arXiv subjects
Publications and source records attributed to Xiangyun Fu.
In this paper, we study the BSW process of the slowly evaporating charged black hole. It can be found that the BSW process will also arise near black hole horizon when the evaporation of charged black hole is very slow. But now the background black hole does not have to be an extremal black hole, and it will be approximately an extremal black hole unless it is nearly a huge stationary black hole.
In this paper, we study the growth index of matter density perturbations for the power law model in $f(T)$ gravity. Using the parametrization $γ(z)=γ_0+γ_1 {z\over 1+z}$ for the growth index, which approximates the real evolution of $γ(z)$ very well, and the observational data of the growth factor, we find that, at $1σ$ confidence level, the power law model in $f(T)$ gravity is consistent with the observations, since the obtained theoretical values of $γ_0$ and $γ_1$ are in the allowed region.
The growth of matter perturbations in the $f(R)$ model proposed by Starobinsky is studied in this paper. Three different parametric forms of the growth index are considered respectively and constraints on the model are obtained at both the $1σ$ and $2σ$ confidence levels, by using the current observational data for the growth factor. It is found, for all the three parametric forms of the growth index examined, that the Starobinsky model is consistent with the observations only at the $2σ$ confidence level.
The unparticle has been suggested as a candidate of dark matter. We investigated the growth rate of the density perturbation for the unparticle dark matter in the flat Universe. First, we consider the model in which unparticle is the sole dark matter and find that the growth factor can be approximated well by $f=(1+3ω_u)Ω^γ_u$, where $ω_u$ is the equation of state of unparticle. Our results show that the presence of $ω_u$ modifies the behavior of the growth factor $f$. For the second model where unparticle co-exists with cold dark matter, the growth factor has a new approximation $f=(1+3ω_u)Ω^γ_u+αΩ_m$ and $α$ is a function of $ω_u$. Thus the growth factor of unparticle is quite different from that of usual dark matter. These information can help us know more about unparticle and the early evolution of the Universe.
We propose a parametrization for the growth index of the linear matter perturbations, $γ(z)=γ_0+\frac{z}{1+z}γ_1$. The growth factor of the perturbations parameterized as $Ω_m^γ$ is analyzed for both the $w$CDM model and the DGP model with our proposed form for $γ$. We find that $γ_1$ is negative for the $w$CDM model but is positive for the DGP model. Thus it provides another signature to discriminate them. We demonstrate that $Ω_m^γ$ with $γ$ taking our proposed form approximates the growth factor very well both at low and high redshfits for both kinds of models. In fact, the error is below 0.03% for the $Λ$CDM model and 0.18% for the DGP model for all redshifts when $Ω_{m0}=0.27$. Therefore, our parametrization may be robustly used to constrain the growth index of different models with the observational data which include points for redshifts ranging from 0.15 to 3.8, thus providing discriminative signatures for different models.
We study the linear growth of matter perturbations in the DGP model with the growth index $γ$ as a function of redshift. At the linear approximation: $γ(z)\approxγ_0+γ_0^\prime z $, we find that, for $0.2\leqΩ_{m,0}\leq0.35$, $γ_0$ takes the value from 0.658 to 0.671, and $γ_0^\prime$ ranges from 0.035 to 0.042. With three low redshift observational data of the growth factor, we obtain the observational constraints on $γ_0$ and $γ_0'$ for the $ΛCDM$ and DGP models and find that the observations favor the $ΛCDM$ model but at the $1σ$ confidence level both the $ΛCDM$ and DGP models are consistent with the observations.
We study the dynamics of a phantom scalar field dark energy interacting with dark matter in loop quantum cosmology (LQC). Two kinds of coupling of the form $α{ρ_m}{\dotϕ}$ (case I) and $3βH (ρ_ϕ+ρ_m)$ (case II) between the phantom energy and dark matter are examined with the potential for the phantom field taken to be exponential. For both kinds of interactions, we find that the future singularity appearing in the standard FRW cosmology can be avoided by loop quantum gravity effects. In case II, if the phantom field is initially rolling down the potential, the loop quantum effect has no influence on the cosmic late time evolution and the universe will accelerate forever with a constant energy ratio between the dark energy and dark matter.
We study the Brownian motion of a charged test particle driven by quantum electromagnetic fluctuations in the vacuum region near a non-dispersive and non-absorbing dielectric half-space and calculate the mean squared fluctuations in the velocity of the test particle. Our results show that a nonzero susceptibility of the dielectrics has its imprints on the velocity dispersions of the test particles. The most noteworthy feature in sharp contrast to the case of an idealized perfectly conducting interface is that the velocity dispersions in the parallel directions are no longer negative and does not die off in time, suggesting that the potentially problematic negativeness of the dispersions in those directions in the case of perfect conductors is just a result of our idealization and does not occur for real material boundaries.