Cosmological tests of gravity with latest observations

We perform observational tests of modified gravity on cosmological scales following model-dependent and model-independent approaches using the latest astronomical observations, including measurements of the local Hubble constant, cosmic microwave background, the baryonic acoustic oscillations and redshift space distortions derived from galaxy surveys including the SDSS BOSS and eBOSS, as well as the weak lensing observations performed by the CFHTLenS team. Using all data combined, we find a deviation from the prediction of general relativity in both the effective Newton's constant, $\mu(a,k)$, and in the gravitational slip, $\eta(a,k)$. The deviation is at a $3.1\sigma$ level in the joint $\{\mu(a,k),\eta(a,k)\}$ space using a two-parameter phenomenological model for $\mu$ and $\eta$, and it reaches a $3.7\sigma$ level if a general parametrisation is used. This signal, which may be subject to unknown observational systematics, or a sign of new physics, is worth further investigating with forthcoming observations.Comment: 12 pages, 8 figures; ApJ accepte

Acoustic guiding and subwavelength imaging with sharp bending by sonic crystal

A sharp bending scheme for the self-collimation of acoustic waves is proposed by simply truncating the sonic crystals. An all-angle and wide-band 90{\deg}-bending wave guide is demonstrated with nearly perfect transmissions for Gaussian beams at a wide range of incident angles. A 90{\deg}-bended imaging for a point source with a subwavelength resolution of 0 0.37{\lambda} is also realized by the proposed structure. These results will find applicability in the manipulation of acoustic waves by sonic crystals.Comment: 8 pages, 5 figure

Effect of Grain Size on Differential Desorption of Volatile Species and on Non-ideal MHD Diffusivity

We developed a chemical network for modeling the chemistry and non-ideal MHD effects from the collapsing dense molecular clouds to protostellar disks. First, we re-formulated the cosmic-ray desorption rate by considering the variations of desorption rate over the grain size distribution. We find that the differential desorption of volatile species is amplified by the grains larger than 0.1 $\mu$m, because larger grains are heated to a lower temperature by cosmic-rays and hence more sensitive to the variations in binding energies. As a result, atomic nitrogen N is $\sim$2 orders of magnitude more abundant than CO; N$_2$H$^+$ also becomes a few times more abundant than HCO$^+$ due to the increased gas-phase N$_2$. However, the changes in ionization fraction due to freeze-out and desorption only have minor effects on the non-ideal MHD diffusivities. Our chemical network confirms that the very small grains (VSGs: below a few 100 $\AA$) weakens the efficiency of both ambipolar diffusion and Hall effect. In collapsing dense cores, a maximum ambipolar diffusion is achieved when truncating the MRN size distribution at 0.1 $\mu$m, and for a maximum Hall effect, the truncation occurs at 0.04 $\mu$m. We conclude that the grain size distribution is crucial to the differential depletion between CO and N$_2$ related molecules, as well as to the non-ideal MHD diffusivities in dense cores.Comment: 15 pages, 11 figures; Submitted to MNRA

Fast and Accurate Computation of Time-Domain Acoustic Scattering Problems with Exact Nonreflecting Boundary Conditions

This paper is concerned with fast and accurate computation of exterior wave equations truncated via exact circular or spherical nonreflecting boundary conditions (NRBCs, which are known to be nonlocal in both time and space). We first derive analytic expressions for the underlying convolution kernels, which allow for a rapid and accurate evaluation of the convolution with $O(N_t)$ operations over $N_t$ successive time steps. To handle the onlocality in space, we introduce the notion of boundary perturbation, which enables us to handle general bounded scatters by solving a sequence of wave equations in a regular domain. We propose an efficient spectral-Galerkin solver with Newmark's time integration for the truncated wave equation in the regular domain. We also provide ample numerical results to show high-order accuracy of NRBCs and efficiency of the proposed scheme.Comment: 22 pages with 9 figure