Sanjay Jhingan

Delicate f(R) gravity models with a disappearing cosmological constant and observational constraints on the model parameters

We study the f(R) theory of gravity using the metric approach. In particular we investigate the recently proposed model by Hu and Sawicki; Appleby and Battye; and Starobinsky. In this model, the cosmological constant is zero in flat space time. The model passes both the solar system and the laboratory tests. But the model parameters need to be fine-tuned to avoid the finite time singularity recently pointed to in the literature. We check the concordance of this model with the H(z) and baryon acoustic oscillation data. We find that the model resembles the {lambda}CDM at high redshift. However, for some parameter values there are variations in the expansion history of the universe at low redshift.

Dark energy generated from a (super-) string effective action with higher-order curvature corrections and a dynamical dilaton

We investigate the possibility of a dark energy universe emerging from an action with higher-order string loop corrections to Einstein gravity in the presence of a massless dilaton. These curvature corrections (up to

Observational cosmology and the cosmic distance duality relation

R^4

Constraining f(R) gravity models with disappearing cosmological constant

order) are different depending upon the type of (super-) string model which is considered. We find in fact that type II, heterotic, and bosonic strings respond differently to dark energy. A dark energy solution is shown to exist in the case of the bosonic string, while the other two theories do not lead to realistic dark energy universes. A detailed analysis of the dynamical stability of the de Sitter solution is presented for the case of a bosonic string. A general prescription for the construction of a de Sitter solution for the low-energy (super-) string effective action is also indicated. Beyond the low-energy (super-) string effective action, when the higher-curvature correction coefficients depend on the dilaton, the reconstruction of the theory from the expansion history of the universe is done with a corresponding prescription for the scalar potentials.

The Lovelock gravity in the critical spacetime dimension

We study the validity of cosmic distance duality relation between angular diameter and luminosity distances. To test this duality relation we use the latest Union2 Supernovae Type Ia (SNe Ia) data for estimating the luminosity distance. The estimation of angular diameter distance comes from the samples of galaxy clusters (real and mock) and FRIIb radio galaxies. We parameterize the distance duality relation as a function of redshift in six different ways. Our results rule out some of the parameterizations significantly.

Cosmokinetics: A joint analysis of Standard Candles, Rulers and Cosmic Clocks

The f(R) gravity models proposed by Hu-Sawicki and Starobinsky are generic for local gravity constraints to be evaded. The large deviations from these models result in either violation of local gravity constraints or the modifications indistinguishable from cosmological constant. The curvature singularity in these models is generic but can be avoided, provided that proper fine-tuning is imposed on the evolution of scalaron in the high curvature regime. In principle, the problem can be circumvented by incorporating quadratic curvature correction in the Lagrangian, though it might be quite challenging to probe the relevant region numerically.

Cosmic distance duality and cosmic transparency

Abstract It is well known that the vacuum in the Einstein gravity, which is linear in the Riemann curvature, is trivial in the critical ( 2 + 1 = 3 ) dimension because vacuum solution is flat. It turns out that this is true in general for any odd critical d = 2 n + 1 dimension where n is the degree of homogeneous polynomial in Riemann defining its higher order analogue whose trace is the nth order Lovelock polynomial. This is the “curvature” for nth order pure Lovelock gravity as the trace of its Bianchi derivative gives the corresponding analogue of the Einstein tensor as defined by Dadhich (2010) [1] . Thus the vacuum in the pure Lovelock gravity is always trivial in the odd critical ( 2 n + 1 ) dimension which means it is pure Lovelock flat but it is not Riemann flat unless n = 1 and then it describes a field of a global monopole. Further by adding Λ we obtain the Lovelock analogue of the BTZ black hole.

Inhomogeneous dust collapse in D-5 Einstein-Gauss-Bonnet gravity

We study the accelerated expansion of the Universe by using the kinematic approach. In this context, we parameterize the deceleration parameter, q(z), in a model independent way. Assuming three simple parameterizations we reconstruct q(z). We do the joint analysis with combination of latest cosmological data consisting of standard candles (Supernovae Union2 sample), standard ruler (CMB/BAO), cosmic clocks (age of passively evolving galaxies) and Hubble (H(z)) data. Our results support the accelerated expansion of the Universe.

Exploring scalar field dynamics with Gaussian processes

We compare distance measurements obtained from two distance indicators, Supernovae observations (standard candles) and Baryon acoustic oscillation data (standard rulers). The Union2 sample of supernovae with BAO data from SDSS, 6dFGS and the latest BOSS and WiggleZ surveys is used in search for deviations from the distance duality relation. We find that the supernovae are brighter than expected from BAO measurements. The luminosity distances tend to be smaller then expected from angular diameter distance estimates as also found in earlier works on distance duality, but the trend is not statistically significant. This further constrains the cosmic transparency.

Bound orbits and gravitational theory

We consider a Lemaitre-Tolman-Bondi type space-time in Einstein gravity with the Gauss-Bonnet combination of quadratic curvature terms, and present an exact solution in closed form. It turns out that the presence of the coupling constant of the Gauss-Bonnet terms {alpha}>0 completely changes the causal structure of the singularities from the analogous general relativistic case. The gravitational collapse of inhomogeneous dust in the five-dimensional Gauss-Bonnet extended Einstein equations leads to formation of a massive, but weak, timelike singularity which is forbidden in general relativity. Interestingly, this is a counterexample to three conjectures, viz., cosmic censorship conjecture, hoop conjecture, and Seiferts conjecture.