733 research outputs found
The radial gauge propagators in quantum gravity
We give a general procedure for extracting the propagators in gauge theories
in presence of a sharp gauge fixing and we apply it to derive the propagators
in quantum gravity in the radial gauge, both in the first and in the second
order formalism in any space-time dimension. In the three dimensional case such
propagators vanish except for singular collinear contributions, in agreement
with the absence of propagating gravitons.Comment: 38 pages, 1 fig. not available, LATEX, IFUP-TH-30/9
The large-scale structure of passive scalar turbulence
We investigate the large-scale statistics of a passive scalar transported by
a turbulent velocity field. At scales larger than the characteristic
lengthscale of scalar injection, yet smaller than the correlation length of the
velocity, the advected field displays persistent long-range correlations due to
the underlying turbulent velocity. These induce significant deviations from
equilibrium statistics for high-order scalar correlations, despite the absence
of scalar flux.Comment: 4 pages, 6 figure
New supersymmetric Wilson loops in ABJ(M) theories
We present two new families of Wilson loop operators in N= 6 supersymmetric
Chern-Simons theory. The first one is defined for an arbitrary contour on the
three dimensional space and it resembles the Zarembo's construction in N=4 SYM.
The second one involves arbitrary curves on the two dimensional sphere. In both
cases one can add certain scalar and fermionic couplings to the Wilson loop so
it preserves at least two supercharges. Some previously known loops, notably
the 1/2 BPS circle, belong to this class, but we point out more special cases
which were not known before. They could provide further tests of the
gauge/gravity correspondence in the ABJ(M) case and interesting observables,
exactly computable by localizationComment: 9 pages, no figure. arXiv admin note: text overlap with
arXiv:0912.3006 by other author
Bed load at low shields stress on arbitrarily sloping beds: Failure of the Bagnold hypothesis
Abstract According to the Bagnold hypothesis for equilibrium bedload transport, a necessary constraint for the maintenance of equilibrium bedload transport is that the fluid shear stress at the bed must be reduced to the critical, or threshold value associated with incipient motion of grains. It was shown in a companion paper to the present one, however that the Bagnold hypothesis breaks down when applied to equilibrium bedload transport on beds with transverse slopes above a relatively modest value that is well below the angle of repose. This failure motivates the abandonment of the Bagnold constraint, even for nearly horizontal beds. The constraint is replaced with an entrainment formulation, according to which a dynamic equilibrium is maintained by a balance between entrainment of bed grains into the bedload layer and deposition of bedload grains onto the bed. The entrainment function is formulated so that the entrainment rate is an increasing function of the excess of the fluid shear stress at the bed over the threshold value. The formulation is implemented with the aid of a unique set of laboratory data that characterizes equilibrium bedload transport at relatively low shear stresses for streamwise angles of bed inclination varying from nearly 0 ° to 22°. The formulation is shown to provide a description of bedload transport on nearly horizontal beds that fits the data as well as that resulting from the Bagnold constraint. The entrainment formulation has the added advantage of not requiring the unrealistically high dynamic coefficient of Coulomb friction resulting from the Bagnold constraint. Finally
Fermi-Walker gauge in 2+1 dimensional gravity.
It is shown that the Fermi-Walker gauge allows the general solution of
determining the metric given the sources, in terms of simple quadratures. We
treat the general stationary problem providing explicit solving formulas for
the metric and explicit support conditions for the energy momentum tensor. The
same type of solution is obtained for the time dependent problem with circular
symmetry. In both cases the solutions are classified in terms of the invariants
of the Wilson loops outside the sources. The Fermi-Walker gauge, due to its
physical nature, allows to exploit the weak energy condition and in this
connection it is proved that, both for open and closed universes with
rotational invariance, the energy condition imply the total absence of closed
time like curves. The extension of this theorem to the general stationary
problem, in absence of rotational symmetry is considered. At present such
extension is subject to some assumptions on the behavior of the determinant of
the dreibein in this gauge. PACS number: 0420Comment: 28 pages, RevTex, no figure
On shape dependence of holographic entanglement entropy in AdS4/CFT3 with Lifshitz scaling and hyperscaling violation
We study the divergent terms and the finite term in the expansion of the holographic entanglement entropy as the ultraviolet cutoff vanishes for smooth spatial regions having arbitrary shape, when the gravitational background is a four dimensional asymptotically Lifshitz spacetime with hyperscaling violation, in a certain range of the hyperscaling parameter. Both static and time dependent backgrounds are considered. For the coefficients of the divergent terms and for the finite term, analytic expressions valid for any smooth entangling curve are obtained. The analytic results for the finite terms are checked through a numerical analysis focussed on disks and ellipses
Granular size segregation in underwater sand ripples
We report an experimental study of a binary sand bed under an oscillating
water flow. The formation and evolution of ripples is observed. The appearance
of a granular segregation is shown to strongly depend on the sand bed
preparation. The initial wavelength of the mixture is measured. In the final
steady state, a segregation in volume is observed instead of a segregation at
the surface as reported before. The correlation between this phenomenon and the
fluid flow is emphasised. Finally, different ``exotic'' patterns and their
geophysical implications are presented.Comment: 8 page
Area-preserving Structure and Anomalies in 1+1-dimensional Quantum Gravity
We investigate the gauge-independent Hamiltonian formulation and the
anomalous Ward identities of a matter-induced 1+1-dimensional gravity theory
invariant under Weyl transformations and area-preserving diffeomorphisms, and
compare the results to the ones for the conventional diffeomorphism-invariant
theory. We find that, in spite of several technical differences encountered in
the analysis, the two theories are essentially equivalent.Comment: 9 pages, LaTe
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