76 research outputs found
Strange Horizons: Understanding Causal Barriers Beyond General Relativity
No full textThis thesis explores two avenues into understanding the physics of black holes and horizons beyond general relativity, via analogue models and Lorentz violating theories. Analogue spacetimes have wildly different dynamics to general relativity; this means time-independent black hole solutions have fewer symmetries, allowing the possibility of non-Killing horizons in stationary solutions. Surface gravity is one of the most important quantities characterizing black holes, with many physically distinct definitions. In the case of non-Killing horizons these different definitions of surface gravity are truly different quantities. This also has application to modified theories of gravity, where there is no reason to expect all horizons to be Killing horizons. In Lorentz violating theories, the situation becomes even stranger, as Killing horizons are at best low energy barriers, but for superluminal dispersion relations a true causal barrier, the universal horizon, may be present. Universal horizons are extremely interesting as they seem to be linked to the thermodynamic consistency of Lorentz-violating theories. Hence, we investigate the nature of these universal horizons via a ray tracing study, and delve into what happens near both the universal and Killing horizons. From this study we determine the surface gravity of universal horizons by the peeling properties of rays near the horizon. As the surface gravity is strongly linked to the properties of Hawking radiation, we investigate whether, and at what temperature these horizons radiate. Finally, we combine our investigations of universal horizons and analogue spacetimes, and ask why we have not seen a universal horizon in studies of analogue gravity. We examine some possibilities to include an aether distinct from the velocity flow characterizing analogue spacetimes, laying the groundwork for an analogue universal horizon
Analogue black holes in relativistic BECs: Mimicking Killing and universal horizons
No full textRelativistic Bose-Einstein condensates (rBECs) have recently become a
well-established system for analogue gravity. Indeed, while such
relativistic systems cannot be yet realized experimentally, they provide
an interesting framework for mimicking metrics for which no analogue is
yet available, thus paving the way for further theoretical and numerical
explorations. In this vein, we here discuss black holes in rBECs and
explore how their features relate to the bulk properties of the system.
We then propose the coupling of external fields to the rBEC as a way to
mimic nonmetric features. In particular, we use a Proca field to
simulate an aether field, as found in Einstein-aether or Ho.
rava-Lifshitz gravity. This allows us to mimic a universal horizon, the
causal barrier relevant for superluminal modes in these modified
gravitational theories
Vorticity in analog gravity
No full textIn the analog gravity framework, the acoustic disturbances in a moving
fluid can be described by an equation of motion identical to a
relativistic scalar massless field propagating in curved space-time.
This description is possible only when the fluid under consideration is
barotropic, inviscid, and irrotational. In this case, the propagation of
the perturbations is governed by an acoustic metric that depends
algebrically on the local speed of sound, density, and the background
flow velocity, the latter assumed to be vorticity-free. In this work we
provide a straightforward extension in order to go beyond the
irrotational constraint. Using a charged-relativistic and
nonrelativistic-Bose- Einstein condensate as a physical system, we show
that in the low-momentum limit and performing the eikonal approximation
we can derive a d'Alembertian equation of motion for the charged phonons
where the emergent acoustic metric depends on flow velocity in the
presence of vorticity
Ray tracing Einstein-Aether black holes: Universal versus Killing horizons
No full textViolating Lorentz invariance, and so implicitly permitting some form of super-luminal communication, necessarily alters the notion of a black hole. Nevertheless, in both Einstein-æther gravity and Hořava-Lifshitz gravity, there is still a causally disconnected region in black-hole solutions, now being bounded by a “universal horizon,” which traps excitations of arbitrarily high velocities. To better understand the nature of these black holes, and their universal horizons, we study ray trajectories in these spacetimes. We find evidence that Hawking radiation is associated with the universal horizon, while the “lingering” of ray trajectories near the Killing horizon hints at reprocessing there. In doing this we solve an apparent discrepancy between the surface gravity of the universal horizon and the associated temperature derived by the tunneling method. These results advance the understanding of these exotic horizons and provide hints for a full understanding of black-hole thermodynamics in Lorentz-violating theories
Applications of, and Extensions to, Selected Exact Solutions in General Relativity
No full textIn this thesis we consider several aspects of general relativity relating to
exact solutions of the Einstein equations. In the first part gravitational
plane waves in the Rosen form are investigated, and we develop a formalism
for writing down any arbitrary polarisation in this form. In addition to
this we have extended this algorithm to an arbitrary number of dimensions,
and have written down an explicit solution for a circularly polarized Rosen
wave. In the second part a particular, ultra-local limit along an arbitrary
timelike geodesic in any spacetime is constructed, in close analogy with the
well-known lightlike Penrose limit. This limit results in a Bianchi type I
spacetime. The properties of these spacetimes are examined in the context
of this limit, including the Einstein equations, stress-energy conservation and
Raychaudhuri equation. Furthermore the conditions for the Bianchi type I
spacetime to be diagonal are explicitly set forward, and the effect of the limit
on the matter content of a spacetime are examined
Identity in Britain : a cradle-to-grave atlas /
No full textSixty million people live in Britain. Imagine sixty million. Imagine a map of sixty million. What would that map look like and what story would it tell us about identity in Britain today? Bethan Thomas and Danny Dorling have brought together this outstanding atlas to provide us with a unique visual picture of identity and geography combined. Identity in Britain explores our changing identities as we progress from infancy to old age and tells the story of the myriad geographies of life in Britain. Features and benefits include: over 280 full colour, detailed maps analysis of the contemporary neighbourhood geographies of people in Britain at various life stages clear introduction and how-to-use guide making the atlas highly accessible for a wide range of users locational reference maps to aid interpretation of the maps on each page Accompanying web resources, including locational cartograms Unlike conventional atlases of human geography, it allows us to see a range of data on a single map; further it allows us to easily see what social mixing does not occur as well as what does. Never before have we had such a vivid geographical picture of identity in Britain today. The atlas is essential reading for those interested in contemporary human identity and the social geography of early twenty first century Britain. It is also an invaluable resource for researchers working in a wide range of statutory and voluntary organisations, policy makers, journalists, politicians, students and academics.Title from publisher's bibliographic system (viewed on 06 Apr 2023).Sixty million people live in Britain. Imagine sixty million. Imagine a map of sixty million. What would that map look like and what story would it tell us about identity in Britain today? Bethan Thomas and Danny Dorling have brought together this outstanding atlas to provide us with a unique visual picture of identity and geography combined. Identity in Britain explores our changing identities as we progress from infancy to old age and tells the story of the myriad geographies of life in Britain. Features and benefits include: over 280 full colour, detailed maps analysis of the contemporary neighbourhood geographies of people in Britain at various life stages clear introduction and how-to-use guide making the atlas highly accessible for a wide range of users locational reference maps to aid interpretation of the maps on each page Accompanying web resources, including locational cartograms Unlike conventional atlases of human geography, it allows us to see a range of data on a single map; further it allows us to easily see what social mixing does not occur as well as what does. Never before have we had such a vivid geographical picture of identity in Britain today. The atlas is essential reading for those interested in contemporary human identity and the social geography of early twenty first century Britain. It is also an invaluable resource for researchers working in a wide range of statutory and voluntary organisations, policy makers, journalists, politicians, students and academics
The effectiveness of Nurture Groups at supporting social and emotional outcomes: a systematic review
No full textBackground: Nurture Groups are a school-based attachment-focused intervention for young people with social and emotional, or mental health needs who may have experienced adversity. The aim of the current review was to systematically evaluate the evidence for Nurture group provision to improve social and emotional outcomes in children and young people across primary and secondary school settings.Methods: studies were included if they had been published in a peer-reviewed journal, participants were children and young people of school age, the study design was quantitative, evaluating the effectiveness of a Nurture group intervention and had at least one outcome measure related to social and emotional outcomes. Screening was conducted by the first author and a voluntary research assistant. Conflicts were resolved via discussion. A systematic search across six databases identified 14 studies for inclusion. Results: findings suggest that Nurture Group provision is, overall, effective at improving pupils’ social and emotional outcomes. However, improvements were not found consistently across both sections of the Boxall Profile, the primary tool used to measure progress within Nurture Groups. Differential effects were also found in relation to some pupil-level characteristics, namely age, baseline score, and gender. Discussion: the findings highlighted the need to conduct further research to examine such factors, as well as to explore the possible mechanisms for change underlying Nurture Group provision.<br/
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