Agus Suroso

Comment on “Gauss-Bonnet inflation”

Recently, an interesting inflationary scenario, named Gauss-Bonnet inflation, is proposed by Kanti et al. [1, 2]. In the model, there is no inflaton potential but the inflaton couples to the Guass-Bonnet term. In the case of quadratic coupling, they find inflation occurs with graceful exit. The scenario is attractive because of the natural set-up. However, we show there exists the gradient instability in the tensor perturbations in this inflationary model. We further prove the no-go theorem for the Gauss-Bonnet inflation without an inflaton potential.

Cosmological model with nonminimal derivative coupling of scalar fields in five dimensions

We study a nonminimal derivative coupling (NMDC) of scalar field, where the scalar field is coupled to curvature tensor in the five dimensional universal extra dimension model. We apply the Einstein equation and find its solution. First, we consider a special case of pure free scalar field without NMDC and we find that for static extradimension, the solution is equivalent to the standard cosmology with stiff matter. For a general case of pure free scalar field with NMDC, we find that the de Sitter solution is the solution of our model. For this solution, the scalar field evolves linearly in time. In the limit of small Hubble parameter, the general case give us the same solution as in the pure free scalar field. Finally, we perform a dynamical analysis to determine the stability of our model. We find that the extradimension, if it exist, can not be static and always shrinks with the expansion of four dimensional spacetime.

Some Aspects of Spherical Symmetric Extremal Dyonic Black Holes in 4d N = 1 Supergravity

In this paper, we study several aspects of extremal spherical symmetric black hole solutions of four-dimensional N = 1 supergravity coupled to vector and chiral multiplets with the scalar potential turned on. In the asymptotic region, the complex scalars are fixed and regular which can be viewed as the critical points of the black hole and the scalar potentials with vanishing scalar charges. It follows that the asymptotic geometries are of a constant and nonzero scalar curvature which are generally not Einstein. These spaces could also correspond to the near horizon geometries which are the product spaces of a two anti-de Sitter surface and the two sphere if the value of the scalars in both regions coincide. In addition, we prove the local existence of nontrivial radius dependent complex scalar fields which interpolate between the horizon and the asymptotic region. We finally give some simple ℂn-models with both linear superpotential and gauge couplings.

Nonminimal derivative coupling in five dimensional universal extra dimensions and recovering the cosmological constant

We study a nonminimal derivative coupling (NMDC) of scalar field, where the scalar field is coupled to curvature tensor in the five dimensional universal extra dimension (UED) model. We choose an ansatz metric which contain a homogenous extra spatial dimension and solve the Einstein equation in the bulk. Under two assumptions concerning the scalar field and scale factors, we get the de Sitter universe and recover the cosmological constant from the coupling constant of NMDC. We also discuss how the extra dimension evolve in time.

Accelerating universe from nonminimal derivative coupling in 5D universal extra dimension

We study a nonminimal derivative coupling of scalar field, where the scalar field is coupled to curvature tensor in the five dimensional bulk in the context of universal extra dimension model. We apply the Einstein equation in the bulk and show that the acelerated expanding solution for the four dimensional scale factor a(t) is exist. Then, we study four different cases of the solution which is correspond to four different types of evolution of the extra dimension scale factor b.

Power function inflation potential analysis for cosmological model with Gauss-Bonnet term

Inflation is still an interesting topic in the study of our universe. An interesting inflation scenario named Gauss-Bonnet inflation proposed without inflation potential has been shown unstable. In this work, we consider the general power function inflation potential, V(ϕ) = mϕ(n) in the model, then the solution is analyzed according to the inflation scenario. Using the stability condition, inflation potential with m positive and 0 ≤ n < 5 give proper solution for the inflation scenario.

Energy Conditions of the Five Dimensional with NMDC and Acelerating Universe

The Energy condition is studied for five dimensional cosmological model with nonminimal derivative coupling (NMDC) between scalar field and curvature tensor. We assume that the scale factors of three dimensional space (a(t)) and the extra dimension (b(t)) is related by b(t) = (a(t))γ, where γ is a constant. We apply the Null Energy Condition (NEC), Weak Energy Condition (WEC), Strong Energy Condition (SEC) and Dominant Energy Condition (DEC) to our model and investigate some constraint in order the energy condition violated. The constraint that we found is appropriate with cosmological model in which the four-dimensional universe expands with positive acceleration and the extra dimension decays.

Energy conditions for the four dimensional cosmological model with nonminimal derivative coupling of scalar field

The energy conditions is a set of linear equations of energy density ρ and pressure p which ensure the the field(s) that we used in our model is physically “reasonable”. We study the energy conditions for four dimensional nonminimal derivative coupling of scalar field and curvature tensor. Considering the scalar field as a perfect fluid, we find some constraint for the coupling constant ξ in order the energy conditions is satisfied or violated. We find that strong energy conditions (SEC) is violated if −1/9H2 ≤ ξ < 1/18H2. For de Sitter solution a ∝ eH0t for some constant H0, we find that while null, weak, and dominant energy conditions violated when ξ<−[12H02(2+9H02)]−1. The accelerating universe is exist for the power law solution (a ∝ tp for constant p) if ξ < 0.The energy conditions is a set of linear equations of energy density ρ and pressure p which ensure the the field(s) that we used in our model is physically “reasonable”. We study the energy conditions for four dimensional nonminimal derivative coupling of scalar field and curvature tensor. Considering the scalar field as a perfect fluid, we find some constraint for the coupling constant ξ in order the energy conditions is satisfied or violated. We find that strong energy conditions (SEC) is violated if −1/9H2 ≤ ξ < 1/18H2. For de Sitter solution a ∝ eH0t for some constant H0, we find that while null, weak, and dominant energy conditions violated when ξ<−[12H02(2+9H02)]−1. The accelerating universe is exist for the power law solution (a ∝ tp for constant p) if ξ < 0.

Cosmological perturbation analysis of nonminimal derivative coupling of scalar and tensor fields in five dimensions

Perturbation is studied for non-minimal derivative coupling between a scalar fields and curvature tensor in five dimensions. We apply this analysis in flat universal five dimensional spacetime, generate the field equations and consider special cases at early universe, to get an exact form of the perturbation parameters. The solutions is used to explain the coupling parameter of non-minimal derivative coupling with curvature tensor. We find that for case of static extradimension at early time, the coupling parameter is approximately have value of ξ ≈ 0, 0274.

Stability analysis of the nonminimal derivative coupling in five dimensional universal extra dimensions

We study a nonminimal derivative coupling (NMDC) of scalar field, where the scalar field is coupled to curvature tensor in the five dimensional universal extra dimension model. We apply the Einstein equation in and find the stability of our model. From the stability analysis, we find that the extradimension, if it exist, can not be static and always shrink with expansion of four dimensional spacetime.