60 research outputs found
Gravitational Lensing Signature of Long Cosmic Strings
The gravitational lensing by long, wiggly cosmic strings is shown to produce
a large number of lensed images of a background source. In addition to pairs of
images on either side of the string, a number of small images outline the
string due to small-scale structure on the string. This image pattern could
provide a highly distinctive signature of cosmic strings. Since the optical
depth for multiple imaging of distant quasar sources by long strings may be
comparable to that by galaxies, these image patterns should be clearly
observable in the next generation of redshift surveys such as the Sloan Digital
Sky Survey.Comment: 4 pages, revtex with 3 postscript figures include
Gauge Invariant Density and Temperature Perturbations in the Quasi-Newtonian Formulation
We give an improved formalism for calculating the evolution of density
fluctuations and temperature perturbations in flat universes. Our equations are
general enough to treat the perturbations in collisionless relics like massive
neutrinos. We find this formulation to be simpler to use than gauge dependent
and other gauge-invariant formalisms. We show how to calculate temperature
fluctuations (including multipole moments) and transfer functions, including
the case of collisionless relics like massive neutrinos. We call this formalism
"quasi-Newtonian" because the equations for the potential and cold matter
fluctuation evolution have the same form as the Newtonian gravitational
equations in an expanding space. The density fluctuation variable also has the
same form inside and outside of the horizon which allows the initial conditions
to be specified in a simple intuitive way. Our sample calculations demonstrate
how to use these equations in cosmological models which have hot, cold, and
mixed dark matter and adiabatic (isentropic) or isocurvature modes. We also
give an approximation which may be used to get transfer functions quickly.Comment: 1 LaTEX text file (29 pages), and a uuencoded gzip'ed tar file
containing 7 Postscript figure
Wave effect in gravitational lensing by a cosmic string
The wave effect in the gravitational lensing phenomenon by a straight cosmic
string is investigated. The interference pattern is expressed in terms of a
simple formula. We demonstrate that modulations of the interfered wave
amplitude can be a unique signature of the wave effect. We briefly mention a
possible chance of detecting the wave effect in future gravitational wave
observatories.Comment: 4 pages, 1 figur
Cosmic Strings Lens Phenomenology: Model of Poisson Energy Distribution
We present a novel approach for investigating lens phenomenology of cosmic
strings in order to elaborate detection strategies in galaxy deep field images.
To account for the complexity of the projected energy distribution of string
networks we assume their lens effects to be similar to those of a straight
string carrying a {\em random} lineic energy distribution. In such a model we
show that, unlike the case of uniform strings, critical phenomena naturally
appear. We explore the properties of the critical lines and caustics. In
particular, assuming that the energy coherence length along the string is much
smaller than the observation scale, we succeeded in computing the total length
of critical lines per unit string length and found it to be . The length of the associated caustic lines can also be computed to be
. The picture we obtain here for the
phenomenology of cosmic string detection is clearly at variance with common
lore.Comment: 10 pages, 5 figures. Minor correction
Constraints on Self-Interacting Dark Matter
We consider the growth of density perturbations in the presence of
self--interacting dark matter, SIDM, proposed by Carlson, Machacek and Hall
(1992). We determine the range of values for the coupling constant
and the particle mass , for which the power spectrum lies in the
``allowed" range based on constraints from the IRAS galaxy survey and damped
Lyman-- systems. Our results show that no combination of parameters
can meet both limits. We consider constraints on the \2-2 scatterings which
keep the SIDM particles in pressure equilibrium, and we show that if such
interactions maintain pressure equilibrium down to the present, they will be
strong enough to disrupt galaxy mergers and may lead to stripping of galaxy
halos as galaxies move through the dark matter background of these particles.
Hence, we also investigate the evolution of large-scale structure in the SIDM
model when the particles drop out of pressure equilibrium at some higher
redshift. The resulting free-streaming leads to an additional suppression of
small-scale perturbations, but it does not significantly affect our results.Comment: 19 pages compressed post-script (3 figures included
Cosmic Strings in a Braneworld Theory with Metastable Gravitons
If the graviton possesses an arbitrarily small (but nonvanishing) mass,
perturbation theory implies that cosmic strings have a nonzero Newtonian
potential. Nevertheless in Einstein gravity, where the graviton is strictly
massless, the Newtonian potential of a cosmic string vanishes. This discrepancy
is an example of the van Dam--Veltman--Zakharov (VDVZ) discontinuity. We
present a solution for the metric around a cosmic string in a braneworld theory
with a graviton metastable on the brane. This theory possesses those features
that yield a VDVZ discontinuity in massive gravity, but nevertheless is
generally covariant and classically self-consistent. Although the cosmic string
in this theory supports a nontrivial Newtonian potential far from the source,
one can recover the Einstein solution in a region near the cosmic string. That
latter region grows as the graviton's effective linewidth vanishes (analogous
to a vanishing graviton mass), suggesting the lack of a VDVZ discontinuity in
this theory. Moreover, the presence of scale dependent structure in the metric
may have consequences for the search for cosmic strings through gravitational
lensing techniques.Comment: 18 pages, 2 figures, revtex. Improved discussion of interpolating
solution. To be published in Phys. Rev.
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