6,803 research outputs found
The many-body reciprocal theorem and swimmer hydrodynamics
We present a reinterpretation and extension of the reciprocal theorem for
swimmers, extending its application from the motion of a single swimmer in an
unbounded domain to the general setting, giving results for both swimmer
interactions and general hydrodynamics. We illustrate the method for a squirmer
near a planar surface, recovering standard literature results and extending
them to a general squirming set, to motion in the presence of a ciliated
surface, and expressions for the flow field throughout the domain. Finally, we
present exact results for the hydrodynamics in two dimensions which shed light
on the near-field behaviour.Comment: 6 pages, 6 figure
Capturing the phase diagram of (2+1)-dimensional CDT using a balls-in-boxes model
We study the phase diagram of a one-dimensional balls-in-boxes (BIB) model
that has been proposed as an effective model for the spatial-volume dynamics of
(2+1)-dimensional causal dynamical triangulations (CDT). The latter is a
statistical model of random geometries and a candidate for a nonperturbative
formulation of quantum gravity, and it is known to have an interesting phase
diagram, in particular including a phase of extended geometry with classical
properties. Our results corroborate a previous analysis suggesting that a
particular type of potential is needed in the BIB model in order to reproduce
the droplet condensation typical of the extended phase of CDT. Since such a
potential can be obtained by a minisuperspace reduction of a (2+1)-dimensional
gravity theory of the Ho\v{r}ava-Lifshitz type, our result strengthens the link
between CDT and Ho\v{r}ava-Lifshitz gravity.Comment: 21 pages, 7 figure
Flavor stability analysis of dense supernova neutrinos with flavor-dependent angular distributions
Numerical simulations of the supernova (SN) neutrino self-induced flavor
conversions, associated with the neutrino-neutrino interactions in the deepest
stellar regions, have been typically carried out assuming the "bulb-model". In
this approximation, neutrinos are taken to be emitted half-isotropically by a
common neutrinosphere. In the recent Ref. \cite{Mirizzi:2011tu} we have removed
this assumption by introducing flavor-dependent angular distributions for SN
neutrinos, as suggested by core-collapse simulations. We have found that in
this case a novel multi-angle instability in the self-induced flavor
transitions can arise. In this work we perform an extensive study of this
effect, carrying out a linearized flavor stability analysis for different SN
neutrino energy fluxes and angular distributions, in both normal and inverted
neutrino mass hierarchy. We confirm that spectra of different nu species which
cross in angular space (where F_{\nu_e}=F_{\nu_x} and
F_{\bar\nu_e}=F_{\bar\nu_x}) present a significant enhancement of the flavor
instability, and a shift of the onset of the flavor conversions at smaller
radii with respect to the case of an isotropic neutrino emission. We also
illustrate how a qualitative (and sometimes quantitative) understanding of the
dynamics of these systems follows from a stability analysis.Comment: (v2: revised version. 10 pages, 10 eps figures. References updated.
Figures imrproved. Matches the version published in PRD.
Supersymmetric AdS Backgrounds in String and M-theory
We first present a short review of general supersymmetric compactifications
in string and M-theory using the language of G-structures and intrinsic
torsion. We then summarize recent work on the generic conditions for
supersymmetric AdS_5 backgrounds in M-theory and the construction of classes of
new solutions. Turning to AdS_5 compactifications in type IIB, we summarize the
construction of an infinite class of new Sasaki-Einstein manifolds in dimension
2k+3 given a positive curvature Kahler-Einstein base manifold in dimension 2k.
For k=1 these describe new supergravity duals for N=1 superconformal field
theories with both rational and irrational R-charges and central charge. We
also present a generalization of this construction, that has not appeared
elsewhere in the literature, to the case where the base is a product of
Kahler-Einstein manifolds.Comment: LaTeX, 35 pages, to appear in the proceedings of the 73rd Meeting
between Physicists and Mathematicians "(A)dS/CFT correspondence", Strasbourg,
September 11-13, 200
Sasaki-Einstein Metrics on S^2 x S^3
We present a countably infinite number of new explicit co-homogeneity one
Sasaki-Einstein metrics on S^2 x S^3, in both the quasi-regular and irregular
classes. These give rise to new solutions of type IIB supergravity which are
expected to be dual to N=1 superconformal field theories in four-dimensions
with compact or non-compact R-symmetry and rational or irrational central
charges, respectively.Comment: 20 pages. v2: references added, typos corrected. v3: minor typos
correcte
Fivebranes Wrapped on SLAG Three-Cycles and Related Geometry
We construct ten-dimensional supergravity solutions corresponding to the near
horizon limit of IIB fivebranes wrapping special Lagrangian three-cycles of
constant curvature. The case of branes wrapping a three-sphere provides a
gravity dual of pure N=2 super-Yang-Mills theory in D=3. The non-trivial part
of the solutions are seven manifolds that admit two G_2 structures each of
which is covariantly constant with respect to a different connection with
torsion. We derive a formula for the generalised calibration for this general
class of solutions. We discuss analogous aspects of the geometry that arises
when fivebranes wrap other supersymmetric cycles which lead to Spin(7) and
SU(N) structures. In some cases there are two covariantly constant structures
and in others one.Comment: v2: 26 pages, 3 figures, 1 table. Section 7 slightly expanded,
references adde
Current behavior of a quantum Hamiltonian ratchet in resonance
We investigate the ratchet current that appears in a kicked Hamiltonian
system when the period of the kicks corresponds to the regime of quantum
resonance. In the classical analogue, a spatial-temporal symmetry should be
broken to obtain a net directed current. It was recently discovered that in
quantum resonance the temporal symmetry can be kept, and we prove that breaking
the spatial symmetry is a necessary condition to find this effect.
Moreover, we show numerically and analytically how the direction of the
motion is dramatically influenced by the strength of the kicking potential and
the value of the period. By increasing the strength of the interaction this
direction changes periodically, providing us with a non-expected source of
current reversals in this quantum model. These reversals depend on the kicking
period also, though this behavior is theoretically more difficult to analyze.
Finally, we generalize the discussion to the case of a non-uniform initial
condition.Comment: 6 pages, 4 figure
General static spherically symmetric solutions in Horava gravity
We derive general static spherically symmetric solutions in the Horava theory
of gravity with nonzero shift field. These represent "hedgehog" versions of
black holes with radial "hair" arising from the shift field. For the case of
the standard de Witt kinetic term (lambda =1) there is an infinity of solutions
that exhibit a deformed version of reparametrization invariance away from the
general relativistic limit. Special solutions also arise in the anisotropic
conformal point lambda = 1/3.Comment: References adde
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