Jian-hua He

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

In this paper, we study the nonlinear matter power spectrum in a specific family of f(R) models that can reproduce the ΛCDM background expansion history, using high resolution N-body simulations based on the ecosmog code. We measure the matter power spectrum in the range of 0.05h  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 generalized parametrized post-Friedman fitting formula, which works well for the models studied here.

Can background cosmology hold the key for modified gravity tests

Modified gravity theories are a popular alternative to dark energy as a possible explanation for the observed accelerating cosmic expansion, and their cosmological tests are currently an active research field. Studies in recent years have been increasingly focused on testing these theories in the nonlinear regime, which is computationally demanding. Here we show that, under certain circumstances, a whole class of theories can be ruled out by using background cosmology alone. This is possible because certain classes of models (i) are fundamentally incapable of producing specific background expansion histories, and (ii) said histories are incompatible with local gravity tests. As an example, we demonstrate that a popular class of models, f(R) gravity, would not be viable if observations suggest even a slight deviation of the background expansion history from that of the ΛCDM paradigm.

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 (the mass that would have produced the same gravitational effect assuming standard gravity as the cluster actually does in f(R) gravity) 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-cold-dark-matter models depends mainly on Ωb/Ωm 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 Ωb/Ωm than that inferred from other observations such as the cosmic microwave background. Our quick calculation shows that the Hu–Sawicki n = 1 f(R) model with |f¯R0|=5×10−5|f¯R0|=5×10−5 is in tension with the gas fraction data of the 42 clusters analysed by Allen et al. We also discuss the implications for other modified gravity models.

Revisiting the screening mechanism in

We reexamine the screening mechanism in

f(R)

f(R)

gravity

gravity using N-body simulations. By explicitly examining the relation between the extrascalar field

Effective Dark Matter Halo Catalog in f(R) Gravity.

\ensuremath{\delta}{f}_{R}

Speeding up

and the gravitational potential

N

\ensuremath{\phi}

-body simulations of modified gravity: Chameleon screening models

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