Behavior Of Cosmological Perturbations Research Articles

Emergent universe and Genesis from the DHOST cosmology

In this article, we present an emergent universe scenario that can be derived from DHOST cosmology. The universe starts asymptotically Minkowski in the far past just like the regular Galileon Genesis, but evolves to a radiation dominated period at the late stage, and therefore, the universe has a graceful exit which is absent in the regular Galileon Genesis. We analyze the behavior of cosmological perturbations and show that both the scalar and tensor modes are free from the gradient instability problem. We further analyze the primordial scalar spectrum generated in various situations and discuss whether a scale invariance can be achieved.

A new approach to cosmological perturbations in f(R) models

We propose an analytic procedure that allows to determine quantitatively the deviation in the behavior of cosmological perturbations between a given f(R) modified gravity model and a ΛCDM reference model. Our method allows to study structure formation in these models from the largest scales, of the order of the Hubble horizon, down to scales deeply inside the Hubble radius, without employing the so-called “quasi-static” approximation. Although we restrict our analysis here to linear perturbations, our technique is completely general and can be extended to any perturbative order.

Exactly solvable model for cosmological perturbations in dilatonic brane worlds

We construct a model where cosmological perturbations are analytically solved based on dilatonic brane worlds. A bulk scalar field has an exponential potential in the bulk and an exponential coupling to the brane tension. The bulk scalar field yields a power-law inflation on the brane. The exact background metric can be found including the back-reaction of the scalar field. Then exact solutions for cosmological perturbations which properly satisfy the junction conditions on the brane are derived. These solutions provide us an interesting model to understand the connection between the behavior of cosmological perturbations on the brane and the geometry of the bulk. Using these solutions, the behavior of an anisotropic stress induced on the inflationary brane by bulk gravitational fields is investigated.

Damping of gravitational waves and density perturbations in the early Universe

Since the discovery of the large angular scale anisotropies in the microwave background radiation, the behaviour of cosmological perturbations (especially, density perturbations and gravitational waves) has been of great interest. In this study, after a detailed and rigorous treatment of the behaviour of gravitational waves in viscous cosmic media, we conclude that the damping of cosmological gravitational waves of long wavelengths is negligible for most cases of physical interest. A preliminary analysis suggests that similar results hold for density perturbations in the long wavelength limit. Therefore, long wavelength cosmological perturbations have not been practically affected by viscous processes,and are good probes of the very early Universe.