Cosmological dynamics with modified induced gravity on the normal DGP branch

In this Letter we investigate cosmological dynamics on the normal branch of a DGP-inspired scenario within a phase space approach where induced gravity is modified in the spirit of f(R)$f(R)$-theories. We apply the dynamical system analysis to achieve the stable solutions of the scenario in the normal DGP branch. Firstly, we consider a general form of the modified induced gravity and we show that there is a standard de Sitter point in phase space of the model. Then we prove that this point is stable attractor only for those f(R)$f(R)$ functions that account for late-time cosmic speed-up.     

A stabilized warped brane world with equal gravity strengths on the branes

We discuss a stabilized brane world model with two branes, allowing a solution to the hierarchy problem due to the warped extra dimension and having a remarkable feature: the strength of gravitational interaction is of the same order on both branes, contrary to the case of the Randall-Sundrum model with a hierarchical difference of the gravitational strength on the branes. The solution also admits the existence of two branes with equal gravity strengths. The article is published in the original.     

Tachyon-Chaplygin inflation on the brane

Tachyon-brane inflationary universe model in the context of a Chaplygin gas equation of state is studied. General conditions for this model to be realizable are discussed. In the high-energy limit and by using an exponential potential we describe in great details the characteristic of this model. Recent observational data from the Wilkinson Microwave Anisotropy Probe experiment are employed to restrict the parameters of the model.     

Intermediate inflation on the brane

Brane inflationary universe model in the context of intermediate inflation is studied. General conditions for this model to be realizable are discussed. In the high-energy limit we describe in great details the characteristic of this model.     

Tachyon-polytropic inflation on the brane

We consider the tachyon-brane inflationary universe model in the context of a polytropic gas equation of state. In slow-roll approximation, we discuss general conditions of this model. For exponential potential, in high-energy limit the characteristics of the model are presented. By using the seven-year Wilkinson Microwave Anisotropy Probe (WMAP7) observational data, we constrain the cosmological parameters of the model.     

Chaplygin inflation on the brane

Brane inflationary universe model in the context of a Chaplygin gas equation of state is studied. General conditions for this model to be realizable are discussed. In the high-energy limit and by using a chaotic potential we describe in great details the characteristic of this model. The parameters of the model are restricted by using recent astronomical observations.     

Modified gravity on the brane and dark energy

We analyze the dynamics of an AdS₅ braneworld with matter fields when gravity is allowed to deviate from the Einstein form on the brane. We consider exact five-dimensional warped solutions which are associated with conformal bulk fields of weight –4 and describe on the brane the following three dynamics: those of inhomogeneous dust, of generalized dark radiation, and of homogeneous polytropic dark energy. We show that, with modified gravity on the brane, the existence of such dynamical geometries requires the presence of non-conformal matter fields confined to the brane.     

Embedding of FRW cosmology in DGP scenario with a non-minimally coupled scalar field on the brane

We construct a DGP inspired braneworld scenario where a scalar field non-minimally coupled to the induced Ricci curvature is present on the brane. We show that this model allows for an embedding of the standard Friedmann cosmology in the sense that the cosmological evolution of the background metric on the brane can be described by the standard Friedmann equation.     

Inverse-scattering theory and the density perturbations from inflation

We show how to use inverse-scattering theory as the basis for the inflationary reconstruction program, the goal of which is to gain information about the physics which drives inflation. Inverse-scattering theory provides an effective and well-motivated procedure, having a sound mathematical basis and being of sufficient generality that it can be considered the foundation for a nonparametric reconstruction program. We show how simple properties of the power spectrum translate directly into statements about the evolution of the background geometry during inflation.     

Chaplygin gas in decelerating DGP gravity and the age of the oldest star

Accelerating Chaplygin gas combined with the decelerating braneworld Dvali–Gabadadze–Porrati (DGP) model can produce an overall accelerated expansion of the order of magnitude seen. Both models have similar asymptotic properties at early and late cosmic times, and are characterized by a length scale. Taking the length scales to be proportional one obtains a combined model with three free parameters, one more than the ΛCDM model, which fits supernovae data equally well. We further constrain it by the CMB shift parameter, and by requiring that the model yields a longer age of the Universe than that of the oldest star HE 1523-0901, t_∗=13.4±0.8(stat)±1.8(syst)$t_{\ast }=13.4\pm 0.8(\mathrm{stat})\pm 1.8(\mathrm{syst})$. In contrast to generalized DGP and Chaplygin gas models, this is a genuine alternative to the cosmological constant model because it does not reduce to it in any limit of the parameter space.     

Viscous modified gravity on a RS brane embedded in AdS<Subscript>5</Subscript>

We consider a modified gravity fluid on a Randall–Sundrum II brane situated at y=0, the action containing a power α of the scalar curvature. As is known from 4D spatially flat modified gravity, the presence of bulk viscosity may drive the cosmic fluid into the phantom region (w<−1) and thereafter inevitably into the Big Rip singularity, even if it is initially nonviscous and lies in the quintessence region (w>−1). The condition for this to occur is that the bulk viscosity contains the power (2α−1) of the scalar expansion. We combine this with the 5D RS II model, and we find that the Big Rip, occurring for α>1/2, carries over to the metric for the bulk metric, |y|>0. Actually, the scale factors on the brane and in the bulk become simply proportional to each other.     

Correlated perturbations from inflation and the cosmic microwave background

We compare the latest cosmic microwave background data with theoretical predictions including correlated adiabatic and cold dark matter (CDM) isocurvature perturbations with a simple power-law dependence. We find that there is a degeneracy between the amplitude of correlated isocurvature perturbations and the spectral tilt. A negative (red) tilt is found to be compatible with a larger isocurvature contribution. Estimates of the baryon and CDM densities are found to be almost independent of the isocurvature amplitude. The main result is that current microwave background data do not exclude a dominant contribution from CDM isocurvature fluctuations on large scales.     

On the cosmological viability of the Hu-Sawicki type modified induced gravity

It has been shown recently that the normal branch of a DGP braneworld scenario self-accelerates if the induced gravity on the brane is modified in the spirit of f(R) modified gravity. Within this viewpoint, we investigate cosmological viability of the Hu-Sawicki type modified induced gravity. Firstly, we present a dynamical system analysis of a general f(R)-DGP model. We show that in the phase space of the model, there exist three standard critical points; one of which is a de Sitter point corresponding to accelerating phase of the universe expansion. The stability of this point depends on the effective equation of state parameter of the curvature fluid. If we consider the curvature fluid to be a canonical scalar field in the equivalent scalar-tensor theory, the mentioned de Sitter phase is unstable, otherwise it is an attractor, stable phase. We show that the effective equation of state parameter of the model realizes an effective phantom-like behavior. A cosmographic analysis shows that this model, which admits a stable de Sitter phase in its expansion history, is a cosmologically viable scenario.     

Low-scale gravity mediation in warped extra dimension and collider phenomenology on hidden sector

We propose a new scenario of gravity-mediated supersymmetry breaking (gravity mediation) in a supersymmetric Randall-Sundrum model, where the gravity mediation takes place at a low scale due to the warped metric. We investigate collider phenomenology involving the hidden sector field, and find a possibility that the hidden sector field can be produced at the LHC and the ILC. The hidden sector may no longer be hidden. Talk presented by N Okada. The talk is based on the work of H Itoh, N Okada and T Yamashita, hep-ph/0606156.     

Constraints on brane inflation after Planck 2015: Impacts of the latest local measurement of the Hubble constant

We investigate the observational constraints on three typical brane inflation models by considering the latest local measurement of the Hubble constant in the global fit. We also employ other observational data, including the Planck 2015 CMB data, the BICEP2/Keck Array B-mode data, and the baryon acoustic oscillations data, in our analysis. Previous studies have shown that the addition of the latest local H_0 measurement favors a larger spectral index, and can exert a significant influence on the model selection of inflation. In this work, we investigate its impacts on the status of brane inflation models. We find that, when the direct H_0 measurement is considered, the prototype model of brane inflation is still in good agreement with the current observational data within the 2σ level range. For the KKLMMT model, the consideration of the H_0 measurement allows the range of the parameter β to be amplified to O(10~(-2)), which slightly alleviates the fine-tuning problem. For the IR DBI model, the addition of the H_0 measurement does not provide a better fit. These results show that the consideration of the new H_0 prior can exert a considerable influence on the brane inflation models. At last, we show that, when β≤1.1, the equilateral non-Gaussianity in the IR DBI inflation model is compatible with the current CMB data at the 1σ level.