4,184 research outputs found
Doppler peaks from active perturbations
We examine how the qualitative structure of the Doppler peaks in the angular
power spectrum of the cosmic microwave anisotropy depends on the fundamental
nature of the perturbations which produced them. The formalism of Hu and
Sugiyama is extended to treat models with cosmic defects. We discuss how
perturbations can be ``active'' or ``passive'' and ``incoherent'' or
``coherent'', and show how causality and scale invariance play rather different
roles in these various cases. We find that the existence of secondary Doppler
peaks and the rough placing of the primary peak unambiguously reflect these
basic properties.Comment: uufile, 8pages, 3 figures. Now available at
http://euclid.tp.ph/Papers/index.html; Changes: URL added, Eqn. (8) expanded,
grant numbers include
Theoretical Study of Fluid Membranes of Spherical Topology with Internal Degrees of Freedom
A theoretical study of vesicles of topological genus zero is presented. The
bilayer membranes forming the vesicles have various degrees of intrinsic
(tangent-plane) orientational order, ranging from smectic to hexatic,
frustrated by curvature and topology. The field-theoretical model for these
`-atic' surfaces has been studied before in the low temperature (mean-field)
limit. Work presented here includes the effects of thermal fluctuations. Using
the lowest Landau level approximation, the coupling between order and shape is
cast in a simple form, facilitating insights into the behaviour of vesicles.
The order parameter contains vortices, whose effective interaction potential is
found, and renormalized by membrane fluctuations. The shape of the phase space
has a counter-intuitive influence on this potential. A criterion is established
whereby a vesicle of finite rigidity may be burst by its own in-plane order,
and an analogy is drawn with flux exclusion from a type-I superconductor.Comment: 34 pages + 4 Postscript figures. Uses RevTe
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