Unruh effect for a Dvali-Gabadadze-Porrati Brane

In braneworld cosmology the brane accelerates in the bulk, and hence it perceives Unruh radiations in the bulk. We discuss the Unruh effect for a Dvali-Gabadadze-Porrati (DGP) brane. We find that the Unruh temperature is proportional to the acceleration of the brane, but chemical potential appears in the distribution function for massless modes. The Unruh temperature does not vanish even at the limit $r_c\to \infty$, which means the gravitational effect of the 5th dimension vanishes. The Unruh temperature equals the $geometric$ temperature when the the density of matter on the brane goes to zero for branch $\epsilon=1$, no matter what the value of the cross radius $r_c$ and the spatial curvature of the brane take. And if the state equation of the matter on the brane satisfies $p=-\rho$, the Unruh temperature always equals the geometric temperature of the brane for both the two branches, which is also independent of the cross radius and the spatial curvature. The Unruh temperature is always higher than geometric temperature for a dust dominated brane.Comment: 11 pages, 2 eps figures, the Green function for DGP brane is correcte

A unification of RDE model and XCDM model

In this Letter, we propose a new generalized Ricci dark energy (NGR) model to unify Ricci dark energy (RDE) and XCDM. Our model can distinguish between RDE and XCDM by introducing a parameter $\beta$ called weight factor. When $\beta=1$, NGR model becomes the usual RDE model. The XCDM model is corresponding to $\beta=0$. Moreover, NGR model permits the situation where neither $\beta=1$ nor $\beta=0$. We then perform a statefinder analysis on NGR model to see how $\beta$ effects the trajectory on the $r-s$ plane. In order to know the value of $\beta$, we constrain NGR model with latest observations including type Ia supernovae (SNe Ia) from Union2 set (557 data), baryonic acoustic oscillation (BAO) observation from the spectroscopic Sloan Digital Sky Survey (SDSS) data release 7 (DR7) galaxy sample and cosmic microwave background (CMB) observation from the 7-year Wilkinson Microwave Anisotropy Probe (WMAP7) results. With Markov Chain Monte Carlo (MCMC) method, the constraint result is $\beta$=$0.08_{-0.21}^{+0.30}(1\sigma)_{-0.28}^{+0.43}(2\sigma)$, which manifests the observations prefer a XCDM universe rather than RDE model. It seems RDE model is ruled out in NGR scenario within $2\sigma$ regions. Furthermore, we compare it with some of successful cosmological models using AIC information criterion. NGR model seems to be a good choice for describing the universe.Comment: 12 pages, 7 figures, 2 tables. Accepted for publication in PL

Constraints on cosmological models from lens redshift data

Strong lensing has developed into an important astrophysical tool for probing both cosmology and galaxies (their structures, formations, and evolutions). Now several hundreds of strong lens systems produced by massive galaxies have been discovered, which may form well-defined samples useful for statistical analyses. To collect a relatively complete lens redshift data from various large systematic surveys of gravitationally lensed quasars and check the possibility to use it as a future complementarity to other cosmological probes. We use the distribution of gravitationally-lensed image separations observed in the Cosmic Lens All-Sky Survey (CLASS), the PMN-NVSS Extragalactic Lens Survey (PANELS), the Sloan Digital Sky Survey (SDSS) and other surveys, considering a singular isothermal ellipsoid (SIE) model for galactic potentials as well as improved new measurements of the velocity dispersion function of galaxies based on the SDSS DR5 data and recent semi-analytical modeling of galaxy formation, to constrain two dark energy models ($\Lambda$CDM and constant $w$) under a flat universe assumption. We find that the current lens redshift data give a relatively weak constraint on the model parameters. However, by combing the redshift data with the baryonic acoustic oscillation peak and the comic macrowave background data, we obtain more stringent results, which show that the flat $\Lambda$ CDM model is still included at 1$\sigma$.Comment: 18 pages, 6 figures, 1 table, A&A accepte

Constraints on f(R) cosmologies from strong gravitational lensing systems

f(R) gravity is thought to be an alternative to dark energy which can explain the acceleration of the universe. It has been tested by different observations including type Ia supernovae (SNIa), the cosmic microwave background (CMB), the baryon acoustic oscillations (BAO) and so on. In this Letter, we use the Hubble constant independent ratio between two angular diameter distances $D=D_{ls}/D_s$ to constrain f(R) model in Palatini approach $f(R)=R-\alpha H^2_0(-\frac{R}{H^2_0})^\beta$. These data are from various large systematic lensing surveys and lensing by galaxy clusters combined with X-ray observations. We also combine the lensing data with CMB and BAO, which gives a stringent constraint. The best-fit results are $(\alpha,\beta)=(-1.50,0.696)$ or $(\Omega_m,\beta)=(0.0734,0.696)$ using lensing data only. When combined with CMB and BAO, the best-fit results are $(\alpha,\beta)=(-3.75,0.0651)$ or $(\Omega_m,\beta)=(0.286,0.0651)$. If we further fix $\beta=0$ (corresponding to $\Lambda$CDM), the best-fit value for $\alpha$ is $\alpha$=$-4.84_{-0.68}^{+0.91}(1\sigma)_{-0.98}^{+1.63}(2\sigma)$ for the lensing analysis and $\alpha$=$-4.35_{-0.16}^{+0.18}(1\sigma)_{-0.25}^{+0.3}(2\sigma)$ for the combined data, respectively. Our results show that $\Lambda$CDM model is within 1$\sigma$ range.Comment: 9 pages, 2 figures, 2 table

Measuring dark matter-neutrino relative velocity on cosmological scales

We present a new method to measure neutrino masses using the dark matter-neutrino relative velocity. The relative motion between dark matter and neutrinos results in a parity-odd bispectrum which can be measured from cross-correlation of different cosmic fields. This new method is not affected by most systematics which are parity even and not limited by the knowledge of optical depth to the cosmic microwave background. We estimate the detectability of the relative velocity effect and find that the minimal sum of neutrino masses could be detected at high significance with upcoming surveys.Comment: 6 pages, 2 figures, 1 table, published versio