Constraints on Dirac-Born-Infeld type dark energy models from varying alpha

We study the variation of the effective fine structure constant alpha for Dirac-Born-Infeld (DBI) type dark energy models. The DBI action based on string theory naturally gives rise to a coupling between gauge fields and a scalar field responsible for accelerated expansion of the universe. This leads to the change of alpha due to a dynamical evolution of the scalar field, which can be compatible with the recently observed cosmological data around the redshift $\tilde{z} \lesssim 3$. We place constraints on several different DBI models including exponential, inverse power-law and rolling massive scalar potentials. We find that these models can satisfy the varying alpha constraint provided that mass scales of the potentials are fine-tuned. When we adopt the mass scales which are motivated by string theory, both exponential and inverse power-law potentials give unacceptably large change of alpha, thus ruled out from observations. On the other hand the rolling massive scalar potential is compatible with the observationally allowed variation of alpha. Therefore the information of varying alpha provides a powerful way to distinguish between a number of string-inspired DBI dark energy models.Comment: 11 pages, 6 figure

Generation of electromagnetic fields in string cosmology with a massive scalar field on the anti D-brane

We study the generation of electromagnetic fields in a string-inspired scenario associated with a rolling massive scalar field $\phi$ on the anti-D3 branes of KKLT de Sitter vacua. The 4-dimensional DBI type effective action naturally gives rise to the coupling between the gauge fields and the inflaton $\phi$, which leads to the production of cosmological magnetic fields during inflation due to the breaking of conformal invariance. We find that the amplitude of magnetic fields at decoupling epoch can be larger than the limiting seed value required for the galactic dynamo. We also discuss the mechanism of reheating in our scenario and show that gauge fields are sufficiently enhanced for the modes deep inside the Hubble radius with an energy density greater than that of the inflaton.Comment: 4 pages and 2 eps figures, minor clarifications added and typos correcte

Prospects of inflation with perturbed throat geometry

We study brane inflation in a warped deformed conifold background that includes general possible corrections to the throat geometry sourced by coupling to the bulk of a compact Calabi-Yau space. We focus specifically, on the perturbation by chiral operator of dimension 3/2 in the CFT. We find that the effective potential in this case can give rise to required number of e-foldings and the spectral index $n_S$ consistent with observation. The tensor to scalar ratio of perturbations is generally very low in this scenario. The COBE normalization, however, poses certain difficulties which can be circumvented provided model parameters are properly fine tuned. We find the numerical values of parameters which can give rise to enough inflation, observationally consistent values of density perturbations, scalar to tensor ratio of perturbations and the spectral index $n_S$.Comment: 7 pages and nine figures; typos corrected, minor comments and clarifications added, revised version to appear in PL

Constraining f(R)f(R) gravity models with disappearing cosmological constant

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 either result into violation of local gravity constraints or the modifications are not distinguishable 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.Comment: 9 pages and 4 figures, minor clarifications and corrections added, final version to appear in PR

Holographic field theory models of dark energy in interaction with dark matter

We discuss two lagrangian interacting dark energy models in the context of the holographic principle. The potentials of the interacting fields are constructed. The models are compared with CMB distance information, baryonic acoustic oscilations, lookback time and the Constitution supernovae sample. For both models the results are consistent with a non vanishing interaction between dark sectors - with more than three standard deviations of confidence for one of them. Moreover, in both cases, the sign of coupling is consistent with dark energy decaying into dark matter, alleviating the coincidence problem.Comment: arXiv admin note: substantial text overlap with arXiv:0912.399

Quintessential Inflation in a thawing realization

We study quintessential inflation with an inverse hyperbolic type potential $V(\phi) = {V_0}/{\cosh \left( {\phi^n}/{\lambda^n} \right)}$, where $V_0$, $\lambda$ and "n" are parameters of the theory. We obtain a bound on $\lambda$ for different values of the parameter n. The spectral index and the tensor-to-scalar-ratio fall in the $1 \sigma$ bound given by the Planck 2015 data for $n \geq 5$ for certain values of $\lambda$. However for $3 \leq n < 5$ there exist values of $\lambda$ for which the spectral index and the tensor-to-scalar-ratio fall only within the $2 \sigma$ bound of the Planck data. Furthermore, we show that the scalar field with the given potential can also give rise to late time acceleration if we invoke the coupling to massive neutrino matter. We also consider the instant preheating mechanism with Yukawa interaction and put bounds on the coupling constants for our model using the nucleosynthesis constraint on relic gravity waves produced during inflation.Comment: 11 page