10,966 research outputs found
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, , and in the
gravitational slip, . The deviation is at a level in the
joint space using a two-parameter phenomenological
model for and , and it reaches a 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 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 2 orders of magnitude more abundant
than CO; NH also becomes a few times more abundant than HCO due to
the increased gas-phase N. 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 ) 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 m, and for a
maximum Hall effect, the truncation occurs at 0.04 m. We conclude that the
grain size distribution is crucial to the differential depletion between CO and
N 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
operations over 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
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