2,166 research outputs found
Primordial Power Spectra from Anisotropic Inflation
We examine cosmological perturbations in a dynamical theory of inflation in
which an Abelian gauge field couples directly to the inflaton, breaking
conformal invariance. When the coupling between the gauge field and the
inflaton takes a specific form, inflation becomes anisotropic and anisotropy
can persist throughout inflation, avoiding Wald's no-hair theorem. After
discussing scenarios in which anisotropy can persist during inflation, we
calculate the dominant effects of a small persistent anisotropy on the
primordial gravitational wave and curvature perturbation power spectra using
the "in-in" formalism of perturbation theory. We find that the primordial power
spectra of cosmological perturbations gain significant direction dependence and
that the fractional direction dependence of the tensor power spectrum is
suppressed in comparison to that of the scalar power spectrum.Comment: 25 pages, 2 figures; References added, typos corrected and some
discussion expanded; version submitted for publication in PR
Classical stability of a homogeneous, anisotropic inflating space-time
We study the classical stability of an anisotropic space-time seeded by a
spacelike, fixed norm, dynamical vector field in a vacuum-energy-dominated
inflationary era. It serves as a model for breaking isotropy during the
inflationary era. We find that, for a range of parameters, the linear
differential equations for small perturbations about the background do not have
a growing mode. We also examine the energy of fluctuations about this
background in flat-space. If the kinetic terms for the vector field do not take
the form of a field strength tensor squared then there is a negative energy
mode and the background is unstable. For the case where the kinetic term is of
the form of a field strength tensor squared we show that perturbations about
the background have positive energy at lowest order.Comment: 12 pages, no figures; references added, content in section V revised
and some clarification made in text; minor typos corrected, v4 closely
resembles version published in Phys. Rev. D; in v5 - incorrect argument in
section V removed and one reference adde
Sigma-Model Aether
Theories of low-energy Lorentz violation by a fixed-norm "aether" vector
field with two-derivative kinetic terms have a globally bounded Hamiltonian and
are perturbatively stable only if the vector is timelike and the kinetic term
in the action takes the form of a sigma model. Here we investigate the
phenomenological properties of this theory. We first consider the propagation
of modes in the presence of gravity, and show that there is a unique choice of
curvature coupling that leads to a theory without superluminal modes.
Experimental constraints on this theory come from a number of sources, and we
examine bounds in a two-dimensional parameter space. We then consider the
cosmological evolution of the aether, arguing that the vector will naturally
evolve to be orthogonal to constant-density hypersurfaces in a
Friedmann-Robertson-Walker cosmology. Finally, we examine cosmological
evolution in the presence of an extra compact dimension of space, concluding
that a vector can maintain a constant projection along the extra dimension in
an expanding universe only when the expansion is exponential.Comment: 8 pages, 2 figures; fixed minor typo and changed references in v
Compensator improvement for multivariable control systems
A theory and the associated numerical technique are developed for an iterative design improvement of the compensation for linear, time-invariant control systems with multiple inputs and multiple outputs. A strict constraint algorithm is used in obtaining a solution of the specified constraints of the control design. The result of the research effort is the multiple input, multiple output Compensator Improvement Program (CIP). The objective of the Compensator Improvement Program is to modify in an iterative manner the free parameters of the dynamic compensation matrix so that the system satisfies frequency domain specifications. In this exposition, the underlying principles of the multivariable CIP algorithm are presented and the practical utility of the program is illustrated with space vehicle related examples
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