ΛCDM Background Research Articles

Gravitational waves in bigravity cosmology

In this paper we study gravitational wave perturbations in a cosmological setting of bigravity which can reproduce the ΛCDM background and large scale structure. We show that in general gravitational wave perturbations are unstable and only for very fine tuned initial conditions such a cosmology is viable. We quantify this fine tuning. We argue that similar fine tuning is also required in the scalar sector in order to prevent the tensor instability to be induced by second order scalar perturbations. Finally, we show that due to this power law instability, models of bigravity can lead to a large tensor to scalar ratio even for low scale inflation.

Cosmological Solutions of Tensor–Vector Theories of Gravity by Varying the Space-Time–Matter Coupling Constant

We consider tensor–vector theories by varying the space-time–matter coupling constant (varying Einstein velocity) in a spatially flat FRW universe. We examine the dynamics of this model by dynamical system method assuming a ΛCDM background and we find some exact solutions by considering the character of critical points of the theory and their stability conditions. Then we reconstruct the potential V(A 2) and the coupling Z(A 2) by demanding a background ΛCDM cosmology. Also we set restrictions on the varying Einstein velocity to solve the horizon problem. This gives a selection rule for choosing the appropriate stable solution. We will see that it is possible to produce the background expansion history H(z) indicated by observations. Finally we will discuss the behavior of the speed of light (c E) for those solutions.

Constraints on dissipative unified dark matter

Modern cosmology suggests that the Universe contains two dark components — dark matter and dark energy — both unkown in laboratory physics and both lacking direct evidence. Alternatively, a unified dark sector, described by a single fluid, has been proposed. Dissipation is a common phenomenon in nature and it thus seems natural to consider models dominated by a viscous dark fluid. We focus on the study of bulk viscosity, as isotropy and homogeneity at large scales implies the suppression of shear viscosity, heat flow and diffusion. The generic ansatz ξ∝ρν for the coefficient of bulk viscosity (ρ denotes the mass/energy density), which for ν = −1/2 mimics the ΛCDM background evolution, offers excellent fits to supernova and H(z) data. We show that viscous dark fluids suffer from large contributions to the integrated Sachs-Wolfe effect (generalising a previous study by Li & Barrow) and a suppression of structure growth at small-scales (as seen from a generalized Meszaros equation). Based on recent observations, we conclude that viscous dark fluid models (with ξ∝ρν and neglecting baryons) are strongly challenged.

Parameterizing scalar-tensor theories for cosmological probes

We study the evolution of density perturbations for a class of f(R) models which closely mimic ΛCDM background cosmology. Using the quasi-static approximation, and the fact that these models are equivalent to scalar-tensor gravity, we write the modified Friedmann and cosmological perturbation equations in terms of the mass M of the scalar field. Using the perturbation equations, we then derive an analytic expression for the growth parameter γ in terms of M, and use our result to reconstruct the linear matter power spectrum. We find that the power spectrum at z ∼ 0 is characterized by a tilt relative to its General Relativistic form, with increased power on small scales. We discuss how one has to modify the standard, constant γ prescription in order to study structure formation for this class of models. Since γ is now scale and time dependent, both the amplitude and transfer function associated with the linear matter power spectrum will be modified. We suggest a simple parameterization for the mass of the scalar field, which allows us to calculate the matter power spectrum for a broad class of f(R) models.

Formula omitted] Cosmological solutions with varying speed of light

We consider f(R) modified gravity theories for describing varying speed of light in a spatially flat FRW model, and find some exact solutions. Also we examine the dynamics of this model by dynamical system method assuming a ΛCDM background and we find some exact solutions by considering the character of critical points of the theory in both formalisms. The behaviour of the speed of light is obtained.

Cosmological solutions of time varying speed of light theories

We consider scalar–tensor theory for describing varying speed of light in a spatially flat FRW space–time. We find some exact solutions in the metric and Palatini formalisms. Also we examine the dynamics of this theory by dynamical system method assuming a ΛCDM background and we find some exact solutions by considering the character of critical points of the theory in both formalisms. We show that for any attractor the form of non-minimal coupling coefficient is quadratic in terms of the scalar field Ψ. Also we show that only attractors of the de Sitter era satisfy the horizon criteria.