121,190 research outputs found
Measuring the cosmic proper distance from fast radio bursts
The cosmic proper distance is a fundamental distance in the Universe.
Unlike the luminosity and angular diameter distances, which correspond to the
angular size, the proper distance is the length of light path from the source
to observer. However, the proper distance has not been measured before. The
recent redshift measurement of a repeat fast radio burst (FRB) can shed light
on the proper distance. We show that the proper distance-redshift relation can
indeed be derived from dispersion measures (DMs) of FRBs with measured
redshifts. From Monte Carlo simulations, we find that about 500 FRBs with DM
and redshift measurements can tightly constrain the proper distance-redshift
relation. We also show that the curvature of our Universe can be constrained
with a model-independent method using this derived proper distance-redshift
relation and the observed angular diameter distances. Owing to the high event
rate of FRBs, hundreds of FRBs can be discovered in the future by upcoming
instruments. The proper distance will play an important role in investigating
the accelerating expansion and the geometry of the Universe.Comment: 9 pages, 4 figures, proof versio
Transonic rotor noise: Theoretical and experimental comparisons
Two complementary methods of describing the high speed rotor noise problem are discussed. The first method uses the second order transonic potential equation to define and characterize the nature of the aerodynamic and acoustic fields and to explain the appearance of radiating shock waves. The second employs the Ffowcs Williams and Hawkings equation to successfully calculate the acoustic far field. Good agreement between theoretical and experimental waveforms is shown for transonic hover tip Mach numbers from 0.8 to 0.9
Helicopter impulsive noise: Theoretical and experimental status
The theoretical and experimental status of helicopter impulsive noise is reviewed. The two major source mechanisms of helicopter impulsive noise are addressed: high-speed impulsive noise and blade-vortex interaction impulsive noise. A thorough physical explanation of both generating mechanism is presented together with model and full-scale measurements of the phenomena. Current theoretical prediction methods are compared with experimental findings of isolated rotor tests. The noise generating mechanism of high speed impulsive noise are fairly well understood - theory and experiment compare nicely over Mach number ranges typical of today's helicopters. For the case of blade-vortex interaction noise, understanding of noise generating mechanisms and theoretical comparison with experiment are less satisfactory. Several methods for improving theory-experiment are suggested
Quadrature entanglement and photon-number correlations accompanied by phase-locking
We investigate quantum properties of phase-locked light beams generated in a
nondegenerate optical parametric oscillator (NOPO) with an intracavity
waveplate. This investigation continuous our previous analysis presented in
Phys.Rev.A 69, 05814 (2004), and involves problems of continuous-variable
quadrature entanglement in the spectral domain, photon-number correlations as
well as the signatures of phase-locking in the Wigner function. We study the
role of phase-localizing processes on the quantum correlation effects. The
peculiarities of phase-locked NOPO in the self-pulsing instability operational
regime are also cleared up. The results are obtained in both the
P-representation as a quantum-mechanical calculation in the framework of
stochastic equations of motion, and also by using numerical simulation based on
the method of quantum state diffusion.Comment: Subm. to PR
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