15,912 research outputs found
Structure formation by cosmic strings with a cosmological constant
Final published version.Comment: 4 Page
A Closed-Form Expression for the Gravitational Radiation Rate from Cosmic Strings
We present a new formula for the rate at which cosmic strings lose energy
into gravitational radiation, valid for all piecewise-linear cosmic string
loops. At any time, such a loop is composed of straight segments, each of
which has constant velocity. Any cosmic string loop can be arbitrarily-well
approximated by a piecewise-linear loop with sufficiently large. The
formula is a sum of polynomial and log terms, and is exact when the
effects of gravitational back-reaction are neglected. For a given loop, the
large number of terms makes evaluation ``by hand" impractical, but a computer
or symbolic manipulator yields accurate results. The formula is more accurate
and convenient than previous methods for finding the gravitational radiation
rate, which require numerical evaluation of a four-dimensional integral for
each term in an infinite sum. It also avoids the need to estimate the
contribution from the tail of the infinite sum. The formula has been tested
against all previously published radiation rates for different loop
configurations. In the cases where discrepancies were found, they were due to
errors in the published work. We have isolated and corrected both the analytic
and numerical errors in these cases. To assist future work in this area, a
small catalog of results for some simple loop shapes is provided.Comment: 29 pages TeX, 16 figures and computer C-code available via anonymous
ftp from directory pub/pcasper at alpha1.csd.uwm.edu, WISC-MILW-94-TH-10,
(section 7 has been expanded, two figures added, and minor grammatical
changes made.
Wiggly Relativistic Strings
We derive the equations of motion for general strings, i.e. strings with
arbitrary relation between tension and energy per unit length
. The renormalization of and that results from
averaging out small scale wiggles on the string is obtained in the general case
to lowest order in the amount of wiggliness. For Nambu-Goto strings we find
deviations from the equation of state in
higher orders. Finally we argue that wiggliness may radically modify the gauge
cosmic string scenario.Comment: 10 pages, LaTeX, UFIFT-HEP-92-1
The spin-orbit alignment of visual binaries
The angle between the stellar spin-axis and the orbital plane of a stellar or
planetary companion has important implications for the formation and evolution
of such systems. A study by Hale (1994) found that binaries with separations au are preferentially aligned while binaries on wider orbits are
frequently misaligned. We aim to test the robustness of the Hale (1994) results
by reanalysing the sample of visual binaries with measured rotation periods
using independently derived stellar parameters and a Bayesian formalism. Our
analysis is based on a combination of data from Hale (1994) and newly obtained
spectroscopic data from the Hertzsprung SONG telescope, combined with
astrometric data from Gaia DR2 and the Washington Double Star Catalog. We
combine measurements of stellar radii and rotation periods to obtain stellar
rotational velocities . Rotational velocities are combined with
measurements of projected rotational velocities to derive posterior
probability distributions of stellar inclination angles . We determine
line-of-sight projected spin-orbit angles by comparing stellar inclination
angles with astrometric orbital inclination angles. We find that the precision
of the available data is insufficient to make inferences about the spin-orbit
alignment of visual binaries. The data are equally compatible with alignment
and misalignment at all orbital separations. We conclude that the previously
reported trend that binaries with separations au are preferentially
aligned is spurious. The spin-orbit alignment distribution of visual binaries
is unconstrained. Based on simulated observations, we predict that it will be
difficult to reach the sufficient precision in , rotation periods, and
orbital inclination required to make robust statistical inferences about the
spin-orbit alignment of visual binaries.Comment: Accepted for publication in A&A. 10 pages, 5 figures, 1 tabl
Cosmogenic radionuclides on LDEF: An unexpected Be-10 result
Following the discovery of the atmospheric derived cosmogenic radionuclide Be-7 on the Long Duration Exposure Facility (LDEF), a search began for other known nuclides produced by similar mechanisms. None of the others have the narrow gamma-ray line emission of Be-7 decay which enabled its rapid detection and quantification. A search for Be-10 atoms on LDEF clamp plates using accelerator mass spectrometry is described. An unexpected result was obtained
Physics at the B Factories
We review recent progress at the two B factories. The first
measurement of CP violation and the prospects for measuring all the angles of
the unitarity triangle are discussed.Comment: To appear in the Proceedings of From the Smallest to the Largest
Distances, a conference in honor of Tranh Thanh Van in Moscow, Russi
Enhancement of laser cooling by the use of magnetic gradients
We present a laser cooling scheme for trapped ions and atoms using a
combination of laser couplings and a magnetic gradient field. In a
Schrieffer-Wolff transformed picture, this setup cancels the carrier and blue
sideband terms completely resulting in an improved cooling behaviour compared
to standard cooling schemes (e.g. sideband cooling) and allowing cooling to the
vibrational ground state. A condition for optimal cooling rates is presented
and the cooling behaviour for different Lamb-Dicke parameters and spontaneous
decay rates is discussed. Cooling rates of one order of magnitude less than the
trapping frequency are achieved using the new cooling method. Furthermore the
scheme turns out to be robust under deviations from the optimal parameters and
moreover provides good cooling rates also in the multi particle case.Comment: 14 pages, 8 figure
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