8,840 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
Q-CSMA: Queue-Length Based CSMA/CA Algorithms for Achieving Maximum Throughput and Low Delay in Wireless Networks
Recently, it has been shown that CSMA-type random access algorithms can
achieve the maximum possible throughput in ad hoc wireless networks. However,
these algorithms assume an idealized continuous-time CSMA protocol where
collisions can never occur. In addition, simulation results indicate that the
delay performance of these algorithms can be quite bad. On the other hand,
although some simple heuristics (such as distributed approximations of greedy
maximal scheduling) can yield much better delay performance for a large set of
arrival rates, they may only achieve a fraction of the capacity region in
general. In this paper, we propose a discrete-time version of the CSMA
algorithm. Central to our results is a discrete-time distributed randomized
algorithm which is based on a generalization of the so-called Glauber dynamics
from statistical physics, where multiple links are allowed to update their
states in a single time slot. The algorithm generates collision-free
transmission schedules while explicitly taking collisions into account during
the control phase of the protocol, thus relaxing the perfect CSMA assumption.
More importantly, the algorithm allows us to incorporate mechanisms which lead
to very good delay performance while retaining the throughput-optimality
property. It also resolves the hidden and exposed terminal problems associated
with wireless networks.Comment: 12 page
Optimal entanglement generation in cavity QED with dissipation
We investigate a two-level atom coupled to a cavity with a strong classical
driving field in a dissipative environment and find an analytical expression of
the time evolution density matrix for the system. The analytical density
operator is then used to study the entanglement between the atom and cavity by
considering the competing process between the atom-field interactions and the
field-environment interactions. It is shown that there is an optimal
interaction time for generating atom-cavity entanglement.Comment: 9 pages, 7 figure
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