94 research outputs found
Average luminosity distance in inhomogeneous universes
The paper studies the correction to the distance modulus induced by
inhomogeneities and averaged over all directions from a given observer. The
inhomogeneities are modeled as mass-compensated voids in random or regular
lattices within Swiss-cheese universes. Void radii below 300 Mpc are
considered, which are supported by current redshift surveys and limited by the
recently observed imprint such voids leave on CMB. The averaging over all
directions, performed by numerical ray tracing, is non-perturbative and
includes the supernovas inside the voids. Voids aligning along a certain
direction produce a cumulative gravitational lensing correction that increases
with their number. Such corrections are destroyed by the averaging over all
directions, even in non-randomized simple cubic void lattices. At low
redshifts, the average correction is not zero but decays with the peculiar
velocities and redshift. Its upper bound is provided by the maximal average
correction which assumes no random cancelations between different voids. It is
described well by a linear perturbation formula and, for the voids considered,
is 20% of the correction corresponding to the maximal peculiar velocity. The
average correction calculated in random and simple cubic void lattices is
severely damped below the predicted maximal one after a single void diameter.
That is traced to cancellations between the corrections from the fronts and
backs of different voids. All that implies that voids cannot imitate the effect
of dark energy unless they have radii and peculiar velocities much larger than
the currently observed. The results obtained allow one to readily predict the
redshift above which the direction-averaged fluctuation in the Hubble diagram
falls below a required precision and suggest a method to extract the background
Hubble constant from low redshift data without the need to correct for peculiar
velocities.Comment: 34 pages, 21 figures, matches the version accepted in JCA
Putting Themselves in the Picture: Using Reflective Diaries in the Teaching of Feminist Geography
The Evolution of Compact Binary Star Systems
We review the formation and evolution of compact binary stars consisting of
white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and
BHs are thought to be the primary astrophysical sources of gravitational waves
(GWs) within the frequency band of ground-based detectors, while compact
binaries of WDs are important sources of GWs at lower frequencies to be covered
by space interferometers (LISA). Major uncertainties in the current
understanding of properties of NSs and BHs most relevant to the GW studies are
discussed, including the treatment of the natal kicks which compact stellar
remnants acquire during the core collapse of massive stars and the common
envelope phase of binary evolution. We discuss the coalescence rates of binary
NSs and BHs and prospects for their detections, the formation and evolution of
binary WDs and their observational manifestations. Special attention is given
to AM CVn-stars -- compact binaries in which the Roche lobe is filled by
another WD or a low-mass partially degenerate helium-star, as these stars are
thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure
Tenure, gender and household structure
Starting with a review of recent literature on gender and housing, this paper goes on to develop a new theory of household structure, in which concepts of gender and generation play a key role. The utility of this theory is then demonstrated in the analysis of data from a survey of households in the City of Salford
Food, feeding and the material everyday geographies of infants: possibilities and potentials
This is an Accepted Manuscript of an article published by Taylor & Francis in Social and Cultural Geography on 16 Jun 2016, available online: http://dx.doi.org/10.1080/14649365.2016.1193889This paper forges an agenda for researching geographies of infants. Scholars have tended to overlook the everyday geographies of very young children. However, outside of geography, infancy is seen as a specifically dynamic period of life, and is subject to sustained research and policy intervention. In particular, early childhood is viewed as a key point in which to intervene to transform enduring, interconnected, societal, educational, and health based inequalities (Department of Health and Department for Education, 2013; US Early Head Start Act, 2007). Food and feeding are seen as critical both to the health of infants now and of the children and adults they become (NHS Start 4 Life, 2010). However, much policy and research under-theorises the importance of socio-spatial contexts and the subjectivity/agency of infants. There is, then, an urgent need for geographers to put infants onto the agenda, to inform and challenge these dominant accounts. Researching with infants necessitates not just critiquing modern, liberal notions of an autonomous subject/agent, but developing a new way of understanding subjectivity and agency. Drawing upon Lupton’s (2013) notion of infant-carer interembodiment, I suggest a way forward with reference to the material geographies of infant feeding
Working the Spaces of Neoliberal Subjectivity: Psychotherapeutic Technologies, Professionalisation and Counselling
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