15,144 research outputs found
Properties of Information Carrying Waves in Cosmology
Recently we studied the effects of information carrying waves propagating
through isotropic cosmologies. By information carrying we mean that the waves
have an arbitrary dependence on a function. We found that the waves introduce
shear and anisotropic stress into the universe. We then constructed explicit
examples of pure gravity wave perturbations for which the presence of this
anisotropic stress is essential and the null hypersurfaces playing the role of
the histories of the wave-fronts in the background space-time are shear-free.
Motivated by this result we now prove that these two properties are true for
all information carrying waves in isotropic cosmologies.Comment: 15 pages, Latex File, accepted for publication in Physical Review
Gravitational Lensing by Cold Dark Matter Catastrophes
Intrinsically cold particle dark matter inevitably creates halos with sharp
discontinuities in projected surface density caused by the projection of fold
catastrophes onto the sky. In principle, these imperfections can be detected
and measured with gravitational lensing through discontinuities in image
magnification and image structure. Lens solutions are discussed for the most
common universal classes of discontinuities. Edges caused by cold particles
such as condensed axions and thermal WIMPs are very sharp, respectively about
and of the halo scale. Their structure can be resolved by
stellar and quasi-stellar sources which show sudden changes in brightness or
even sudden disappearances (sometimes within hours) as edges are crossed.
Images of extended objects such as edge-on galaxies or jets can show sudden
bends at an edge, or stretched, parity-inverted reflection symmetry about a
sharp line. Observational strategies and prospects are briefly discussed.Comment: 9 pages, AASTeX. Final version, with explanatory figure added, to be
published in the Astrophysical Journa
Shear-Free Gravitational Waves in an Anisotropic Universe
We study gravitational waves propagating through an anisotropic Bianchi I
dust-filled universe (containing the Einstein-de-Sitter universe as a special
case). The waves are modeled as small perturbations of this background
cosmological model and we choose a family of null hypersurfaces in this
space-time to act as the histories of the wavefronts of the radiation. We find
that the perturbations we generate can describe pure gravitational radiation if
and only if the null hypersurfaces are shear-free. We calculate the
gauge-invariant small perturbations explicitly in this case. How these differ
from the corresponding perturbations when the background space-time is
isotropic is clearly exhibited.Comment: 32 pages, accepted for publication in Physical Review
Metric Perturbation Approach to Gravitational Waves in Isotropic Cosmologies
Gravitational waves in isotropic cosmologies were recently studied using the
gauge-invariant approach of Ellis-Bruni. We now construct the linearised metric
perturbations of the background Robertson-Walker space-time which reproduce the
results obtained in that study. The analysis carried out here also facilitates
an easy comparison with Bardeen.Comment: 29 pages, Latex file, accepted for publication in Physical Review
Single-color two-photon spectroscopy of Rydberg states in electric fields
Rydberg states of atomic helium with principal quantum numbers ranging from
n=20 to n=100 have been prepared by non-resonance-enhanced single-color
two-photon excitation from the metastable 2 {^3}S{_1} state. Photoexcitation
was carried out using linearly and circularly polarized pulsed laser radiation.
In the case of excitation with circularly polarized radiation, Rydberg states
with azimuthal quantum number |m_{\ell}|=2 were prepared in zero electric
field, and in homogeneous electric fields oriented parallel to the propagation
axis of the laser radiation. In sufficiently strong electric fields, individual
Rydberg-Stark states were resolved spectroscopically, highlighting the
suitability of non-resonance-enhanced multiphoton excitation schemes for the
preparation of long-lived high-|m_{\ell}| hydrogenic Rydberg states for
deceleration and trapping experiments. Applications of similar schemes for
Doppler-free excitation of positronium atoms to Rydberg states are also
discussed
Gravitational Wave Propagation in Isotropic Cosmologies
We study the propagation of gravitational waves carrying arbitrary
information through isotropic cosmologies. The waves are modelled as small
perturbations of the background Robertson-Walker geometry. The perfect fluid
matter distribution of the isotropic background is, in general, modified by
small anisotropic stresses. For pure gravity waves, in which the perturbed Weyl
tensor is radiative (i.e. type N in the Petrov classification), we construct
explicit examples for which the presence of the anisotropic stress is shown to
be essential and the histories of the wave-fronts in the background
Robertson-Walker geometry are shear-free null hypersurfaces. The examples
derived in this case are analogous to the Bateman waves of electromagnetic
theory.Comment: 27 pages, accepted for publication in Phys.Rev.
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