Search

We discuss axion dark matter detection via two mechanisms: spontaneous decays and resonant conversion in neutron star magnetospheres. For decays, we show that the brightness temperature signal, rather than flux, is a less ambiguous measure for selecting candidate objects. This is owing principally to the finite beam width of telescopes which prevents one from being sensitive to the total flux from the object. With this in mind, we argue that the large surface-mass-density of the galactic centre or the Virgo cluster centre offers the best chance of improving current constraints on the axion-photon coupling via spontaneous decays. For the neutron star case, we first carry out a detailed study of mixing in magnetised plasmas. We derive transport equations for the axion-photon system via a controlled gradient expansion, allowing us to address inhomogeneous mass-shell constraints for arbitrary momenta. We then derive a non-perturbative Landau-Zener formula for the conversion probability valid across the range of relativistic and non-relativistic axions and show that the standard perturbative resonant conversion amplitude is a truncation of this result in the non-adiabatic limit. Our treatment reveals that that infalling dark matter axions typically convert non-adiabatically in magnetospheres. We describe the limitations of one-dimensional mixing equations and explain how three-dimensional effects activate new photon polarisations, including longitudinal modes and illustrate these arguments with numerical simulations in higher dimensions. We find that the bandwidth of the radio signal is dominated by Doppler broadening from the relative motion of the neutron star with respect to the observer. Therefore, we conclude that the radio signal from the resonant decay is weaker than previously thought, which means one relies on local density peaks to probe weaker axion-photon couplings., Comment: 31 pages, 14 figures and 2 tables. Accepted by Phys. Rev. D. Comments welcome

We review the Equation of State (EoS) approach to dark sector perturbations and apply it to $f(\mathcal{R})$ gravity models of dark energy. We show that the EoS approach is numerically stable and use it to set observational constraints on designer models. Within the EoS approach we build an analytical understanding of the dynamics of cosmological perturbations for the designer class of $f(\mathcal{R})$ gravity models, characterised by the parameter $B_0$ and the background equation of state of dark energy $w$. When we use the Planck Cosmic Microwave Background (CMB) temperature anisotropy, polarisation and lensing data as well as the Baryonic Acoustic Oscillation (BAO) data from SDSS and WiggleZ, we find $B_0

We study the nature of potentials in scalar field based models for dark energy - with both canonical and noncanonical kinetic terms. We calculate numerically, and using an analytic approximation around $a\approx 1$, potentials for models with constant equation-of-state parameter, $w_{\phi}$. We find that for a wide range of models with canonical and noncanonical kinetic terms there is a simple approximation for the potential that holds when the scale factor is in the range $0.6\lesssim a\lesssim 1.4$. We discuss how this form of the potential can also be used to represent models with non-constant $w_{\phi}$ and, hence, how it could be used in reconstruction from cosmological data., Comment: 17 pages, 6 figures. Accepted by Phys. Rev. D

We examine the dynamics of scalar perturbations in closed Friedmann-Lema\^itre-Robertson- Walker (FLRW) universes in the framework of Brane World theory with a timelike extra dimension. In this scenario, the unperturbed Friedmann equations contain additional terms arising from the bulk-brane interaction that implement non-singular bounces in the models with a cosmological constant and non-interacting perfect fluids. The structure of the phase-space of the models allows for two basic configurations, namely, one bounce solutions or eternal universes. Assuming that the matter content of the model is given by dust and radiation, we derive the dynamical field equations for scalar hydrodynamical perturbations considering either a conformally flat (de Sitter) bulk or a perturbed bulk. We perform a numerical analysis which can shed some light on the study of cosmological scalar perturbations in bouncing brane world models. From a mathematical point of view we show that although the bounce enhances the amplitudes of scalar perturbations for one bounce models in the case of a de Sitter bulk, the amplitudes of the perturbations remain sufficiently small and bounded relative to the background values up to a certain scale. For one bounce models in the case of a perturbed bulk the amplitudes of all perturbations (apart from the Weyl fluid energy density) remain sufficiently small and bounded relative to the background values for any scale of the perturbations. We also discuss and compare the stability and bounded behaviour of the perturbations in the late accelerated phase of one bounce solutions. For eternal universes we argue that some of these features are maintained only for early times (typically of the order of the first bounce). In this sense we show that eternal solutions are highly unstable configurations considering the background model of this paper., Comment: 15 pages, 29 figures. Accepted for publication in Physical Review D