17 research outputs found
Real sector of the nonminimally coupled scalar field to self-dual gravity
A scalar field nonminimally coupled to gravity is studied in the canonical
framework, using self-dual variables. The corresponding constraints are first
class and polynomial. To identify the real sector of the theory, reality
conditions are implemented as second class constraints, leading to three real
configurational degrees of freedom per space point. Nevertheless, this
realization makes non-polynomial some of the constraints. The original complex
symplectic structure reduces to the expected real one, by using the appropriate
Dirac brackets. For the sake of preserving the simplicity of the constraints,
an alternative method preventing the use of Dirac brackets, is discussed. It
consists of converting all second class constraints into first class by adding
extra variables. This strategy is implemented for the pure gravity case.Comment: Latex file, 22 pages, no figure
Reality conditions for Ashtekar gravity from Lorentz-covariant formulation
We show the equivalence of the Lorentz-covariant canonical formulation
considered for the Immirzi parameter to the selfdual Ashtekar
gravity. We also propose to deal with the reality conditions in terms of Dirac
brackets derived from the covariant formulation and defined on an extended
phase space which involves, besides the selfdual variables, also their
anti-selfdual counterparts.Comment: 14 page
Background independence in a nutshell
We study how physical information can be extracted from a background
independent quantum system. We use an extremely simple `minimalist' system that
models a finite region of 3d euclidean quantum spacetime with a single
equilateral tetrahedron. We show that the physical information can be expressed
as a boundary amplitude. We illustrate how the notions of "evolution" in a
boundary proper-time and "vacuum" can be extracted from the background
independent dynamics.Comment: 19 pages, 19 figure
The Entropy of the Rotating Charged Black Threebrane from a Brane-Antibrane System
We show that a model based on a D3-brane--anti-D3-brane system at finite
temperature, proposed previously as a microscopic description of the
non-rotating black threebrane of type IIB supergravity arbitrarily far from
extremality, can also successfully reproduce the entropy of the rotating
threebrane with arbitrary charge (including the neutral case, which corresponds
to the Kerr black hole in seven dimensions). Our results appear to confirm in
particular the need for a peculiar condition on the energy of the two gases
involved in the model, whose physical interpretation remains to be elucidated.Comment: 12 pages, references added in section 1 and