568 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
Microarcsecond instability of the celestial reference frame
The fluctuation of the angular positions of reference extragalactic radio and
optical sources under the influence of the irregular gravitational field of
visible Galactic stars is considered. It is shown that these angular
fluctuations range from a few up to hundreds of microarcseconds. This leads to
a small rotation of the celestial reference frame. The nondiagonal coefficients
of the rotation matrix are of the order of a microarcsecond. The temporal
variation of these coefficients due to the proper motion of the foreground
stars is of the order of one microsecond per 20 years. Therefore, the celestial
reference frame can be considered inertial and homogeneous only to
microarcsecond accuracy. Astrometric catalogues with microarcsecond accuracy
will be unstable, and must be reestablished every 20 years.Comment: 5 pages, 2 figures, accepted to MNRA
Pulsar timing arrays as imaging gravitational wave telescopes: angular resolution and source (de)confusion
Pulsar timing arrays (PTAs) will be sensitive to a finite number of
gravitational wave (GW) "point" sources (e.g. supermassive black hole
binaries). N quiet pulsars with accurately known distances d_{pulsar} can
characterize up to 2N/7 distant chirping sources per frequency bin \Delta
f_{gw}=1/T, and localize them with "diffraction limited" precision \delta\theta
\gtrsim (1/SNR)(\lambda_{gw}/d_{pulsar}). Even if the pulsar distances are
poorly known, a PTA with F frequency bins can still characterize up to
(2N/7)[1-(1/2F)] sources per bin, and the quasi-singular pattern of timing
residuals in the vicinity of a GW source still allows the source to be
localized quasi-topologically within roughly the smallest quadrilateral of
quiet pulsars that encircles it on the sky, down to a limiting resolution
\delta\theta \gtrsim (1/SNR) \sqrt{\lambda_{gw}/d_{pulsar}}. PTAs may be
unconfused, even at the lowest frequencies, with matched filtering always
appropriate.Comment: 7 pages, 1 figure, matches Phys.Rev.D versio
Detection of gravitational waves from the QCD phase transition with pulsar timing arrays
If the cosmological QCD phase transition is strongly first order and lasts
sufficiently long, it generates a background of gravitational waves which may
be detected via pulsar timing experiments. We estimate the amplitude and the
spectral shape of such a background and we discuss its detectability prospects.Comment: 7 pages, 5 figs. Version accepted by PR
Black Holes of a Minimal Size in String Gravity
A lower limit for a neutral black hole size is obtained in the frames of the
string gravity model with the second order curvature correction. It is shown
that this effect remains when the third order curvature correction is also
taken into account and argued that such restriction does exist in all
perturbative orders of curvature expansions.Comment: 6 LaTeX pages, 1 PostScript figure (epsfig.sty), minor changes in the
text and references, submitted to Int.J.Mod.Phy
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