Physics, School of http://hdl.handle.net/1842/1192019-09-13T19:46:33Z2019-09-13T19:46:33ZWomen's representation and experiences in the high performance computing communityFrantzana, Athinahttp://hdl.handle.net/1842/361272019-09-10T10:56:58Z2019-11-26T00:00:00ZWomen's representation and experiences in the high performance computing community
Frantzana, Athina
Gender imbalance in STEM disciplines (Science, Technology, Engineering and
Mathematics) has been a research subject for years. Studies have shown potential
reasons leading to the underrepresentation of women in such disciplines and have
suggested why and how to improve the gender balance and women's experiences in
these areas. The High Performance Computing (HPC) community, which spans
various STEM subjects and relies on advanced scientific research, might present a
similar picture. The aim of this thesis is to understand the gender demographics
of the HPC community, to identify the underlying reasons of a potential gender
imbalance, and to suggest effective ways of improvement.
Since HPC is such a broad community, to obtain a first picture of the proportion
of women in the HPC community, we decided to examine historical demographics
of two different settings which are potential indicators of the participation and
contribution of women in the community, namely HPC-related conferences and
HPC training courses. From the analysis of these quantitative data, we found that
women were fewer than men in all the categories of conference participation that
we examined, and that women were outnumbered by men at all levels and years of
the courses examined. Our study reveals an underrepresentation of women in the
HPC community, along the lines of what already observed in STEM disciplines.
Additionally, we conducted a survey in order to further understand the reasons of
the gender imbalance and to find out from the people within the HPC community
what could be done to address the issue. Results from our survey indicate that
the clear majority of both women and men forming the HPC community come
from a STEM background, which is considered as the main reason of women's
underrepresentation by the participants of this study. We also discovered that
women are less likely to receive training and to develop software, both crucial
factors for using HPC facilities for research purposes. Gender differences are
also found in the impact of parenthood on career progression; the perception of
gender discrimination in workplace and conference environments; the importance
of gender balance, mentoring, role models and Equality and Diversity awareness
in the HPC community.
Similar findings and gender differences are also highlighted and confirmed by the
results of further qualitative approaches of this study. We conducted interviews
and focus group discussions with selected and recommended individuals of the
community, to support and interpret previously obtained data, and to stimulate
new ideas or hypotheses for future work. According to the interviewees and the
participants to focus group discussions, one of the main challenges of the HPC
community is its image of a closed, inaccessible, "geeky" area, which focuses
on the size, speed and power of supercomputers, rather than on their use for
solving problems in research and in life. This might be one reason that makes the
community unattractive to women. Also, of significant importance for the current
diversity status of the community is the fact that HPC is not well-promoted
as a research tool, especially to more gender-balanced non-STEM subjects, in
combination with the lack of formal (HPC and programming) training and of
women in senior positions.
This thesis forms the first step to understand the womens representation and
experiences within the HPC community. All the topics studied, and the evidence
gathered in this thesis have provided significant insight to enable further research
on the best practices for improvement in the HPC community and related STEM
fields.
2019-11-26T00:00:00ZConsequences of dissipative dynamics in the early universeHernández Jiménez., Rafaelhttp://hdl.handle.net/1842/361192019-09-10T10:18:47Z2019-11-26T00:00:00ZConsequences of dissipative dynamics in the early universe
Hernández Jiménez., Rafael
Warm inflation presents an exceptional description of the early universe cosmology.
It is a scenario of an inflationary dynamics in which the state of the universe during
inflation is not the vacuum state, but rather an excited statistical thermal state. It
introduces dissipation into the inflationary dynamics which can be well explained by
first principles of a quantum multi-field theory. Besides, this approach has several
attractive features. For instance, the additional friction may ease the required flatness
of the inflaton potential. Besides, even if radiation is subdominant during inflation,
may smoothly become the leading component if the ratio of dissipation Q ≳ 1 at the
end of inflation (ϵeff ~ 1 + Q), with no need for a separate reheating period. It
also may explain the nature of the classical inhomogeneities observed in the CMB,
since for WI the fluctuations of the inflaton are thermally induced; hence there is
no need to explain the troublesome quantum-to-classical transition problem of the
standard inflation picture, cold inflation, due to the purely quantum origin of the
density perturbations. Furthermore, one well established key aspect is the prediction
for a low tensor-to-scalar ratio, which now we see is consistent with Planck legacy.
Taking into account above encouraging warm inflation characteristics, in this thesis we
will describe both warm inflation model building and the confrontation of theory with
observation. We will examine two basic models: The Warm Little Inflaton scenario
and the distributed mass model. In each case, we determine the parametric regimes in
which the dynamical evolution is consistent for 50-60 e-folds of inflation, taking into
account thermal corrections to the scalar potential (if necessary). In the first model
we consider three distinct types of scalar potentials for the inflaton, namely chaotic
inflation with a quartic monomial potential, a Higgs-like symmetry breaking potential
and a non-renormalizable plateau-like potential. On the other hand, the distributed
mass model is examined for various mass distributions considering a chaotic quartic
potential. Both scenarios are theoretically and observationally successful for a broad
range of parameter values. Indeed, they agree remarkably with the Planck legacy data.
The Warm Little Inflaton is undoubtedly the simplest realisation of warm inflation within a concrete quantum field theory construction, since it requires only a small
number of fields; in particular, the inflation is directly coupled to just two light fields.
Distributed mass models can be viewed as realisations of the landscape property of
string theory, with the mass distributions coming from the underlying spectra of the
theory, which themselves would be affected by the vacuum of the theory.
2019-11-26T00:00:00ZCosmology with underdensities of the cosmic webDemchenko, Vasiliy Grigoriyvichhttp://hdl.handle.net/1842/361082019-09-09T12:21:05Z2019-08-19T00:00:00ZCosmology with underdensities of the cosmic web
Demchenko, Vasiliy Grigoriyvich
This thesis explores the utilisation of underdense regions of the Cosmic Web
as a tool for studying cosmology. Underdensities, known as cosmic voids,
provide a complementary approach for understanding the large-scale structure
of our Universe, as well as providing a unique environment to explore the
effects of dark energy. An application of the spherical model to void evolution
is presented, showcasing its ability to provide non-linear density and velocity
profiles for voids. This methodology is then applied in order to reconstruct the
initial conditions of the void using a late-time void density profile. Using this
reconstruction, the spherical model yields non-linear late-time velocity profiles
which are used to predict redshift-space distortions around voids, showing the
capacity to constrain cosmological parameters using measurements in the non-linear
regime. Furthermore, this thesis investigates how cosmologists can utilise
the advances of weak gravitational lensing to exploit the underdensities of the
Universe. A study of the weak lensing measurement around voids is presented
with a focus on the SLICS simulation suite and the KiDS and GAMA surveys.
A watershed void finding algorithm, zobov, is applied to both the simulations
and data, showing that the geometry of the GAMA survey does not lend well to
extracting a weak lensing signal from voids due to limitations from both the survey
volume and geometry. In contrast, projected underdensities, known as troughs,
and the full, projected density spectrum, known as Density Split Statistics, are
both shown to yield great potential as cosmological tools. The final chapter
of this thesis investigates the use of this novel approach to explore non-ΛCDM
cosmologies using the cosmo-SLICS simulations, showing how underdensities can
potentially constrain the equation-of-state of dark energy with a higher precision
than overdense regions. Chapter 1 of this thesis provides a brief overview of
cosmology, while Chapter 2 introduces the theory of weak gravitational lensing.
Chapter 3 discusses the spherical model applied to void evolution and redshift-space
distortions around voids, while Chapter 4 explores the weak gravitational
lensing signal around voids in simulations and data. Chapter 5 utilises a suite of
simulations to investigate the sensitivity of Density Split Statistics to dark energy
models.
2019-08-19T00:00:00ZTwo-reggeon exchange to all loop ordersReichel, Joschahttp://hdl.handle.net/1842/361072019-09-06T11:12:51Z2019-11-26T00:00:00ZTwo-reggeon exchange to all loop orders
Reichel, Joscha
In the limit of very large centre-of-mass energy s the two-parton scattering amplitude
is well approximated by the exchange of reggeons in the t channel. In particular, the
leading contributions to the signature-even two-reggeon exchange are described by the
famous Balitsky–Fadeev–Kuraev–Lipatov (BFKL) evolution equation. In this thesis
we demonstrate that it is possible to solve this equation iteratively, and in this way
calculate the associated amplitude to any loop order in perturbative QCD.
The key idea is to analyse the evolution of the two-reggeon wavefunction in two complementary
regions. The so-called soft region is characterised by the small momentum of
one of the reggeons. There, the wavefunction obeys a simplified evolution equation and
evaluates to a polynomial in the soft momentum. This region is the exclusive source of
the singularities of the signature-even amplitude. Consequently, the complementary region
is described by purely finite integrals which can be evaluated without dimensional
regularisation, directly in terms of a class of iterated polylogarithms. The contributions
from both regions are combined and shown to recover the result of the full BFKL
evolution. All the above methods are algorithmic and work to any loop order.
We resum the singularities of the amplitude to all loop orders and match the result to the
predictions made by the soft factorisation theorem to shed light on the universal infrared
behaviour of two-parton scattering. This lets us extract the all-order soft anomalous
dimension in the high-energy limit whose properties we analyse in detail. In particular,
it turns out to be an entire function of the coupling which can be approximated by a
simple oscillating function well beyond the perturbative regime.
The finite terms of the signature-even amplitude show intricate combinations of transcendental
numbers. At low loop orders they are the well-known values of the Riemann
zeta function evaluated at integer arguments. However, examining the amplitude at
eleven loops and beyond reveals that a broader class of numbers — so-called single-valued
multiple zeta values — is needed to describe the two-reggeon exchange. Moreover,
finite terms that originate in the soft region are readily resummed and allow us to
derive a modified evolution equation for the complementary hard region. In fact, there
are more hints of all-order resummation which we discuss towards the end of this thesis
hoping they will inspire future research in this area.
2019-11-26T00:00:00Z