arXiv subjects

Jian-hua He

Publications and source records attributed to Jian-hua He.

28 records · Page 2Linked to original sources

arXiv2015

Effective dark matter power spectra in $f(R)$ gravity

Using N-body simulations, we measure the power spectrum of the effective dark matter density field, which is defined through the modified Poisson equation in $f(R)$ cosmologies. We find that when compared to the conventional dark matter power spectrum, the effective power spectrum deviates more significantly from the $Λ$CDM model. For models with $f_{R0}=-10^{-4}$, the deviation can exceed 150\% while the deviation of the conventional matter power spectrum is less than 50\%. Even for models with $f_{R0}=-10^{-6}$, for which the conventional matter power spectrum is very close to the $Λ$CDM prediction, the effective power spectrum shows sizeable deviations. Our results indicate that traditional analyses based on the dark matter density field may seriously underestimate the impact of $f(R)$ gravity on galaxy clustering. We therefore suggest the use of the effective density field in such studies. In addition, based on our findings, we also discuss several possible methods of making use of the differences between the conventional and effective dark matter power spectra in $f(R)$ gravity to discriminate the theory from the $Λ$CDM model.

astro-ph.CO↗

arXiv2015

Cluster gas fraction as a test of gravity

We propose a new cosmological test of gravity, by using the observed mass fraction of X-ray emitting gas in massive galaxy clusters. The cluster gas fraction, believed to be a fair sample of the average baryon fraction in the Universe, is a well-understood observable, which has previously mainly been used to constrain background cosmology. In some modified gravity models, such as $f(R)$ gravity, gas temperature in a massive cluster is determined by the effective mass of that cluster, which can be larger than its true mass. On the other hand, X-ray luminosity is determined by the true gas density, which in both modified gravity and $Λ$CDM models depends mainly on $Ω_{\rm b}/Ω_{\rm m}$ and hence the true total cluster mass. As a result, the standard practice of combining gas temperatures and X-ray surface brightnesses of clusters to infer their gas fractions can, in modified gravity models, lead to a larger - in $f(R)$ gravity this can be $1/3$ larger - value of $Ω_{\rm b}/Ω_{\rm m}$ than that inferred from other observations such as the CMB. A quick calculation shows that the Hu-Sawicki $n=1$ $f(R)$ model with $|\bar{f}_{R0}|=3\sim5\times10^{-5}$ is in tension with the gas fraction data of the 42 clusters analysed by Allen et al. (2008). We also discuss the implications for other modified gravity models.

astro-ph.CO↗

arXiv2015

Effective Dark Matter Halo catalog in $f(R)$ gravity

We introduce the idea of {\it effective} dark matter halo catalog in $f(R)$ gravity, which is built using the {\it effective} density field. Using a suite of high resolution N-body simulations, we find that the dynamical properties of halos, such as the distribution of density, velocity dispersion, specific angular momentum and spin, in the effective catalog of $f(R)$ gravity closely mimic those in the $Λ$CDM model. Thus, when using effective halos, an $f(R)$ model can be viewed as a $Λ$CDM model. This effective catalog therefore provides a convenient way for studying the baryonic physics, the galaxy halo occupation distribution and even semi-analytical galaxy formation in $f(R)$ cosmologies.

astro-ph.CO↗

arXiv2014

Revisiting the screening mechanism in $f(R)$ gravity

We reexamine the screening mechanism in $f(R)$ gravity using N-body simulations. By explicitly examining the relation between the extra scalar field $δf_R$ and the gravitational potential $ϕ$ in the perturbed Universe, we find that the relation between these two fields plays an important role in understanding the screening mechanism. We show that the screening mechanism in $f(R)$ gravity depends mainly on the depth of the potential well, and find a useful condition for identifying unscreened halos in simulations. We also discuss the potential application of our results to real galaxy surveys.

astro-ph.CO↗

arXiv2014

Weighing neutrinos in $f(R)$ gravity in light of BICEP2

We constrain the neutrino mass in $f(R)$ gravity using the latest observations from the Planck, BAO and BICEP2 data. We find that the measurement on the B-modes can break the degeneracy between the massive neutrinos and the $f(R)$ gravity. We find a non-zero value of the Compton wavelengths $B_{0}$ at a $68\%$ confidence level for the $f(R)$ model in the presence of massive neutrinos when the BICEP2 data is used. Furthermore, the tension on the tensor-to-scalar ratios between the measured values from Plank and BICEP2 is significantly reconciled in our model.

astro-ph.CO↗

arXiv2014

Weighing Neutrinos in $f(R)$ gravity

We constrain the neutrino properties in $f(R)$ gravity using the latest observations from cosmic microwave background(CMB) and baryon acoustic oscillation(BAO) measurements. We first constrain separately the total mass of neutrinos $\sum m_ν$ and the effective number of neutrino species $N_{\rm eff}$. Then we constrain $N_{\rm eff}$ and $\sum m_ν$ simultaneously. We find $\sum m_ν<0.462 {\rm eV}$ at a $95\%$ confidence level for the combination of Planck CMB data, WMAP CMB polarization data, BAO data and high-$l$ data from the Atacama Cosmology Telescope and the South Pole Telescope. We also find $N_{\rm eff}=3.32^{+0.54}_{-0.51}$ at a $95\%$ confidence level for the same data set. When constraining $N_{\rm eff}$ and $\sum m_ν$ simultaneously, we find $N_{\rm eff}=3.58^{+0.72}_{-0.69}$ and $\sum m_ν<0.860{\rm eV}$ at a $95\%$ confidence level, respectively.

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arXiv2013

Revisiting the matter power spectra in $f(R)$ gravity

In this paper, we study the non-linear matter power spectrum in a specific family of $f(R)$ models that can reproduce the $Λ$CDM background expansion history, using high resolution $N$-body simulations based on the {\sc ecosmog} code. We measure the matter power spectrum in the range of $0.05h{\rm Mpc}^{-1} 10^{-3}$, we find no chameleon screening in dense regions at late times ($z<3$), which means that those models could be ruled out due to the factor-of-$1/3$ enhancement to the strength of Newtonian gravity. We also give the best-fit parameters for a generalised PPF fitting formula which works well for the models studied here.

astro-ph.CO↗

arXiv2012

Revisiting f(R) gravity models that reproduce $Λ$CDM expansion

We reconstruct an $f(R)$ gravity model that gives rise to the particular $Λ$CDM background evolution of the universe. We find well-defined, real-valued analytical forms for the $f(R)$ model to describe the universe both in the early epoch from the radiation to matter dominated eras and the late time acceleration period. We further examine the viability of the derived $f(R)$ model and find that it is viable to describe the evolution of the universe in the past and there does not exist the future singularity in the Lagrangian.

astro-ph.CO↗

arXiv2012

Testing $f(R)$ dark energy model with the large scale structure

In this work, we further investigate the family of $f(R)$ dark energy models that can exactly mimic the same background expansion history as that of the $Λ$CDM model. We study the large scale structure in the $f(R)$ gravity using the full set cosmological perturbation equations. We investigate the structure formation in both the spatially flat and curved Universe. We also confront our model with the latest observations and conduct a Markov Chain Monte Carlo analysis on the parameter space.

astro-ph.CO↗

arXiv2012

Modeling $f(R)$ gravity in terms of mass dilation rate

We review the conformal equivalence in describing the background expansion of the universe by $f(R)$ gravity both in the Jordan frame and the Einstein frame. In the Jordan frame, we present the general analytic expression for $f(R)$ models that have the same expansion history as the $Λ$CDM model. This analytic form can provide further insights on how cosmology can be used to test the $f(R)$ gravity at the largest scales. Moreover we present a systematic and self-consistent way to construct the viable $f(R)$ model in Jordan frame using the mass dilation rate function from the Einstein frame through the conformal transformation. In addition, we extend our study to the linear perturbation theories and we further exhibit the equivalence of the $f(R)$ gravity presented in the Jordan frame and Einstein frame in the perturbed space-time. We argue that this equivalence has solid physics root.

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