All-sky signals from recombination to reionization with the SKA

Cosmic evolution in the hydrogen content of the Universe through recombination and up to the end of reionization is expected to be revealed as subtle spectral features in the uniform extragalactic cosmic radio background. The redshift evolution in the excitation temperature of the 21-cm spin flip transition of neutral hydrogen appears as redshifted emission and absorption against the cosmic microwave background. The precise signature of the spectral trace from cosmic dawn and the epoch of reionization are dependent on the spectral radiance, abundance and distribution of the first bound systems of stars and early galaxies, which govern the evolution in the spin-flip level populations. Redshifted 21 cm from these epochs when the spin temperature deviates from the temperature of the ambient relic cosmic microwave background results in an all-sky spectral structure in the 40-200 MHz range, almost wholly within the band of SKA-Low. Another spectral structure from gas evolution is redshifted recombination lines from epoch of recombination of hydrogen and helium; the weak all-sky spectral structure arising from this event is best detected at the upper end of the 350-3050 MHz band of SKA-mid. Total power spectra of SKA interferometer elements form the measurement set for these faint signals from recombination and reionization; the inter-element interferometer visibilities form a calibration set. The challenge is in precision polarimetric calibration of the element spectral response and solving for additives and unwanted confusing leakages of sky angular structure modes into spectral modes. Herein we discuss observing methods and design requirements that make possible these all-sky SKA measurements of the cosmic evolution of hydrogen.Comment: Accepted for publication in the SKA Science Book 'Advancing Astrophysics with the Square Kilometre Array', to appear in 201

The effect of temperature-dependent solubility on the onset of thermosolutal convection in a horizontal porous layer

We consider the onset of thermosolutal (double-diffusive) convection of a binary fluid in a horizontal porous layer subject to fixed temperatures and chemical equilibrium on the bounding surfaces, in the case when the solubility of the dissolved component depends on temperature. We use a linear stability analysis to investigate how the dissolution or precipitation of this component affects the onset of convection and the selection of an unstable wavenumber; we extend this analysis using a Galerkin method to predict the structure of the initial bifurcation and compare our analytical results with numerical integration of the full nonlinear equations. We find that the reactive term may be stabilizing or destabilizing, with subtle effects particularly when the thermal gradient is destabilizing but the solutal gradient is stabilizing. The preferred spatial wavelength of convective cells at onset may also be substantially increased or reduced, and strongly reactive systems tend to prefer direct to subcritical bifurcation. These results have implications for geothermal-reservoir management and ore prospecting

Demonstration of an inductively coupled ring trap for cold atoms

We report the first demonstration of an inductively coupled magnetic ring trap for cold atoms. A uniform, ac magnetic field is used to induce current in a copper ring, which creates an opposing magnetic field that is time-averaged to produce a smooth cylindrically symmetric ring trap of radius 5 mm. We use a laser-cooled atomic sample to characterize the loading efficiency and adiabaticity of the magnetic potential, achieving a vacuum-limited lifetime in the trap. This technique is suitable for creating scalable toroidal waveguides for applications in matter-wave interferometry, offering long interaction times and large enclosed areas

Cosmic Reionization and the 21-cm signal: Comparison between an analytical model and a simulation

We measure several properties of the reionization process and the corresponding low-frequency 21-cm signal associated with the neutral hydrogen distribution, using a large volume, high resolution simulation of cosmic reionization. The brightness temperature of the 21-cm signal is derived by post-processing this numerical simulation with a semi-analytical prescription. Our study extends to high redshifts (z ~ 25) where, in addition to collisional coupling, our post-processed simulations take into account the inhomogeneities in the heating of the neutral gas by X-rays and the effect of an inhomogeneous Lya radiation field. Unlike the well-studied case where spin temperature is assumed to be significantly greater than the temperature of the cosmic microwave background due to uniform heating of the gas by X-rays, spatial fluctuations in both the Lya radiation field and X-ray intensity impact predictions related to the brightness temperature at z > 10, during the early stages of reionization and gas heating. The statistics of the 21-cm signal from our simulation are then compared to existing analytical models in the literature and we find that these analytical models provide a reasonably accurate description of the 21-cm power spectrum at z < 10. Such an agreement is useful since analytical models are better suited to quickly explore the full astrophysical and cosmological parameter space relevant for future 21-cm surveys. We find, nevertheless, non-negligible differences that can be attributed to differences in the inhomogeneous X-ray heating and Lya coupling at z > 10 and, with upcoming interferometric data, these differences in return can provide a way to better understand the astrophysical processes during reionization.Comment: Major paper revision to match version accepted for publication in ApJ. Simulation now fully includes fluctuations in the X-ray heating and the Lya radiation field. 18 pages, 13 figure

Social tourism and well-being in later life

Social tourism research has tended to concentrate on its benefits for young families and people with disabilities; few studies have explored its wellbeing value for economically disadvantaged older people. Based on participant-driven interviews during a UK social tourism trip, this paper informs understandings of social tourism as a set of experiences and illuminates the links between wellbeing and social tourism opportunities for older people. It reveals that social tourism presents older individuals with occasions for escape, respite, companionship, and reminiscence and for renegotiation of self-identity following spousal bereavement, but that these trips can be anxiously anticipated. The study suggests a research agenda is needed which explores the physiological, psychological, social and spiritual impacts of social tourism on older people’s wellbeing

Instability of the salinity profile during the evaporation of saline groundwater

We investigate salt transport during the evaporation and upflow of saline groundwater. We describe a model in which a sharp evaporation-precipitation front separates regions of soil saturated with an air-vapour mixture and with saline water. We then consider two idealised problems. We first investigate equilibrium configurations of the fresh-water system when the depth of the soil layer is finite, obtaining results for the location of the front and for the upflow of water induced by the evaporation. Motivated by these results, we develop a solution for a propagating front in a soil layer of infinite depth, and we investigate the gravitational stability of the salinity profile which develops below the front, obtaining marginal linear stability conditions in terms of a Rayleigh number and a dimensionless salt saturation parameter. Applying our findings to realistic parameter regimes, we predict that salt fingering is unlikely to occur in low-permeability soils, but is likely in high-permeability (sandy) soils under conditions of relatively low evaporative upflow