29,867 research outputs found
Scale dependence of cosmological backreaction
Due to the non-commutation of spatial averaging and temporal evolution,
inhomogeneities and anisotropies (cosmic structures) influence the evolution of
the averaged Universe via the cosmological backreaction mechanism. We study the
backreaction effect as a function of averaging scale in a perturbative approach
up to higher orders. We calculate the hierarchy of the critical scales, at
which 10% effects show up from averaging at different orders. The dominant
contribution comes from the averaged spatial curvature, observable up to scales
of 200 Mpc. The cosmic variance of the local Hubble rate is 10% (5%) for
spherical regions of radius 40 (60) Mpc. We compare our result to the one from
Newtonian cosmology and Hubble Space Telescope Key Project data.Comment: 6 pages, 2 figures; v3: substantial modifications, new figure
Occam's razor meets WMAP
Using a variety of quantitative implementations of Occam's razor we examine
the low quadrupole, the ``axis of evil'' effect and other detections recently
made appealing to the excellent WMAP data. We find that some razors {\it fully}
demolish the much lauded claims for departures from scale-invariance. They all
reduce to pathetic levels the evidence for a low quadrupole (or any other low
cut-off), both in the first and third year WMAP releases. The ``axis of
evil'' effect is the only anomaly examined here that survives the humiliations
of Occam's razor, and even then in the category of ``strong'' rather than
``decisive'' evidence. Statistical considerations aside, differences between
the various renditions of the datasets remain worrying
Slow-roll inflation with a Gauss-Bonnet correction
We consider slow-roll inflation for a single scalar field with an arbitrary
potential and an arbitrary nonminimal coupling to the Gauss-Bonnet term. By
introducing a combined hierarchy of Hubble and Gauss-Bonnet flow functions, we
analytically derive the power spectra of scalar and tensor perturbations. The
standard consistency relation between the tensor-to-scalar ratio and the
spectral index of tensor perturbations is broken. We apply this formalism to a
specific model with a monomial potential and an inverse monomial Gauss-Bonnet
coupling and constrain it by the 7-year Wilkinson Microwave Anisotropy Probe
data. The Gauss-Bonnet term with a positive (or negative) coupling may lead to
a reduction (or enhancement) of the tensor-to-scalar ratio and hence may revive
the quartic potential ruled out by recent cosmological data.Comment: 7 pages, 2 figures, RevTeX, references added, published versio
Kinematic and morphological modeling of the bipolar nebula Sa2-237
We present [OIII]500.7nm and Halpha+[NII] images and long-slit, high
resolution echelle spectra in the same spectral regions of Sa2--237, a possible
bipolar planetary nebula. The image shows a bipolar nebula of about 34" extent,
with a narrow waist, and showing strong point symmetry about the central
object, indicating it's likely binary nature. The long slit spectra were taken
over the long axis of the nebula, and show a distinct ``eight'' shaped pattern
in the velocity--space plot, and a maximum projected outflow velocity of
V=106km/s, both typical of expanding bipolar planetary nebulae. By model
fitting the shape and spectrum of the nebula simultaneously, we derive the
inclination of the long axis to be 70 degrees, and the maximum space velocity
of expansion to be 308 km/s. Due to asymmetries in the velocities we adopt a
new value for the system's heliocentric radial velocity of -30km/s. We use the
IRAS and 21cm radio fluxes, the energy distribution, and the projected size of
Sa2-237 to estimate it's distance to be 2.1+-0.37kpc. At this distance Sa2-237
has a luminosity of 340 Lsun, a size of 0.37pc, and -- assuming constant
expansion velocity -- a nebular age of 624 years. The above radial velocity and
distance place Sa2--237 in the disk of the Galaxy at z=255pc, albeit with
somewhat peculiar kinematics.Comment: 10pp, 4 fig
Kaluza-Klein electrically charged black branes in M-theory
We present a class of Kaluza-Klein electrically charged black p-brane
solutions of ten-dimensional, type IIA superstring theory. Uplifting to eleven
dimensions these solutions are studied in the context of M-theory. They can be
interpreted either as a p+1 extended object trapped around the eleventh
dimension along which momentum is flowing or as a boost of the following
backgrounds: the Schwarzschild black (p+1)-brane or the product of the
(10-p)-dimensional Euclidean Schwarzschild manifold with the (p+1)-dimensional
Minkowski spacetime.Comment: 16 pages, uses latex and epsf macro, figures include
Chiral Models in Noncommutative N=1/2 Four Dimensional Superspace
We derive the component Lagrangian for a generic N=1/2 supersymmetric chiral
model with an arbitrary number of fields in four space-time dimensions. We then
investigate a toy model in which the deformation parameter modifies the
undeformed potential near the origin of the field space in a way which suggests
possible physical applications.Comment: Final version to appear in Physical Review D. We have added a new
section in which we show that our component action can be resummed. The final
answer takes a simple compact form. We also show how differences emerge for
the modified Kaehler structure with respect to the ones found in the
literature. We have also added two reference
Resonant Scattering and Recombination in CAL 87
The eclipsing supersoft X-ray binary CAL 87 has been observed with Chandra on
August 13/14, 2001 for nearly 100 ksec, covering two full orbital cycles and
three eclipses. The shape of the eclipse light curve derived from the
zeroth-order photons indicates that the size of the X-ray emission region is
about 1.5 solar radii. The ACIS/LETG spectrum is completely dominated by
emission lines without any noticeable continuum. The brightest emission lines
are significantly redshifted and double-peaked, suggestive of emanating in a
2000 km/s wind. We model the X-ray spectrum by a mixture of recombination and
resonant scattering. This allows us to deduce the temperature and luminosity of
the ionizing source to be kT = 50-100 eV and L_X = 5E37 erg/s.Comment: To appear in Proceedings of IAU Coll. 194 "Compact binaries in the
Galaxy and beyond" (Rev. Mex. A&A Conf. Series), eds. G. Tovmassian and E.
Sio
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