825 research outputs found
Nonlinear Structure Formation, Backreaction and Weak Gravitational Fields
There is an ongoing debate in the literature concerning the effects of
averaging out inhomogeneities (``backreaction'') in cosmology. In particular,
some simple models of structure formation studied in the literature seem to
indicate that the backreaction can play a significant role at late times, and
it has also been suggested that the standard perturbed FLRW framework is no
longer a good approximation during structure formation, when the density
contrast becomes nonlinear. In this work we use Zalaletdinov's covariant
averaging scheme (macroscopic gravity or MG) to show that as long as the metric
of the Universe can be described by the perturbed FLRW form, the corrections
due to averaging remain negligibly small. Further, using a fully relativistic
and reasonably generic model of pressureless spherical collapse, we show that
as long as matter velocities remain small (which is true in our model), the
perturbed FLRW form of the metric can be explicitly recovered. Together, these
results imply that the backreaction remains small even during nonlinear
structure formation, and we confirm this within the toy model with a numerical
calculation.Comment: 8 pages, eas format, talk given at July 2008 CRAL-IPNL conference on
"Dark Energy and Dark Matter
Spontaneous breaking of conformal invariance in theories of conformally coupled matter and Weyl gravity
We study the theory of Weyl conformal gravity with matter degrees of freedom
in a conformally invariant interaction. Specifically, we consider a triplet of
scalar fields and SO(3) non-abelian gauge fields, i.e. the Georgi-Glashow model
conformally coupled to Weyl gravity. We show that the equations of motion admit
solutions spontaneously breaking the conformal symmetry and the gauge symmetry,
providing a mechanism for supplying a scale in the theory. The vacuum solution
corresponds to anti-de-Sitter space-time, while localized soliton solutions
correspond to magnetic monopoles in asymptotically anti-de-Sitter space-time.
The resulting effective action gives rise to Einstein gravity and the residual
U(1) gauge theory. This mechanism strengthens the reasons for considering
conformally invariant matter-gravity theory, which has shown promising
indications concerning the problem of missing matter in galactic rotation
curves.Comment: 20 pages, 1 figure, revised and added reference
Flight Mechanics of a Tail-less Articulated Wing Aircraft
This paper explores the flight mechanics of a Micro Aerial Vehicle (MAV) without a vertical tail. The key to stability and control of such an aircraft lies in the ability to control the twist and dihedral angles of both wings independently. Specifically, asymmetric dihedral can be used to control yaw whereas antisymmetric twist can be used to control roll. It has been demonstrated that wing dihedral angles can regulate sideslip and speed during a turn maneuver. The role of wing dihedral in the aircraft's longitudinal performance has been explored. It has been shown that dihedral angle can be varied symmetrically to achieve limited control over aircraft speed even as the angle of attack and flight path angle are varied. A rapid descent and perching maneuver has been used to illustrate the longitudinal agility of the aircraft. This paper lays part of the foundation for the design and stability analysis of an agile flapping wing aircraft capable of performing rapid maneuvers while gliding in a constrained environment
Entropy of Null Surfaces and Dynamics of Spacetime
The null surfaces of a spacetime act as one-way membranes and can block information for a corresponding family of observers (time-like curves). Since lack of information can be related to entropy, this suggests the possibility of assigning an entropy to the null surfaces of a spacetime. We motivate and introduce such an entropy functional in terms of the normal to the null surface and a fourth-rank divergence free tensor with the algebraic symmetries of the curvature tensor. Extremising this entropy then leads to field equations for the background metric of the spacetime. When is constructed from the metric alone, these equations are identical to Einstein's equations with an undetermined cosmological constant (which arises as an integration constant). More generally, if is allowed to depend on both metric and curvature in a polynomial form, one recovers the Lanczos-Lovelock gravity. In all these cases: (a) We only need to extremise the entropy associated with the null surfaces; the metric is not a dynamical variable in this approach. (b) The extremal value of the entropy agrees with standard results, when evaluated on-shell for a solution admitting a horizon. The role of full quantum theory of gravity will be to provide the specific form of which should be used in the entropy functional. With such an interpretation, it seems reasonable to interpret the Lanczos-Lovelock type terms as quantum corrections to classical gravity
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