1,406 research outputs found
Biases in the Quasar Mass-Luminosity Plane
We find that the recently reported departure from the Eddington luminosity
limit for the highest quasar black hole masses at a given redshift is an
artifact due to biases in black hole mass measurements. This sub-Eddington
boundary (with non-unity slope) in the quasar mass-luminosity plane was
initially reported by Steinhardt & Elvis (2010a) using the FWHM-based black
hole mass catalogue of Shen et al. (2008). However, the significance of the
boundary is reduced when the FWHM-based mass-scaling relationship is
recalibrated following Wang et al. (2009) and using the most updated
reverberation mapping estimates of black hole masses. Furthermore, this
boundary is not seen using mass estimates based on the line dispersion of the
same quasars' MgII emission lines. Thus, the initial report of a sub-Eddington
boundary with non-unity slope was due to biases in estimating masses using the
FWHM of a fit of one or two Gaussians to quasar MgII emission lines. We provide
evidence that using the line dispersion of the MgII line produces less biased
black hole mass estimates.Comment: 11 pages, 10 figures, Accepted for publication in MNRA
Cosmological Imprint of an Energy Component with General Equation of State
We examine the possibility that a significant component of the energy density
of the universe has an equation-of-state different from that of matter,
radiation or cosmological constant (). An example is a cosmic scalar
field evolving in a potential, but our treatment is more general. Including
this component alters cosmic evolution in a way that fits current observations
well. Unlike , it evolves dynamically and develops fluctuations,
leaving a distinctive imprint on the microwave background anisotropy and mass
power spectrum.Comment: revised version, with added references, to appear in Phys. Rev. Lett.
(4 pages Latex, 2 postscript figures
Non-linear Brane Dynamics in 6 Dimensions
We consider a dynamical brane world in a six dimensional spacetime containing
a singularity. Using the Israel conditions we study the motion of a 4-brane
embedded in this setup. We analize the brane behavior when its position is
perturbed about a fixed point and solve the full non-linear dynamics in the
several possible scenarios. We also investigate the possible gravitational
shortcuts and calculate the delay between graviton and photon signals and the
ratio of the corresponding subtended horizons.Comment: 5 pages, 2 figures. Contribution to the Proceedings of
"Renormalization Group and Anomalies in Gravitation and Cosmology", Ouro
Preto, Brazil, March 200
The cosmic gravitational wave background in a cyclic universe
Inflation predicts a primordial gravitational wave spectrum that is slightly
``red,'' i.e., nearly scale-invariant with slowly increasing power at longer
wavelengths. In this paper, we compute both the amplitude and spectral form of
the primordial tensor spectrum predicted by cyclic/ekpyrotic models. The
spectrum is blue and exponentially suppressed compared to inflation on long
wavelengths. The strongest observational constraint emerges from the
requirement that the energy density in gravitational waves should not exceed
around 10 per cent of the energy density at the time of nucleosynthesis.Comment: 4 pages, 3 figuer
Dark-Energy Dynamics Required to Solve the Cosmic Coincidence
Dynamic dark energy (DDE) models are often designed to solve the cosmic
coincidence (why, just now, is the dark energy density , the same
order of magnitude as the matter density ?) by guaranteeing for significant fractions of the age of the universe. This
typically entails ad-hoc tracking or oscillatory behaviour in the model.
However, such behaviour is neither sufficient nor necessary to solve the
coincidence problem. What must be shown is that a significant fraction of
observers see . Precisely when, and for how long, must a
DDE model have in order to solve the coincidence? We
explore the coincidence problem in dynamic dark energy models using the
temporal distribution of terrestrial-planet-bound observers. We find that any
dark energy model fitting current observational constraints on and
the equation of state parameters and , does have for a large fraction of observers in the universe. This demotivates DDE
models specifically designed to solve the coincidence using long or repeated
periods of .Comment: 16 pages, 8 figures, Submitted to Phys. Rev.
Density Perturbations and the Cosmological Constant from Inflationary Landscapes
An anthropic understanding of the cosmological constant requires that the
vacuum energy at late time scans from one patch of the universe to another. If
the vacuum energy during inflation also scans, the various patches of the
universe acquire exponentially differing volumes. In a generic landscape with
slow-roll inflation, we find that this gives a steeply varying probability
distribution for the normalization of the primordial density perturbations,
resulting in an exponentially small fraction of observers measuring the COBE
value of 10^-5. Inflationary landscapes should avoid this "\sigma problem", and
we explore features that can allow them to do that. One possibility is that,
prior to slow-roll inflation, the probability distribution for vacua is
extremely sharply peaked, selecting essentially a single anthropically allowed
vacuum. Such a selection could occur in theories of eternal inflation. A second
possibility is that the inflationary landscape has a special property: although
scanning leads to patches with volumes that differ exponentially, the value of
the density perturbation does not vary under this scanning. This second case is
preferred over the first, partly because a flat inflaton potential can result
from anthropic selection, and partly because the anthropic selection of a small
cosmological constant is more successful.Comment: 23 page
Holography and Variable Cosmological Constant
An effective local quantum field theory with UV and IR cutoffs correlated in
accordance with holographic entropy bounds is capable of rendering the
cosmological constant (CC) stable against quantum corrections. By setting an IR
cutoff to length scales relevant to cosmology, one easily obtains the currently
observed rho_Lambda ~ 10^{-47} GeV^4, thus alleviating the CC problem. It is
argued that scaling behavior of the CC in these scenarios implies an
interaction of the CC with matter sector or a time-dependent gravitational
constant, to accommodate the observational data.Comment: 7 pages, final version accepted by PR
Coupled quintessence and curvature-assisted acceleration
Spatially homogeneous models with a scalar field non-minimally coupled to the
space-time curvature or to the ordinary matter content are analysed with
respect to late-time asymptotic behaviour, in particular to accelerated
expansion and isotropization. It is found that a direct coupling to the
curvature leads to asymptotic de Sitter expansion in arbitrary exponential
potentials, thus yielding a positive cosmological constant although none is
apparent in the potential. This holds true regardless of the steepness of the
potential or the smallness of the coupling constant. For matter-coupled scalar
fields, the asymptotics are obtained for a large class of positive potentials,
generalizing the well-known cosmic no-hair theorems for minimal coupling. In
this case it is observed that the direct coupling to matter does not impact the
late-time dynamics essentially.Comment: 17 pages, no figures. v2: typos correcte
Matter density perturbations in interacting quintessence models
Models with dark energy decaying into dark matter have been proposed to solve
the coincidence problem in cosmology. We study the effect of such coupling in
the matter power spectrum. Due to the interaction, the growth of matter density
perturbations during the radiation dominated regime is slower compared to
non-interacting models with the same ratio of dark matter to dark energy today.
This effect introduces a damping on the power spectrum at small scales
proportional to the strength of the interaction and similar to the effect
generated by ultrarelativistic neutrinos. The interaction also shifts
matter--radiation equality to larger scales. We compare the matter power
spectrum of interacting quintessence models with the measurments of 2dFGRS. We
particularize our study to models that during radiation domination have a
constant dark matter to dark energy ratio.Comment: 11 pages, 4 figures, accepted for publication in Phys. Rev.
Quantum effects and superquintessence in the new age of precision cosmology
Recent observations of Type Ia supernova at high redshifts establish that the
dark energy component of the universe has (a probably constant) ratio between
pressure and energy density . The
conventional quintessence models for dark energy are restricted to the range
, with the cosmological constant corresponding to .
Conformally coupled quintessence models are the simplest ones compatible with
the marginally allowed superaccelerated regime (). However, they are
known to be plagued with anisotropic singularities.
We argue here that the extension of the classical approach to the
semiclassical one, with the inclusion of quantum counterterms necessary to
ensure the renormalization, can eliminate the anisotropic singularities
preserving the isotropic behavior of conformally coupled superquintessence
models. Hence, besides of having other interesting properties, they are
consistent candidates to describe the superaccelerated phases of the universe
compatible with the present experimental data.Comment: 7 pages. Essay selected for "Honorable Mention" in the 2004 Awards
for Essays on Gravitation, Gravity Research Foundatio
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