Fundamental physics with black holes and scalar fields

Thesis

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.

Authors: Bamber, J

• DOI: 10.5287/ora-4joqooy0e
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: dark matter; numerical relativity; general relativity; black holes; gravitational waves; scalar fields; fifth forces
• Subjects: Astrophysics; Black holes (Astronomy); Gravitational waves; Dark matter (Astronomy); General relativity (Physics); Scalar field theory