Spectral Index in Curvaton Scenario

A red tilted primordial power spectrum is preferred by WMAP five-year data and a large positive local-type non-Gaussianity $f_{NL}$ might be observed as well. In this short note we find that a red tilted and large non-Gaussian primordial power spectrum cannot be naturally obtained in curvaton model, because $f_{NL}$ is related to the initial condition of inflation.Comment: 12 pages; refs adde

Weak Gravity Conjecture with Large Extra Dimensions

In the presence of large extra dimensions, the fundamental Planck scale can be much lower than the apparent four-dimensional Planck scale. In this setup, the weak gravity conjecture implies a much more stringent constraint on the UV cutoff for the U(1) gauge theory in four dimensions. This new energy scale may be relevant to LHC.Comment: 8 pages; Refs added, version for publication in PL

An analytic calculation of the growth index for f(R)f(R) dark energy model

We derive the analytic formula of the growth index for $f(R)$ dark energy model where the effect on the growth of matter density perturbation $\delta_m$ from modified gravity (MG) is encoded in the effective Newton coupling constant $G_{\rm eff}$ in MG (or equivalently $g\equiv {G_{\rm eff}/ G}$). Based on the analytic formula, we propose that the parameter $g$ can be directly figured out by comparing the observed growth rate $f_g\equiv d\ln\delta_m/d\ln a$ to the prediction of $f_g$ in general relativity.Comment: 5 pages, 1 figure; refs added; minor revised, version accepted by EPJ