17,484 research outputs found
A Geometrical Test of the Cosmological Energy Contents Using the Lyman-alpha Forest
In this Letter we explore a version of the test of cosmological geometry
proposed by Alcock and Paczynski (1979), using observations of the Lyman-alpha
forest in the spectra of close quasar pairs. By comparing the correlations in
absorption in one quasar spectrum with correlations between the spectra of
neighboring quasars one can determine the relation of the redshift distance
scale to the angle distance scale at the redshift of the absorbers, . Since this relationship depends on the parameters of the cosmological
model, these parameters may be determined using the Lyman-alpha forest. While
this test is relatively insensitive to the density parameter in a
dust-dominated universe, it is more sensitive to the presence of a matter
component with large negative pressure (such as a cosmological constant
) and its equation of state. With only 25 pairs of quasar spectra at
angular separations , one can discriminate between an open universe () and an flat
(-dominated) universe at the level. The S/N can be enhanced
by considering quasar pairs at smaller angular separations, but requires proper
modeling of nonlinear redshift space distortions. Here the correlations and
redshift space distortions are modeled using linear theory.Comment: 13 pages, 2 ps figures, submitted to ApJ
Theory of the evolutionary minority game
We present a theory which describes a recently introduced model of an
evolving, adaptive system in which agents compete to be in the minority. The
agents themselves are able to evolve their strategies over time in an attempt
to improve their performance. The present theory explicitly demonstrates the
self-interaction, or so-called market impact, that agents in such systems
experience
Microcausality of spin-induced noncommutative theories
In this brief report, the microcausility of quantum field theory on
spin-induced noncom- mutative spacetime is discussed. It is found that for
spacelike seperation the microcausality is not obeyed by the theory generally.
It means that Lorentz covariance can not guaran- tee microcausality in quantum
field thoery. We also give some comments about quantum field thoeries on such
noncommutative spacetime and the relations between noncommutative spacetime and
causality.Comment: 9 pages, no figur
Migration and Growth of Protoplanetary Embryos I: Convergence of Embryos in Protoplanetary Disks
According to the core-accretion scenario, planets form in protostellar disks
through the condensation of dust, coagulation of planetesimals, and emergence
of protoplanetary embryos. At a few AU in a minimum mass nebula, embryos'
growth is quenched by dynamical isolation due to the depletion of planetesimals
in their feeding zone. However, embryos with masses () in the range of a
few Earth masses () migrate toward a transition radius between the
inner viscously heated and outer irradiated regions of their natal disk. Their
limiting isolation mass increases with the planetesimals surface density. When
, embryos efficiently accrete gas and evolve into cores of
gas giants. We use numerical simulation to show that, despite streamline
interference, convergent embryos essentially retain the strength of
non-interacting embryos' Lindblad and corotation torque by their natal disks.
In disks with modest surface density (or equivalently accretion rates), embryos
capture each other in their mutual mean motion resonances and form a convoy of
super Earths. In more massive disks, they could overcome these resonant
barriers to undergo repeated close encounters including cohesive collisions
which enable the formation of massive cores.Comment: 9 pages, 6 figures, accepted for publication in Ap
Black Hole Entropy and Viscosity Bound in Horndeski Gravity
Horndeski gravities are theories of gravity coupled to a scalar field, in
which the action contains an additional non-minimal quadratic coupling of the
scalar, through its first derivative, to the Einstein tensor or the analogous
higher-derivative tensors coming from the variation of Gauss-Bonnet or Lovelock
terms. In this paper we study the thermodynamics of the static black hole
solutions in dimensions, in the simplest case of a Horndeski coupling to
the Einstein tensor. We apply the Wald formalism to calculate the entropy of
the black holes, and show that there is an additional contribution over and
above those that come from the standard Wald entropy formula. The extra
contribution can be attributed to unusual features in the behaviour of the
scalar field. We also show that a conventional regularisation to calculate the
Euclidean action leads to an expression for the entropy that disagrees with the
Wald results. This seems likely to be due to ambiguities in the subtraction
procedure. We also calculate the viscosity in the dual CFT, and show that the
viscosity/entropy ratio can violate the bound for
appropriate choices of the parameters.Comment: 30 pages, no figure, minor revision
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