Dr Deaglan Bartlett

Abstract:

The concordance model of cosmology predicts a universe which finishes in a finite amount of conformal time at a future conformal boundary. We show that for particular cases we study, the background variables and perturbations may be analytically continued beyond this boundary and that the "end of the universe"is not necessarily the end of their physical development. Remarkably, these theoretical considerations of the end of the universe might have observable consequences today: perturbation modes consistent with these boundary conditions have a quantized power spectrum which may be relevant to features seen in the large scale cosmic microwave background. Mathematically these cosmological models may either be interpreted as a palindromic universe mirrored in time, a reflecting boundary condition, or a double cover, but are identical with respect to their observational predictions and stand in contrast to the predictions of conformal cyclic cosmologies.

Abstract:

Lorentz invariance violation in quantum gravity (QG) models or a nonzero photon mass, mγ, would lead to an energy-dependent propagation speed for photons, such that photons of different energies from a distant source would arrive at different times, even if they were emitted simultaneously. By developing source-by-source, Monte Carlo-based forward models for such time delays from gamma ray bursts, and marginalizing over empirical noise models describing other contributions to the time delay, we derive constraints on mγ and the QG length scale, ℓQG, using spectral lag data from the BATSE satellite. We find mγ<4.0×10-5 h eV/c2 and ℓQG<5.3×10-18 h GeV-1 at 95% confidence, and demonstrate that these constraints are robust to the choice of noise model. The QG constraint is among the tightest from studies which consider multiple gamma ray bursts and the constraint on mγ, although weaker than from using radio data, provides an independent constraint which is less sensitive to the effects of dispersion by electrons.

Abstract:

Theories of gravity that obey the Weak Equivalence Principle have the same Parametrised Post-Newtonian parameter $\gamma$ for all particles at all energies. The large Shapiro time delays of extragalactic sources allow us to put tight constraints on differences in $\gamma$ between photons of different frequencies from spectral lag data, since a non-zero $\Delta \gamma$ would result in a frequency-dependent arrival time. The majority of previous constraints have assumed that the Shapiro time delay is dominated by a few local massive objects, although this is a poor approximation for distant sources. In this work we consider the cosmological context of these sources by developing a source-by-source, Monte Carlo-based forward model for the Shapiro time delays by combining constrained realisations of the local density field using the Bayesian origin reconstruction from galaxies algorithm with unconstrained large-scale modes. Propagating uncertainties in the density field reconstruction and marginalising over an empirical model describing other contributions to the time delay, we use spectral lag data of Gamma Ray Bursts from the BATSE satellite to constrain $\Delta \gamma < 2.1 \times 10^{-15}$ at $1 \sigma$ confidence between photon energies of $25 {\rm \, keV}$ and $325 {\rm \, keV}$.

Abstract:

Galileons are scalar field theories which obey the Galileon symmetry $\varphi \to \varphi + b + c_\mu x^\mu$ and are capable of self-acceleration if they have an inverted sign for the kinetic term. These theories violate the Strong Equivalence Principle, such that black holes (BHs) do not couple to the Galileon field, whereas non-relativistic objects experience a fifth force with strength $\Delta G / G_{\rm N}$ relative to gravity. For galaxies falling down a gradient in the Galileon field, this results in an offset between the centre of the galaxy and its host supermassive BH. We reconstruct the local gravitational and Galileon fields through a suite of constrained N-body simulations (which we dub CSiBORG) and develop a Monte Carlo-based forward model for these offsets on a galaxy-by-galaxy basis. Using the measured offset between the optical centre and active galactic nucleus of 1916 galaxies from the literature, propagating uncertainties in the input quantities and marginalising over an empirical noise model describing astrophysical and observational noise, we constrain the Galileon coupling to be $\Delta G / G_{\rm N} < 0.16$ at $1\sigma$ confidence for Galileons with crossover scale $r_{\rm C} \gtrsim H_0^{-1}$.

Abstract:

We study the displacements between the centres of galaxies and their supermassive black holes (BHs) in the cosmological hydrodynamical simulation Horizon-AGN, and in a variety of observations from the literature. The BHs in Horizon-AGN feel a subgrid dynamical friction force, sourced by the surrounding gas, which prevents recoiling BHs being ejected from the galaxy. We find that (i) the fraction of spatially offset BHs increases with cosmic time, (ii) BHs live on prograde orbits in the plane of the galaxy with an orbital radius that decays with time but stalls near z = 0, and (iii) the magnitudes of offsets from the galaxy centres are substantially larger in the simulation than in observations. We attribute the stalling of the infall and excessive offset magnitudes to the fact that dynamical friction from stars and dark matter is not modelled in the simulation, and hence provide a way to improve the BH dynamics of future simulations.