21,726 research outputs found
The optimism bias : a cognitive neuroscience perspective
The optimism bias is a well-established psychological phenomenon. Its study has implications that are far reaching in fields as diverse as mental health and economic theory. With the emerging field of cognitive neuroscience and the advent of advanced neuroimaging techniques, it has been possible to investigate the neural basis of the optimism bias and to understand in which neurological conditions this natural bias fails. This review first defines the optimism bias, discusses its implications and reviews the literature that investigates its neural basis. Finally some potential pitfalls in experimental design are discussed.peer-reviewe
Zoll Metrics, Branched Covers, and Holomorphic Disks
We strengthen our previous results regarding the moduli spaces of Zoll
metrics and Zoll projective structures on S^2. In particular, we describe a
concrete, open condition which suffices to guarantee that a totally real
embedding of RP^2 in CP_2 arises from a unique Zoll projective structure on the
2-sphere. Our methods ultimately reflect the special role such structures play
in the initial value problem for the 3-dimensional Lorentzian Einstein-Weyl
equations.Comment: 21 pages, LaTeX2
Yamabe Invariants and Spin^c Structures
The Yamabe Invariant of a smooth compact manifold is by definition the
supremum of the scalar curvatures of unit-volume Yamabe metrics on the
manifold. For an explicit infinite class of 4-manifolds, we show that this
invariant is positive but strictly less than that of the 4-sphere. This is done
by using spin^c Dirac operators to control the lowest eigenvalue of a
perturbation of the Yamabe Laplacian. These results dovetail perfectly with
those derived from the perturbed Seiberg-Witten equations, but the present
method is much more elementary in spirit.Comment: Standard LaTeX fil
Zoll Manifolds and Complex Surfaces
We classify compact surfaces with torsion-free affine connections for which
every geodesic is a simple closed curve. In the process, we obtain completely
new proofs of all the major results concerning the Riemannian case.
In contrast to previous work, our approach is twistor-theoretic, and depends
fundamentally on the fact that, up to biholomorphism, there is only one complex
structure on CP2
Nonlinear Gravitons, Null Geodesics, and Holomorphic Disks
We develop a global twistor correspondence for pseudo-Riemannian conformal
structures of signature (++--) with self-dual Weyl curvature. Near the
conformal class of the standard indefinite product metric on S^2 x S^2, there
is an infinite-dimensional moduli space of such conformal structures, and each
of these has the surprising global property that its null geodesics are all
periodic. Each such conformal structure arises from a family of holomorphic
disks in CP_3 with boundary on some totally real embedding of RP^3 into CP_3.
An interesting sub-class of these conformal structures are represented by
scalar-flat indefinite K\"ahler metrics, and our methods give particularly
sharp results in this more restrictive setting.Comment: 56 pages, LaTeX2
The Einstein-Weyl Equations, Scattering Maps, and Holomorphic Disks
We show that conformally compact, globally hyperbolic, Lorentzian
Einstein-Weyl 3-manifolds are in natural one-to-one correspondence with
orientation-reversing diffeomorphisms of the 2-sphere. The proof hinges on a
holomorphic-disk analog of Hitchin's mini-twistor correspondence.Comment: 11 pages, LaTeX2e. Revised version strengthens result and completes
proo
Radiative cooling of swept up gas in AGN-driven galactic winds and its implications for molecular outflows
We recently used hydro-chemical simulations to demonstrate that molecular
outflows observed in luminous quasars can be explained by molecule formation
within the AGN wind. However, these simulations cover a limited parameter
space, due to their computational cost. We have therefore developed an analytic
model to follow cooling in the shocked ISM layer of an AGN wind. We explore
different ambient densities (), density profile
slopes (), AGN luminosities (), and metallicities (). The swept up gas
mostly cools within ~1 Myr. Based on our previous simulations, we predict that
this gas would produce observable molecular outflows. The instantaneous
momentum boost initially increases as the outflow decelerates. However, it
reaches a maximum of 20, due to work done against the gravitational
potential. The predicted time-averaged observational estimate of the molecular
outflow momentum boost reaches a maximum of , partly due to our
assumed molecular fraction, 0.2, but also because the instantaneous and
observational, time-averaged definitions are not equivalent. Thus recent
observational estimates of order unity momentum boosts do not necessarily rule
out energy-driven outflows. Finally, we find that dust grains are likely to
re-form by accretion of metals after the shocked ISM layer has cooled, assuming
that a small fraction of dust grains swept up after this layer has cooled are
able to mix into the cool phase, and assuming that grain growth remains
efficient in the presence of the strong AGN radiation field. This would enable
rapid molecule formation, as assumed in our models.Comment: 22 pages, 16 figures (including appendices). Accepted for publication
in MNRA
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