Fundamental physics with black holes and scalar fields

One well-motivated proposal to address the limitations of the Standard Model and General Relativity (GR) is the addition of one or more novel scalar fields. In this thesis we will study the interaction of scalar field dark matter (DM) with black holes (BHs), and scalar fields in theories of modified gravity. The Nobel-Prize-winning detection of gravitational waves (GWs) has opened up a new area of astrophysics. Scalar fields can form dense clouds around BHs, so as most GWs are produced from BH binary mergers, one might be able to detect scalar DM with GW astronomy. We will start by exploring the formation of such clouds, conducting novel simulations of scalar DM accretion onto a spinning Kerr BH, characterising the growth, and estimating the potential for GW signals. A binary BH merger can be divided into the early inspiral, the highly relativistic merger, and the post-merger “ringdown”. First we will examine the ringdown, deriving a novel analytic perturbative formula to estimate the shift in the GW quasi-normal mode frequencies due to an accreting cloud. We will show that the contribution from the accretion rate, previously neglected, can dominate the shift. For the early inspiral we will simulate the accretion of scalar DM around BHs on fixed orbits, finding that there is a preferred, quasi-stationary scalar field profile. For the highly relativistic regime we use full Numerical Relativity. We will examine the impact of different initial scalar distributions, showing that the quasi-stationary profile is an attractor solution, and that naively superimposing matter onto a quasi-circular binary can produce unphysical eccentricity. Lastly, we will explore scalar fields beyond GR. Scalar-tensor theories are a popular modified-gravity model, yet they often predict “fifth forces” which are tightly constrained. It has been shown that for scale-invariant gravity the fifth force is highly suppressed. However, this result was obtained in a particular frame, and quantum effects make the choice of frame highly non-trivial. We will discuss how one can apply a covariant formalism to extend the result to all frames, and show that the usual dichotomy of “Jordan” versus “Einstein” frame can be better understood as a geometric continuum

The effect of wave dark matter on equal mass black hole mergers

For dark matter to be detectable with gravitational waves from binary black holes, it must reach higher than average densities in their vicinity. In the case of light (wave-like) dark matter, the density of dark matter between the binary can be significantly enhanced by accretion from the surrounding environment. Here we show that the resulting dephasing effect on the last ten orbits of an equal mass binary is maximized when the Compton wavelength of the scalar particle is comparable to the orbital separation, $2\pi/\mu\sim d$. The phenomenology of the effect is different to the channels that are usually discussed, where dynamical friction (along the orbital path) and radiation of energy and angular momentum drive the dephasing, and is rather dominated by the radial force (the spacetime curvature in the radial direction) towards the overdensity between the black holes. Whilst our numerical studies limit us to scales of the same order, this effect may persist at larger separations and/or particle masses, playing a significant role in the merger history of binaries.Comment: 5 pages, 4 figures, 1 appendix, 1 movie: https://youtu.be/2VJIfqCp7D8 Comments welcome

GRDzhadzha: A code for evolving relativistic matter on analytic metric backgrounds

GRDzhadzha is an open-source code for relativistic simulations of matter fields on curved spacetimes that admit an analytic description (e.g. stationary black holes). It is based on the publicly available 3+1D numerical relativity code GRChombo. Such a description is valid where the density of the matter is small compared to the curvature scale of the spacetime, which is the case for many physical scenarios - for example, dark matter environments. The approach offers significant savings on memory and speed compared to running full numerical relativity simulations, since the metric variables and their derivatives are calculated analytically, and therefore are not evolved or stored on the grid. This brief paper introduces the code and gives details of some applications for which it has already been used.Comment: Submitted for review in the Journal of Open Source Software; Comments welcome; The code can be found at https://github.com/GRChombo/GRDzhadzha.gi

GRChombo: An adaptable numerical relativity code for fundamental physics

GRChombo is an open-source code for performing Numerical Relativity time evolutions, built on top of the publicly available Chombo software for the solution of PDEs. Whilst GRChombo uses standard techniques in NR, it focusses on applications in theoretical physics where adaptability, both in terms of grid structure, and in terms of code modification, are key drivers

The Bristol CMIP6 Data Hackathon

The Bristol CMIP6 Data Hackathon formed part of the Met Office Climate Data Challenge Hackathon series during 2021, bringing together around 100 UK early career researchers from a wide range of environmental disciplines. The purpose was to interrogate the under-utilised but currently most advanced climate model inter-comparison project datasets to develop new research ideas, create new networks and outreach opportunities in the lead up to COP26. Experts in different science fields, supported by a core team of scientists and data specialists at Bristol, had the unique opportunity to explore together interdisciplinary environmental topics summarised in this article