Modelling the Milky Way's globular cluster system

We construct a model for the Galactic globular cluster system based on a realistic gravitational potential and a distribution function (DF) analytic in the action integrals. The DF comprises disc and halo components whose functional forms resemble those recently used to describe the stellar discs and stellar halo. We determine the posterior distribution of our model parameters using a Bayesian approach. This gives us an understanding of how well the globular cluster data constrain our model. The favoured parameter values of the disc and halo DFs are similar to values previously obtained from fits to the stellar disc and halo, although the cluster halo system shows clearer rotation than does the stellar halo. Our model reproduces the generic features of the globular cluster system, namely the density profile, the mean rotation velocity. The fraction of disc clusters coincides with the observed fraction of metal-rich clusters. However, the data indicate either incompatibility between catalogued cluster distances and current estimates of distance to the Galactic Centre, or failure to identify clusters behind the bulge. As the data for our Galaxy's components increase in volume and precision over the next few years, it will be rewarding to revisit the present analysis.Comment: 13 pages accepted by MNRA

Motion in a scalar field

Light scalar fields emerge as a generic prediction in physics beyond the Standard Model. For example, they arise as new degrees of freedom in modified gravity, as Kaluza-Klein modes from extra compactified dimensions, and as Nambu-Goldstone bosons from spontaneously broken symmetries. Far from being just objects of theoretical interest, these scalar fields could also play crucial roles in resolving some of the most important open problems, such as the nature of dark matter and dark energy. Given this ubiquity in modern theoretical physics and their potentially far-reaching implications, efforts to detect or otherwise rule out these hypothetical scalars have burgeoned into a global enterprise in recent years. This thesis contributes to this ongoing effort by updating our understanding of how light scalar fields influence the dynamics of moving bodies, focusing on two novel scenarios. We begin by reanalysing the motion of electrons in laboratory experiments designed to deliver high-precision measurements of the fine-structure constant. The vacuum chambers employed in these setups make them ideal testing grounds for a class of scalar--tensor theories that screen the effects of their scalar mode based on the ambient density. If unscreened, the scalar exerts an attractive “fifth” force on the electron and, moreover, transforms the vacuum cavity into a dielectric medium due to its interactions with electromagnetic fields. Because these effects introduce different amounts of systematic bias into each experiment, good agreement between independent measurements of the fine-structure constant can be used to establish meaningful constraints on the parameter spaces of these models. In the second part of this thesis, we turn to investigate how ambient scalar fields influence the motion of binary black holes. Even though the models we consider are subject to no-hair theorems, the interplay between absorption at the horizons and momentum transfer in the bulk of the spacetime still gives rise to interesting phenomenology. We show that this interaction causes a fraction of the ambient field to be ejected from the system as scalar radiation, while the black holes themselves are seen to feel the effects of an emergent fifth force. Moreover, if the ambient field corotates with the binary, it can extract energy from the orbital motion and grow exponentially through a process akin to superradiance. Although these effects turn out to be highly suppressed in the regime amenable to analytic methods, the novel techniques developed herein lay the groundwork for future studies of these complex gravitational systems.The work in this thesis was funded primarily by a Cambridge International Scholarship from the Cambridge Commonwealth, European and International Trust. Additional funding came in the form of a Research Scholarship and multiple Rouse Ball travel grants from Trinity College, Cambridge, and a research studentship award (Ref. S52/064/19) from the Cambridge Philosophical Society. As a member of the High Energy Physics and Relativity & Gravitation groups in the Department of Applied Mathematics and Theoretical Physics, the author also benefited from the following STFC Consolidated Grants: No. ST/L000385/1, No. ST/L000636/1, No. ST/P000673/1, and No. ST/P000681/1

Gravitational bremsstrahlung from spinning binaries in the post-Minkowskian expansion

We present a novel calculation of the four-momentum that is radiated into gravitational waves during the scattering of two arbitrarily spinning bodies. Our result, which is accurate to leading order in $G$, to quadratic order in the spins, and to all orders in the velocity, is derived by using a Routhian-based worldline effective field theory formalism in concert with a battery of analytic techniques for evaluating loop integrals. While nonspinning binaries radiate momentum only along the direction of their relative velocity, we show that the inclusion of spins generically allows for momentum loss in all three spatial directions. We also verify that our expression for the radiated energy agrees with the overlapping terms from state-of-the-art calculations in post-Newtonian theory.Comment: Version accepted for publication. 5 pages, 2 figures, 1 table + supplemental material. Some minor typos corrected and a few references added relative to v

Constraining spontaneous black hole scalarization in scalar-tensor-Gauss-Bonnet theories with current gravitational-wave data

We examine the constraining power of current gravitational-wave data on scalar-tensor-Gauss-Bonnet theories that allow for the spontaneous scalarization of black holes. In the fiducial model that we consider, a slowly rotating black hole must scalarize if its size is comparable to the new length scale $\lambda$ that the theory introduces, although rapidly rotating black holes of any mass are effectively indistinguishable from their counterparts in general relativity. With this in mind, we use the gravitational-wave event GW190814\,\unicode{x2014}\,whose primary black hole has a spin that is bounded to be small, and whose signal shows no evidence of a scalarized primary\,\unicode{x2014}\,to rule out a narrow region of the parameter space. In particular, we find that values of ${\lambda \in [56, 96]~M_\odot}$ are strongly disfavored with a Bayes factor of $0.1$ or less. We also include a second event, GW151226, in our analysis to illustrate what information can be extracted when the spins of both components are poorly measured.Comment: 8 pages + references, 4 figure

Gravitational waves from binary black holes in a self-interacting scalar dark matter cloud

We investigate the imprints of accretion and dynamical friction on the gravitational-wave signals emitted by binary black holes embedded in a scalar dark matter cloud. As a key feature in this work, we focus on scalar fields with a repulsive self-interaction that balances against the self-gravity of the cloud. To a first approximation, the phase of the gravitational-wave signal receives extra correction terms at $-4$PN and $-5.5$PN orders, relative to the prediction of vacuum general relativity, due to accretion and dynamical friction, respectively. Future observations by LISA and B-DECIGO have the potential to detect these effects for a large range of scalar masses~$m_\mathrm{DM}$ and self-interaction couplings~$\lambda_4$; observations by ET and Advanced~LIGO could also detect these effects, albeit in a more limited region of parameter space. Crucially, we find that even if a dark matter cloud has a bulk density~$\rho_0$ that is too dilute to be detected via the effects of dynamical friction, the imprints of accretion could still be observable because it is controlled by the independent scale $\rho_a = 4 m_{\rm DM}^4 c^3/(3 \lambda_4 \hbar^3)$. In the models we consider, the infalling dark matter increases in density up to this characteristic scale $\rho_a$ near the Schwarzschild radius, which sets the accretion rate and its associated impact on the gravitational~waveform.Comment: 20 pages, 6 figures, 5 table

Spin-orbit effects for compact binaries in scalar-tensor gravity

Abstract: Gravitational waves provide us with a new window into our Universe, and have already been used to place strong constrains on the existence of light scalar fields, which are a common feature in many alternative theories of gravity. However, spin effects are still relatively unexplored in this context. In this work, we construct an effective point-particle action for a generic spinning body that can couple both conformally and disformally to a real scalar field, and we show that requiring the existence of a self-consistent solution automatically implies that if a scalar couples to the mass of a body, then it must also couple to its spin. We then use well-established effective field theory techniques to conduct a comprehensive study of spin-orbit effects in binary systems to leading order in the post-Newtonian (PN) expansion. Focusing on quasicircular nonprecessing binaries for simplicity, we systematically compute all key quantities, including the conservative potential, the orbital binding energy, the radiated power, and the gravitational-wave phase. We show that depending on how strongly each member of the binary couples to the scalar, the spin-orbit effects that are due to a conformal coupling first enter into the phase at either 0.5 PN or 1.5 PN order, while those that arise from a disformal coupling start at either 3.5 PN or 4.5 PN order. This suppression by additional PN orders notwithstanding, we find that the disformal spin-orbit terms can actually dominate over their conformal counterparts due to an enhancement by a large prefactor. Accordingly, our results suggest that upcoming gravitational-wave detectors could be sensitive to disformal spin-orbit effects in double neutron star binaries if at least one of the two bodies is sufficiently scalarised

Gravitational bremsstrahlung from spinning binaries in the post-Minkowskian expansion

International audienceWe present a novel calculation of the four-momentum that is radiated into gravitational waves during the scattering of two arbitrarily spinning bodies. Our result, which is accurate to leading order in G, to quadratic order in the spins, and to all orders in the velocity, is derived by using a Routhian-based worldline effective field theory formalism in concert with a battery of analytic techniques for evaluating loop integrals. While nonspinning binaries radiate momentum only along the direction of their relative velocity, we show that the inclusion of spins generically allows for momentum loss in all three spatial directions. We also verify that our expression for the radiated energy agrees with the overlapping terms from state-of-the-art calculations in post-Newtonian theory