1,725 research outputs found
The spectrum of gravitational waves in Randall-Sundrum braneworld cosmology
We study the generation and evolution of gravitational waves (tensor
perturbations) in the context of Randall-Sundrum braneworld cosmology. We
assume that the initial and final stages of the background cosmological model
are given by de Sitter and Minkowski phases, respectively, and they are
connected smoothly by a radiation-dominated phase. This setup allows us to
discuss the quantum-mechanical generation of the perturbations and to see the
final amplitude of the well-defined zero mode. Using the Wronskian formulation,
we numerically compute the power spectrum of gravitational waves, and find that
the effect of initial vacuum fluctuations in the Kaluza-Klein modes is
subdominant, contributing not more than 10% of the total power spectrum. Thus
it is confirmed that the damping due to the Kaluza-Klein mode generation and
the enhancement due to the modification of the background Friedmann equation
are the two dominant effects, but they cancel each other, leading to the same
spectral tilt as the standard four-dimensional result. Kaluza-Klein gravitons
that escape from the brane contribute to the energy density of the dark
radiation at late times. We show that a tiny amount of the dark radiation is
generated due to this process.Comment: 10 pages, 5 figures; v2: published versio
Cepheids and other short-period variables near the Galactic Centre
We report the result of our near-infrared survey of short-period variable
stars (P<60d) in a field-of-view of 20'x30' towards the Galactic Centre.
Forty-five variables are discovered and we classify the variables based on
their light curve shapes and other evidence. In addition to 3 classical
Cepheids reported previously, we find 16 type II Cepheids, 24 eclipsing
binaries, one pulsating star with P=0.265d (RR Lyr or delta Sct) and one
Cepheid-like variable whose nature is uncertain. Eclipsing binaries are
separated into the foreground objects and those significantly obscured by
interstellar extinction. One of the reddened binaries contains an O-type
supergiant and its light curve indicates an eccentric orbit. We discuss the
nature and distribution of type II Cepheids as well as the distance to the
Galactic Centre based on these Cepheids and other distance indicators. The
estimates of R0(GC) we obtained based on photometric data agree with previous
results obtained with kinematics of objects around the GC. Furthermore, our
result gives a support to the reddening law obtained by Nishiyama and
collaborators, A(Ks)/E(H-Ks)=1.44, because a different reddening law would
result in a rather different distance estimate.Comment: 13 pages, 10 figures, 7tables, accepted for publication in MNRA
Quantum-mechanical generation of gravitational waves in braneworld
We study the quantum-mechanical generation of gravitational waves during
inflation on a brane embedded in a five-dimensional anti-de Sitter bulk. To
make the problem well-posed, we consider the setup in which both initial and
final phases are given by a de Sitter brane with different values of the Hubble
expansion rate. Assuming that the quantum state is in a de Sitter invariant
vacuum in the initial de Sitter phase, we numerically evaluate the amplitude of
quantum fluctuations of the growing solution of the zero mode in the final de
Sitter phase. We find that the vacuum fluctuations of the initial Kaluza-Klein
gravitons as well as of the zero mode gravitons contribute to the final
amplitude of the zero mode on small scales, and the power spectrum is quite
well approximated by what we call the rescaled spectrum, which is obtained by
rescaling the standard four-dimensional calculation following a simple mapping
rule. Our results confirm the speculation raised in Ref.
\cite{Kobayashi:2003cn} before.Comment: 11 pages, 11 figure
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