417 research outputs found
Gravitational strings. Do we see one?
I present a class of objects called gravitational strings (GS) for their
similarity to the conventional cosmic strings: even though the former are just
singularities in flat spacetime, both varieties are equally "realistic", they
may play equally important cosmological r\^ole and their lensing properties are
akin. I argue that the enigmatic object CSL-1 is an evidence in favor of the
existence of GS.Comment: The published version. Minor correction
Modelling of the evolution of a droplet cloud in a turbulent flow
The effects of droplet inertia and turbulent mixing on the droplet number density distribution in a turbulent flow field are studied. A formulation of the turbulent convective diffusion equation for the droplet number density, based on the modified Fully Lagrangian Approach, is proposed. The Fully Lagrangian Approach for the dispersed phase is extended to account for the Hessian of transformation from Eulerian to Lagrangian variables. Droplets with moderate inertia are assumed to be transported and dispersed by large scale structures of a filtered field in the Large Eddy Simulation (LES) framework. Turbulent fluctuations, not visible in the filtered solution for the droplet velocity field, induce an additional diffusion mass flux and hence additional dispersion of the droplets. The Lagrangian formulation of the transport equation for the droplet number density and the modified Fully Lagrangian Approach (FLA) make it possible to resolve the flow regions with intersecting droplet trajectories in the filtered flow field. Thus, we can cope successfully with the problems of multivalued filtered droplet velocity regions and caustic formation. The spatial derivatives for the droplet number density are calculated by projecting the FLA solution on the Eulerian mesh, resulting in a hybrid Lagrangian–Eulerian approach to the problem. The main approximations for the method are supported by the calculation of droplet mixing in an unsteady one-dimensional flow field formed by large-scale oscillations with an imposed small-scale modulation. The results of the calculations for droplet mixing in decaying homogeneous and isotropic turbulence are validated by the results of Direct Numerical Simulations (DNS) for several values of the Stokes number
Colloquium: Comparison of Astrophysical and Terrestrial Frequency Standards
We have re-analyzed the stability of pulse arrival times from pulsars and
white dwarfs using several analysis tools for measuring the noise
characteristics of sampled time and frequency data. We show that the best
terrestrial artificial clocks substantially exceed the performance of
astronomical sources as time-keepers in terms of accuracy (as defined by cesium
primary frequency standards) and stability. This superiority in stability can
be directly demonstrated over time periods up to two years, where there is high
quality data for both. Beyond 2 years there is a deficiency of data for
clock/clock comparisons and both terrestrial and astronomical clocks show equal
performance being equally limited by the quality of the reference timescales
used to make the comparisons. Nonetheless, we show that detailed accuracy
evaluations of modern terrestrial clocks imply that these new clocks are likely
to have a stability better than any astronomical source up to comparison times
of at least hundreds of years. This article is intended to provide a correct
appreciation of the relative merits of natural and artificial clocks. The use
of natural clocks as tests of physics under the most extreme conditions is
entirely appropriate; however, the contention that these natural clocks,
particularly white dwarfs, can compete as timekeepers against devices
constructed by mankind is shown to be doubtful.Comment: 9 pages, 2 figures; presented at the International Frequency Control
Symposium, Newport Beach, Calif., June, 2010; presented at Pulsar Conference
2010, October 12th, Sardinia; accepted 13th September 2010 for publication in
Reviews of Modern Physic
Constraints on the acceleration of the solar system from high-precision timing
Many astronomers have speculated that the solar system contains undiscovered
massive planets or a distant stellar companion. The acceleration of the solar
system barycenter can constrain the mass and position of the putative
companion. In this paper we use the most recent timing data on accurate
astronomical clocks (millisecond pulsars, pulsars in binary systems and
pulsating white dwarfs) to constrain this acceleration. No evidence for
non-zero acceleration has been found; the typical sensitivity achieved by our
method is a/c=a few times 10^{-19} s^{-1}, comparable to the acceleration due
to a Jupiter-mass planet at 200 AU. The acceleration method is limited by the
uncertainties in the distances and by the timing precision for pulsars in
binary systems, and by the intrinsic distribution of the period derivatives for
millisecond pulsars. Timing data provide stronger constraints than residuals in
the motions of comets or planets if the distance to the companion exceeds a few
hundred AU. The acceleration method is also more sensitive to the presence of a
distant companion (> 300-400 AU) than existing optical and infrared surveys. We
outline the differences between the effects of the peculiar acceleration of the
solar system and the background of gravitational waves on high-precision
timing.Comment: 28 pages including 7 figures; submitted to AJ on April 22, 200
Gravitons and Lightcone Fluctuations II: Correlation Functions
A model of a fluctuating lightcone due to a bath of gravitons is further
investigated. The flight times of photons between a source and a detector may
be either longer or shorter than the light propagation time in the background
classical spacetime, and will form a Gaussian distribution centered around the
classical flight time. However, a pair of photons emitted in rapid succession
will tend to have correlated flight times. We derive and discuss a correlation
function which describes this effect. This enables us to understand more fully
the operational significance of a fluctuating lightcone. Our results may be
combined with observational data on pulsar timing to place some constraints on
the quantum state of cosmological gravitons.Comment: 16 pages and two figures, uses eps
CMB Anisotropy Induced by a Moving Straight Cosmic String
We showed that the part of strings could be detected by optical method is
only 20% from the total available amount of such objects, therefore the
gravitational lensing method has to be "completed" by CMB one. We found the
general structure of the CMB anisotropy generated by a cosmic string for simple
model of straight string moving with constant velocity. For strings with
deficit angle 1-2 arcsec the amplitude of generated anisotropy has to be 15-30
muK (the corresponding string linear density is (G mu) ~ 10^{-7} and energy is
GUT one, 10^{15} GeV). To use both radio and optical methods the deficit angle
has to be from 0.1 arcsec to 5-6 arcsec. If cosmic string can be detected by
optical method, the length of corresponding brightness spot of anisotropy has
to be no less than 100 degrees.Comment: 6 pages, 1 Postscript figure, will be published in proceedings of
QUARKS-2008, 15th International Seminar on High Energy Physics, Sergiev
Posad, Russia, 23-29 May, 200
Shapiro Effect as a Possible Cause of the Low-Frequency Pulsar Timing Noise in Globular Clusters
A prolonged timing of millisecond pulsars has revealed low-frequency
uncorrelated noise, presumably of astrophysical origin, in the pulse arrival
time (PAT) residuals for some of them. In most cases, pulsars in globular
clusters show a low-frequency modulation of their rotational phase and spin
rate. The relativistic time delay of the pulsar signal in the curved space time
of randomly distributed and moving globular cluster stars (the Shapiro effect)
is suggested as a possible cause of this modulation.
Given the smallness of the aberration corrections that arise from the
nonstationarity of the gravitational field of the randomly distributed ensemble
of stars under consideration, a formula is derived for the Shapiro effect for a
pulsar in a globular cluster. The derived formula is used to calculate the
autocorrelation function of the low-frequency pulsar noise, the slope of its
power spectrum, and the behavior of the statistic that characterizes
the spectral properties of this noise in the form of a time function. The
Shapiro effect under discussion is shown to manifest itself for large impact
parameters as a low-frequency noise of the pulsar spin rate with a spectral
index of n=-1.8 that depends weakly on the specific model distribution of stars
in the globular cluster. For small impact parameters, the spectral index of the
noise is n=-1.5.Comment: 23 pages, 6 figure
Limits on the Stochastic Gravitational Wave Background from the North American Nanohertz Observatory for Gravitational Waves
We present an analysis of high-precision pulsar timing data taken as part of
the North American Nanohertz Observatory for Gravitational waves (NANOGrav)
project. We have observed 17 pulsars for a span of roughly five years using the
Green Bank and Arecibo radio telescopes. We analyze these data using standard
pulsar timing models, with the addition of time-variable dispersion measure and
frequency-variable pulse shape terms. Sub-microsecond timing residuals are
obtained in nearly all cases, and the best root-mean-square timing residuals in
this set are ~30-50 ns. We present methods for analyzing post-fit timing
residuals for the presence of a gravitational wave signal with a specified
spectral shape. These optimally take into account the timing fluctuation power
removed by the model fit, and can be applied to either data from a single
pulsar, or to a set of pulsars to detect a correlated signal. We apply these
methods to our dataset to set an upper limit on the strength of the
nHz-frequency stochastic supermassive black hole gravitational wave background
of h_c (1 yr^-1) < 7x10^-15 (95%). This result is dominated by the timing of
the two best pulsars in the set, PSRs J1713+0747 and J1909-3744.Comment: To be submitted to Ap
Observing gravitational wave bursts in pulsar timing measurements
We propose a novel method for observing the gravitational wave signature of
super-massive black hole (SMBH) mergers. This method is based on detection of a
specific type of gravitational waves, namely gravitational wave burst with
memory (BWM), using pulsar timing. We study the unique signature produced by
BWM in anomalous pulsar timing residuals. We show that the present day pulsar
timing precision allows one to detect BWM due to SMBH mergers from distances up
to 1 Gpc (for case of equal mass 10^8 Msun SMBH). Improvements in precision of
pulsar timing together with the increase in number of observed pulsars should
eventually lead to detection of a BWM signal due to SMBH merger, thereby making
the proposed technique complementary to the capabilities of the planned LISA
mission.Comment: 9 pages, 1 figure, generally matches the MNRAS versio
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