Author Archives: Alexander Gough

Paper: Making (dark matter) waves – how wave interference can help model cold dark matter

Posted on by

Alex’s first first-author paper is now available on arXiv! An art piece Alex made related to this work also placed 2nd in the Art of Science competition hosted by the SAgE faculty at Newcastle. You can see more images and descriptions in Alex’s twitter thread on the paper (and the associated art piece).

This paper models the dark matter field as a single wavefunction, rather than traditional fluid variables or collisionless particles. We show explicitly how the complex phenomenology of multi-streaming (caused by collisionless particles flowing through each other) is encoded in the interference and oscillations of the wavefunction in a simple toy model. This wave model avoids the infinite density spikes which occur when evolving classical cold dark matter collapse.

Evolution of the position and density of a set of cold dark matter particles under the Zel’dovich approximation.

This paper demonstrates how the oscillations in the wavefunction can be “unwoven” to recover a set of wavefunctions corresponding to the set of classical streams in the Zel’dovich approximation. In the multi-stream region, where the dark matter cannot be described by a perfect fluid, we demonstrate how to separate the wavefunction into an “average part”, which describes the classical fluid behaviour, and an oscillatory “hidden part” which is responsible for producing beyond perfect fluid quantities such as velocity dispersion.

Comparison between the Zel’dovich approximation for cold dark matter and evolution of a wavefunction under the free Schrödinger equation. The wavefunction avoids the infinite density spikes (caustics) seen in classical cold dark matter, and introduces small scale interference to decorate the classical density.
Splitting of the wavefunction into three parts, each corresponding to classical dark matter trajectories.

The dense caustics formed by cold dark matter are replaced with diffraction caustics in the simple wave model, which provide classification of these caustics, and certain universal features related to the wave nature of the model. Such features are akin to those found in truly wavelike models of dark matter, such as fuzzy dark matter and ultralight axions.

The wave field corresponding to a cusp caustic. The scaling of the peak height and fringe widths are universal features, classified by catastrophe theory.

Outreach: 2 articles in Astrobites

Posted on by

Alex has published two more articles for the Astrobites collaboration.

The first article is an interview with the organisers of the Cosmology from Home 2021 conference (which includes fellow Newcastle PGR Niko Sarcevic) about what makes a successful online conference. This includes a discussion about the aims of online conferences and how they may differ from in-person conferences, as well as some practical tips, tricks, and tools for building a conference online from the start.

The second article is a daily paper summary looking at recent work done by Dan Thomas Sankarshana Srinivasan, Francesco Pace, and Richard Battye in a series of two papers about how to build cosmological simulations for modified gravity in a model independent way. Paper I sets up a mathematical framework for extending standard techniques to work in both the large and small scale limit, even through an intermediate regime where both cosmic perturbation theory and Newtonian theory don’t apply. Paper II sets up modified gravity simulations using this framework and through some simple test cases demonstrates the importance of these sorts of techniques for exploring modifications to gravity in the era of next generation surveys.

Outreach: New Astrobites Author

Posted on by

Alex will begin writing for Astrobites starting in 2021. The Astrobites collaboration is a group of astronomy and astrophysics graduate students around the world who write daily summaries of recent astrophysics research, accessible to the undergraduate level. These “daily summary posts” have made up the backbone of Astrobites over the last 10 years, and in more recent years they have begun to also write about things beyond daily summaries, including series of posts about DEI problems in astronomy, mental health in academia, what the day-to-day life in astronomy looks like, and application processes and career advice.

Paper: When galactic foregrounds are allowed to vary

Posted on by

A minimal power-spectrum-based moment expansion for CMB B-mode searches by S. Azzoni, M. H. Abitbol, D. Alonso, A. Gough, N. Katayama, T. Matsumura

Primordial gravitational waves, thought to be sourced by inflation in the very early universe, should imprint themselves on the polarisation signal of the cosmic microwave background (CMB). Direct detection of the “B-mode” polarisation characteristic of these gravitational waves would provide strong evidence for an inflationary period in the early universe, as well as providing insight into the mechanisms driving it. The strength of the tensor perturbations which correspond to gravitational waves is parametrised by the “tensor to scalar ratio”, r. 

Detecting true B mode signal from the CMB is difficult, as the primordial B mode signal is dominated by foreground elements, such as thermal dust emission and synchrotron emission within our own galaxy. These galactic foregrounds can be dealt with by masking—cutting the portions of the sky where the galactic plane is out of the signal—though this has the disadvantage of throwing away true signal. The other approach is to model the foreground elements statistically, with some dependence on frequency.

The standard technique is to characterise the foreground elements’ spectral energy density (a function of frequency) by an amplitude and a spectral index. These spectral indices are usually considered to be constant across the sky, which, given that they are empirically fit parameters, rather than motivated by underlying physics, is not well justified. For small sky fraction surveys, the difference is thought to be negligible, but for future CMB experiments, this assumption should be relaxed. This paper is a proof of concept for an extension beyond assuming a constant spectral index.

Allowing the spectral indices of dust and synchrotron emission to vary across the sky, we propagate forward the effect on the total observed power spectrum in a very general way using a moment expansion. A highly simplified case of this model is then tested against simulations of varying degrees of realism, to see whether this kind of expansion tightens the constraints on r compared to not using the moment expansion. The result is not possible to significantly detect the effects of this variation on the foreground multi-frequency power spectra for Simon’s-Observatory-like sensitivities, at least using the strong simplifications we used to test against simulations. Nevertheless the methodology set up in this paper, as well as potential extensions, will be useful for analysis of ground based CMB experiments such as Simon’s Observatory and CMB-Stage 4.

Outreach: Public talk for Astronomical Society

Posted on by

Remote outreach talk to the Newcastle Astronomical Society, given by Alex Gough

The Skeleton of Our Universe

The goal of this talk is to introduce the topic of my research, understanding the largest structures in the universe, to the members of the Newcastle Astronomical Society. This begins by setting the stage for where cosmology takes place, and winding back the cosmic clock to the early universe and the cosmic microwave background (CMB). From there, understanding that the very early universe is nearly the same everywhere, with only 10 parts per million deviation from the mean density, it becomes an obvious scientific question to understand how those tiny fluctuations grow into the rich structure of galaxies we see today. Understanding this growth, and the role dark matter has to play in it, is the focus of my research.

After touring through the history of the universe, we take a detour into understanding how these huge distances and times are actually measured. This detour provides a link from my work in cosmology to the stellar physics and observations that members of an astronomical society are more familiar with. It also provides a nice opportunity to look at beautiful space pictures.

The end point of the talk is the 6 numbers one needs to measure to construct the universe. These are based on the 6 parameters in ΛCDM (the standard cosmological model), slightly modified to make them more accessible to this general audience. These break down into:

  • 2 numbers from the early universe: the amplitude and scale dependence of the fluctuations in the CMB
  • 2 numbers for the “pie recipe” of the universe: how much dark energy and dark matter do we have
  • 2 timescales for the universe: the age of the universe, and the time you have to wait for the first stars to form.