354 research outputs found
Massive gravity and the suppression of anisotropies and gravitational waves in a matter-dominated contracting universe
We consider a modified gravity model with a massive graviton, but which
nevertheless only propagates two gravitational degrees of freedom and which is
free of ghosts. We show that non-singular bouncing cosmological background
solutions can be generated. In addition, the mass term for the graviton
prevents anisotropies from blowing up in the contracting phase and also
suppresses the spectrum of gravitational waves compared to that of the scalar
cosmological perturbations. This addresses two of the main problems of the
matter bounce scenario.Comment: 20 pages, 2 figure
Automatic hyphenation of afrikaans
A Dissertation Submitted to the Faculty of Science
University of the Witwatersrand, Johannesburg
for the Degree of M aster of Science
November 198
Stringy black-hole gas in -corrected dilaton gravity
We discuss the properties of the gas of primordial `stringy' black holes
possibly formed in the high-curvature phase preceding the bouncing transition
to the phase of standard cosmological evolution. We show that the regime
dominated by such a string-hole gas can be consistently described by explicit
solutions of the string effective action including first-order
corrections. We present a phase space analysis of the stability of such
solutions comparing the results obtained from different actions and including
the possibility of -symmetric configurations.Comment: 17 pages, 2 figures. Replaced to match published versio
Cosmology at the top of the α′ tower
The cosmology of the fully -corrected duality-invariant action for the Neveu-Schwarz sector of string theory is revisited, with special emphasis on its coupling to matter sources. The role of the duality covariant pressure and dilatonic charge of the matter sector is explored in various contexts, from the low-curvature regime to non-perturbative solutions in . We comment on how an infinite tower of corrections allows for fixed-dilaton de Sitter solutions, even in vacuum. We further investigate the necessary conditions for accelerated expansion in the Einstein frame, as well as for non-singular bounces that could resolve the big bang singularity. In particular, explicit examples are constructed, which show that the tower of corrections may support an Einstein-frame non-singular cosmological bouncing background, even when the matter sector respects the null energy condition
Unconventional order-disorder phase transition in improper ferroelectric hexagonal manganites
The improper ferroelectricity in YMnO and other related multiferroic
hexagonal manganites are known to cause topologically protected ferroelectric
domains that give rise to rich and diverse physical phenomena. The local
structure and structural coherence across the ferroelectric transition,
however, were previously not well understood. Here we reveal the evolution of
the local structure with temperature in YMnO using neutron total scattering
techniques, and interpret them with the help of first-principles calculations.
The results show that, at room temperature, the local and average structures
are consistent with the established ferroelectric symmetry. On
heating, both local and average structural analyses show striking anomalies
from K up to the Curie temperature consistent with increasing
fluctuations of the order parameter angle. These fluctuations result in an
unusual local symmetry lowering into a \textit{continuum of structures} on
heating. This local symmetry breaking persists into the high-symmetry non-polar
phase, constituting an unconventional type of order-disorder transition.Comment: 10 pages, 5 figure
Securing Bring-Your-Own-Device (BYOD) programming exams
Traditional pen and paper exams are inadequate for modern university
programming courses as they are misaligned with pedagogies and learning
objectives that target practical coding ability. Unfortunately, many
institutions lack the resources or space to be able to run assessments in
dedicated computer labs. This has motivated the development of
bring-your-own-device (BYOD) exam formats, allowing students to program in a
similar environment to how they learnt, but presenting instructors with
significant additional challenges in preventing plagiarism and cheating. In
this paper, we describe a BYOD exam solution based on lockdown browsers,
software which temporarily turns students' laptops into secure workstations
with limited system or internet access. We combine the use of this technology
with a learning management system and cloud-based programming tool to
facilitate conceptual and practical programming questions that can be tackled
in an interactive but controlled environment. We reflect on our experience of
implementing this solution for a major undergraduate programming course,
highlighting our principal lesson that policies and support mechanisms are as
important to consider as the technology itself.Comment: Accepted by SIGCSE 202
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