Francesca von Braun-Bates

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Francesca von Braun-Bates

D. Phil. student

Curriculum vitae, updated October 2013: PDF.
Old website pages at USyd.
Arxiv: list of publications

Non-linear collapse in modified gravity theories

DPhil research (2012-2015)
Collaborators: Prof. Pedro Ferreira, Dr. Julien Devriendt, Johannes Noller (University of Oxford)

This is my current research, started in October 2012.

Current numerical methods for studying the non-linear growth of structure are firmly entrenched in the Newtonian non relativistic limit. Yet some of the current proposed sources of dark energy, dark matter and modified gravity are inherently relativistic. We wish to build an N-body simulation that correctly incorporates relativistic fields and allows us to study a wide range of statistics, from the formation of clusters, cluster counts and their overall impact on large scale structure. As a first step we would work with a PM code but, if time allows, we would work up to one of the AMR codes that that the group currently works on.

Gravitational lensing of pulsars as a probe of dark matter halos

PhD, changed to MSc (Res.) project (2011-2013)
Collaborators: Prof. Geraint Lewis (University of Sydney)

The thesis is available as a PDF with accompanying slides (also PDF).

A key question in cosmology is the properties of dark matter. The goal of this thesis is to present a gravitational lensing-based observational test for the mapping and characterisation of dark matter halos. Dark matter halos act as gravitational lenses whose mass can be extracted from the variable flux and additional time delay induced in photons passing near or through the halo. The key dependencies of the mass estimate are the density profile adopted for the halo and the underlying cosmology. Mapping of dark matter halos throughout the Milky Way can be performed by examining pulsar signals for variability. I examine the usefulness of this method as a probe of dark matter using millisecond pulsars, since these have both the highest rotational frequencies and the most stability in their periods. Considering both current and future observing capabilities, I determine a threshold for the observability of such time delays.

Bayesian inference of cosmological parameters using Type Ia SNe

BSc. Honours project (2010)
Collaborators: Prof. Geraint Lewis (University of Sydney)

The thesis is available as a PDF with accompanying slides (also PDF).

Dark energy and gravity are equally important in determining the large-scale geometry and evolution of the universe. This evolution can be computationally modelled using the Friedmann ODEs obtained from the general relativistic Friedmann-Robertson-Lemaitre-Walker metric tensor. Under the assumption of a spatially flat universe with its only contents being matter and a time-varying dark energy, the parameters in the Friedmann equations reduce to the Hubble constant, the cosmological matter density, the two-parameter dark energy equation of state and a matter-dark energy interaction parameter. I demonstrate the use of Bayesian inference to estimate values for these cosmological parameters. The parameters are related to observations by redshift-distance relations. I focus on the luminosity distance, which can be measured using Type Ia supernovae. The data used is a selection of three recent SNe compilations and five artificially generated sets: three based on these compilations to see how the parameter uncertainty scales with error and two projected data sets based upon the upcoming Supernova Acceleration Probe. I use the Metropolis-Hastings algorithm, an example of Markov chain Monte Carlo methods, to generate the posterior probability distributions for models with one and two free parameters (although the code I wrote can be used for up to n parameters). Credible regions can be generated for the parameter space from the posterior distributions. I conclude that the one-parameter distributions show that the currently accepted concordance cosmology is a plausible approximation to the parameters, although differences in the light curve fitters which were used to calibrate the data prevent a more decisive estimation. The two-parameter models exhibit a wide range of degeneracies which need to be resolved by combining the supernova-deroved credible regions with those from the cosmic microwave background and baryon acoustic oscillations.

Cosmological parameter estimation in a dark matter-dark energy-interacting universe

BSc. Special Project (2009)
Collaborators: Prof. Geraint Lewis (University of Sydney)

A short report is available as a PDF. This work forms a simplified version (a six-week project) of work which later became my Honours thesis.

Dynamics of Kerr spacetimes

BSc. Talented Student Project (2008)
Collaborators: Prof. Geraint Lewis (University of Sydney)

The presentation slides are available as a Powerpoint 2003 file. The accompanying "popular physics" article is a SXW file. A short project addressing two different challenges in modern cosmology: self-directed research and outreach.

The project solved the geodesic equations of a general Kerr geometry and used this to explore the systematic differences between the trajectories of photons and massive objects. The report and presentation aims to explain this to a layman's audience while sacrificing neither scientific content nor accuracy.

Effects of varying stellar properties on the H-alpha:UV flux ratio

BSc. Talented Student Project (2007)
Collaborators: Prof. John O'Byrne (University of Sydney)

The presentation slides are available as a Powerpoint 2003 file. The accompanying "popular physics" article is a SXW file.

It is possible to for galactic age maps by examining the flux ratio between the H-alpha line in the radio and the hydrogen UV lines: this flux traces the population of (necessarily) young O and A stars in spiral arms compared to those of longer-lived main sequence stars. A vital question in determining the uncertainties in such age maps is whether individual variations in stellar properties cause significant change in the flux. Using the FORTRAN90 program Starburst 99, stellar spectra are obtained for various stellar populations and systematic differneces are highlighted. The results of this six-week project are presented to a scientific audience in an oral presentation and to a general audience via a popular science article.