Black holes and neutron stars in the generalized tensor-vector-scalar theory

Bekenstein's Tensor-Vector-Scalar (TeVeS) theory has had considerable success as a relativistic theory of Modified Newtonian Dynamics (MoND). However, recent work suggests that the dynamics of the theory are fundamentally flawed and numerous authors have subsequently begun to consider a generalization of TeVeS where the vector field is given by an Einstein-Aether action. Herein, I develop strong-field solutions of the generalized TeVeS theory, in particular exploring neutron stars as well as neutral and charged black holes. I find that the solutions are identical to the neutron star and black hole solutions of the original TeVeS theory, given a mapping between the parameters of the two theories, and hence provide constraints on these values of the coupling constants. I discuss the consequences of these results in detail including the stability of such spacetimes as well as generalizations to more complicated geometries.Comment: Accepted for publication in Physical Review

A neutron star progenitor for FRBs? Insights from polarisation measurements

Fast Radio Bursts (FRBs) are intense, millisecond-duration broadband radio transients, the emission mechanisms of which are not understood. Masui et al. recently presented Green Bank Telescope observations of FRB 110523, which displayed temporal variation of the linear polarisation position angle (PA). This effect is commonly seen in radio pulsars and is attributed to a changing projected magnetic field orientation in the emission region as the star rotates. If a neutron star is the progenitor of this FRB, and the emission mechanism is pulsar-like, we show that the progenitor is either rapidly rotating, or the emission originates from a region of complex magnetic field geometry. The observed PA variation could also be caused by propagation effects within a neutron-star magnetosphere, or by spatially varying magnetic fields if the progenitor lies in a dense, highly magnetised environment. Although we urge caution in generalising results from FRB 110523 to the broader FRB population, our analysis serves as a guide to interpreting future polarisation measurements of FRBs, and presents another means of elucidating the origins of these enigmatic ephemera.Comment: 7 pages, 2 figures, submitted to MNRA

The effect of pressure gradients on luminosity distance - redshift relations

Inhomogeneous cosmological models have had significant success in explaining cosmological observations without the need for dark energy. Generally, these models imply inhomogeneous matter distributions alter the observable relations that are taken for granted when assuming the Universe evolves according to the standard Friedmann equations. Moreover, it has recently been shown that both inhomogeneous matter and pressure distributions are required in both early and late stages of cosmological evolution. These associated pressure gradients are required in the early Universe to sufficiently describe void formation, whilst late-stage pressure gradients stop the appearance of anomalous singularities. In this paper we investigate the effect of pressure gradients on cosmological observations by deriving the luminosity distance - redshift relations in spherically symmetric, inhomogeneous spacetimes endowed with a perfect fluid. By applying this to a specific example for the energy density distribution and using various equations of state, we are able to explicitly show that pressure gradients may have a non-negligble effect on cosmological observations. In particular, we show that a non-zero pressure gradient can imply significantly different residual Hubble diagrams for $z\lesssim1$ compared to when the pressure is ignored. This paper therefore highlights the need to properly consider pressure gradients when interpreting cosmological observations.Comment: Accepted for publication in Classical and Quantum Gravit

Neutron Star Merger Remnants: Braking Indices, Gravitational Waves, and the Equation Of State

The binary neutron star merger GW170817/GRB170817A confirmed that at least some neutron star mergers are the progenitors of short gamma-ray bursts. Many short gamma-ray bursts have long-term x-ray afterglows that have been interpreted in terms of post-merger millisecond magnetars---rapidly rotating, highly magnetised, massive neutron stars. We review our current understanding of millisecond magnetars born in short gamma-ray bursts, focusing particularly three main topics. First, whether millisecond magnetars really do provide the most plausible explain for the x-ray plateau. Second, determining and observing the gravitational-wave emission from these remnants. Third, determining the equation of state of nuclear matter from current and future x-ray and gravitational-wave measurements.Comment: Conference Proceedings of the Xiamen-CUSTIPEN Workshop on the EOS of Dense Neutron-Rich Matter in the Era of Gravitational Wave Astronomy (January 3 - 7, 2019, Xiamen, China