatively low redshifts in this model; 21 cm surveys would be more powerful than galaxy surveys for constraining models with 'early' dark energy. Overall, the best constraints come from combining surveys of the two types. This results in extremely tight constraints on dark matter and inflation, and improves constraints on dark energy, as judged by the standard figure of merit, by more than an order of magnitude over either survey alone. © 2009 RAS
High-resolution imaging of the cosmic mass distribution from gravitational lensing of pre-galactic H I
No full textLow-frequency radio observations of neutral hydrogen during and before the epoch of cosmic re-ionization will provide ∼1000 quasi-independent source planes, each of precisely known redshift, if a resolution of ∼1 arcmin or better can be attained. These planes can be used to reconstruct the projected mass distribution of foreground material. Structure in these source planes is linear and Gaussian at high redshift (30 < z < 300) but is non-linear and non-Gaussian during re-ionization. At both epochs, significant power is expected down to subarcsecond scales. We demonstrate that this structure can, in principle, be used to make mass images with a formal signal-to-noise ratio (S/N) per pixel exceeding 10, even for pixels as small as an arcsecond. With an ideal telescope, both resolution and S/N can exceed those of even the most optimistic idealized mass maps from galaxy lensing by more than an order of magnitude. Individual dark haloes similar in mass to that of the Milky Way could be imaged with high S/N out to z ∼ 10. Even with a much less ambitious telescope, a wide-area survey of 21-cm lensing would provide very sensitive constraints on cosmological parameters, in particular on dark energy. These are up to 20 times tighter than the constraints obtainable from comparably sized, very deep surveys of galaxy lensing, although the best constraints come from combining data of the two types. Any radio telescope capable of mapping the 21-cm brightness temperature with good frequency resolution (∼0.05 MHz) over a band of width ≳10 MHz should be able to make mass maps of high quality. The planned Square Kilometre Array may be able to map the mass with moderate S/N down to arcminute scales, depending on the re-ionization history of the universe and the ability to subtract foreground sources. © 2007 RAS
New constraints on macroscopic compact objects as dark matter candidates from gravitational lensing of type Ia supernovae
No full textWe use the distribution, and particularly the skewness, of high redshift type Ia supernovae brightnesses relative to the low redshift sample to constrain the density of macroscopic compact objects (MCOs) in the Universe. The supernova data favor dark matter made of microscopic particles (such as the lightest supersymmetric partner) over MCOs with masses between 10-2M and 1010M at 89% confidence. Future data will greatly improve this limit. Combined with other constraints, MCOs larger than one-tenth the mass of Earth (∼10-7M) can be eliminated as the sole constituent of dark matter. © 2007 The American Physical Society
Smooth particle lensing
No full textWe present a numerical technique to compute the gravitational lensing induced by simulated haloes. It relies on a 2D-Tree domain decomposition in the lens plane combined with a description of N-body particles as extended clouds with a non-singular density. This technique is made fully adaptive by the use of a density-dependent smoothing which allows one to probe the lensing properties of haloes from the densest regions in the centre or in substructures to the low-density regions in the outskirts. 'Smooth Particle Lensing' (SPL) has some promising features. First, the deflection potential, the deflection angles, the convergence and the shear are direct and separate end-products of the SPL calculation and can be computed at an arbitrary distribution of points on the lens plane. Secondly, this flexibility avoids the use of interpolation or a finite differentiation procedure on a grid, does not require padding the region with zeros and focuses the computing power on relevant regions. The SPL algorithm is tested by populating isothermal spheres and ellipsoids with particles and then comparing the lensing calculations to the classical fast Fourier transform based technique and analytic solutions. We assess issues related to the resolution of the lensing code and the limitations set by the simulations themselves. We conclude by discussing how SPL can be used to predict the impact of substructures on strong lensing and how it can be generalized to weak-lensing and cosmic shear simulations. © 2007 RAS
Work disability in an inception cohort of early rheumatoid arthritis (RA) receiving treat-to-target therapy
No full textScientific Poster ARA-P73Wechalekar MD, Lester S, Hill C, Shanahan E, Metcalf R, Shanahan M, Proudman
Relationship between plasma omega-3 fatty acids and clinical outcomes in recent onset rheumatoid arthritis
No full textScientific poster ARA-P54Proudman S, Cleland L, Lee A, Wechalekar M, Metcalf R, Sullivan T, Spargo L, James
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