Bob Osano

How close can an inhomogeneous universe mimic the concordance model

Recently, spatially inhomogeneous cosmological models have been proposed as an alternative to the ΛCDM model, with the aim of reproducing the late time dynamics of the Universe without introducing a cosmological constant or dark energy. This paper investigates the possibility of distinguishing such models from the standard ΛCDM using background or large scale structure data. It also illustrates and emphasizes the necessity of testing the Copernican principle in order to confront the tests of general relativity with the large scale structure.

Gravitational waves generated by second order effects during inflation

The generation of gravitational waves during inflation due to the non-linear coupling of scalar and tensor modes is discussed. Two methods describing gravitational wave perturbations are used and compared: a covariant and local approach, as well as a metric based analysis based on the Bardeen formalism. An application to slow-roll inflation is also described.

Cosmic Electromagnetic Fields due to Perturbations in the Gravitational Field

We use nonlinear gauge-invariant perturbation theory to study the interaction of an inflation produced seed magnetic field with density and gravitational wave perturbations in an almost Friedmann-Lema\^{\i}tre-Robertson-Walker spacetime with zero spatial curvature. We compare the effects of this coupling under the assumptions of poor conductivity, perfect conductivity and the case where the electric field is sourced via the coupling of velocity perturbations to the seed field in the ideal magnetohydrodynamic regime, thus generalizing, improving on and correcting previous results. We solve our equations for long wavelength limits and numerically integrate the resulting equations to generate power spectra for the electromagnetic field variables, showing where the modes cross the horizon. We find that the interaction can seed electric fields with nonzero curl and that the curl of the electric field dominates the power spectrum on small scales, in agreement with previous arguments.

The Decoupling of Scalar-Modes from a Linearly Perturbed Dust-Filled Bianchi Type-I Model

We study linear perturbations about a dust filled Bianchi type-I model with the vorticity set to zero. In comparison to linear perturbations about the Friedmann—Lemaitre—Robertson—Walker models, modes of perturbations about the Bianchi type-I models are coupled. We find that the tensor that represents the background shear needs to be degenerate in order for the scalar-mode perturbations to decouple from the rest of the flow.

Toward the analogue of a thermally generated electromagnetic field

The evolution equations for magnetic and vorticity fields are known to display the same pattern when dissipation and sources terms are neglected. We investigate the analogy between the two fields for non-vanishing dissipation and sources. In addition to the magnetic Reynolds (ReM) and Prandtl (PrM) numbers, we define a new number (SM) that is given by the ratio of the diffusive term to the Biermann battery term and which allows for a different classification of magnetized fluid behavior. Numerical simulations of the two fields are then carried out given a parameter space made of Reynolds, Prandtl, and source numbers. We find it appropriate to present and discuss the findings against Prandtl numbers given that these provide the link between viscous and magnetic diffusion. Our simulations indicate that there exists a range of Prandtl numbers for which the fields remain analogues which raises the important question of how far the analogy goes and also raises the prospect of a theory of analogue magnetism.

Dynamo Theory, Nonlinear Magnetic Fields, and the Euler Potentials

The debate on the role of Euler potentials (EP), , in the dynamo theory is a long-standing one. It is known that the cross-product of gradients of two the potentials may represent magnetic fields lines. However, 2D and 3D dynamo hydromagnetic simulations suggest that their utility as analogues of magnetic field potential is restricted. This raises questions about their utility in the broader context of magneto-genesis and dynamo theories. We reexamine this and find that a reinterpretation of such potentials offers a new insight into the role EP may play in the general evolution of magnetic fields.