1,156 research outputs found

### 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

### Testing the phenomenological interacting dark energy with observational $H(z)$ data

In order to test the possible interaction between dark energy and dark
matter, we investigate observational constraints on a phenomenological
scenario, in which the ratio between the dark energy and matter densities is
proportional to the power law case of the scale factor, $r\equiv
(\rho_X/\rho_m)\propto a^{\xi}$. By using the Markov chain Monte Carlo method,
we constrain the phenomenological interacting dark energy model with the newly
revised $H(z)$ data, as well as the cosmic microwave background (CMB)
observation from the 7-year Wilkinson Microwave Anisotropy Probe (WMAP7)
results, the baryonic acoustic oscillation (BAO) observation from the
spectroscopic Sloan Digital Sky Survey (SDSS) data release 7 (DR7) galaxy
sample and the type Ia supernovae (SNe Ia) from Union2 set. The best-fit values
of the model parameters are
$\Omega_{m0}=0.27_{-0.02}^{+0.02}(1\sigma)_{-0.03}^{+0.04}(2\sigma)$,
$\xi=3.15_{-0.50}^{+0.48}(1\sigma)_{-0.71}^{+0.72}(2\sigma)$, and
$w_X=-1.05_{-0.14}^{+0.15}(1\sigma)_{-0.21}^{+0.21}(2\sigma)$, which are more
stringent than previous results. These results show that the standard
$\Lambda$CDM model without any interaction remains a good fit to the recent
observational data; however, the interaction that the energy transferring from
dark matter to dark energy is slightly favored over the interaction from dark
energy to dark matter. It is also shown that the $H(z)$ data can give more
stringent constraints on the phenomenological interacting scenario when
combined to CMB and BAO observations, and the confidence regions of
$H(z)$+BAO+CMB, SNe+BAO+CMB, and $H(z)$+SNe+BAO+CMB combinations are consistent
with each other.Comment: 6 pages, 4 figures, 1 table. MNRAS in pres

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