arXiv subjects
Publications and source records attributed to S. Fabi.
The Regge-Teitelboim formulation of gravity, which utilizes dynamical embeddings in a background space, effectively introduces source terms in the standard Einstein equations that are not attributable to the energy-momentum tensor. We show that for a simple class of embeddings of the Robertson-Walker metric, these source terms naturally generate cosmic acceleration.
We present the geometric formulation of gravity based on the mathematical structure of a Lie Algebroid. We show that this framework provides the geometrical setting to describe the gauge propriety of gravity.
In this note we demonstrate the equation of motion for test particles in an ambient gravitational field for the teleparallel theory of gravity, considered as a generalized gauge theory, using a computational scheme due to Feynman. It can be thought of as the Wong equation for a generalized gauge theory. The Wong and Lorentz equations become identical when the generators of a generalized non-abelian gauge theory are taken to be the local translation generators.
Upon applying Chamseddine's noncommutative deformation of gravity we obtain the leading order noncommutative corrections to the Robertson-Walker metric tensor. We get an isotropic inhomogeneous metric tensor for a certain choice of the noncommutativity parameters. Moreover, the singularity of the commutative metric at $t=0$ is replaced by a more involved space-time structure in the noncommutative theory. In a toy model we construct a scenario where there is no singularity at $t=0$ at leading order in the noncommutativity parameter. Although singularities may still be present for nonzero $t$, they need not be the source of all time-like geodesics and the result resembles a bouncing cosmology.
In this paper we calculate the zero point energy density experienced by observers on M^4 due to a massless scalar field defined throughout M^4 x T^2_F, where T^2_F are fuzzy extra dimensions. Using the Green's function approach we calculate the energy density for the commutative torus and the fuzzy torus. We calculate then the energy density for the fuzzy torus using the Hamiltonian approach. Agreement is shown between Green's function and Hamiltonian approaches.
We study the possibility that dark energy is a manifestation of the Casimir energy on extra dimensions with the topology of $S^2$. We consider our universe to be $M^4 \times S^2$ and modify the geometry by introducing noncommutativity on the extra dimensions only, i.e. replacing $S^2$ with the fuzzy version $S_{F}^2$. We find the energy density as a function of the size of the representation $M+1$ of the algebra of $S_{F}^2$, and we calculate its value for the $M+1=2$ case. The value of the energy density turns out to be positive, i.e. provides dark energy, and the size of the extra dimensions agrees with the experimental limit. We also recover the correct commutative limit as the noncommutative parameter goes to zero.