Born-Infeld strategy to smooth theories having divergent solutions is applied to teleparallel equivalent of General Relativity. Differing from other theories of modified gravity, modified teleparallelism leads to second order equations, since teleparallel Lagrangian only contains first derivatives of the vierbein. We show that Born-Infeld-modified teleparallelism solves the particle horizon problem in a spatially flat FRW universe by providing an initial exponential expansion without resorting to an inflaton field.
One approach in modern cosmology consists in supposing that dark matter and dark energy are different manifestations of a single `quartessential' fluid. Following such idea, this work presents a study of the evolution of perturbations of density in a flat cosmological model with a modified Chaplygin gas acting as a single component. Our goal is to obtain properties of the model which can be used to distinguish it from another cosmological models which have the same solutions for the general evolution of the scale factor of the universe, without the construction of the power spectrum. Our analytical results, which alone can be used to uniquely characterize the specific model studied in our work, show that the evolution of the density contrast can be seen as composed by a spheroidal wave function. We also present a numerical analysis which clearly indicates as one interesting feature of the model the appearence of peaks in the evolution of the density constrast.
We study the possibility that the current accelerated expansion of the universe is driven by the vacuum energy density of a colored scalar field which is responsible for a phase transition in which the gauge $SU(3)_c$ symmetry breaks. We show that if we are stuck in a $SU(3)_c$-preserving false vacuum, then $SU(3)_c$ symmetry breaking can be accommodated without violating any experimental QCD bounds or bounds from cosmological observations. Moreover, unless there is an unnatural fine-tuning beyond the usual cosmological constant fine-tuning, the true vacuum state of the universe is anti-de Sitter. The model can likely be tested at the LHC. A possible (though not neccessary) consequence of the model is the existence of fractionally charged massive hadrons. The model can be embedded in supersymmetric theories where massive colored scalar fields appear naturally, thus reopening previously closed regions of SUSY parameter space.
We study Chern-Simons (CS) gravity in the parameterized post-Newtonian (PPN) framework through weak-field solutions of the modified field equations for a perfect fluid source. We discover that CS gravity possesses the same PPN parameters as general relativity, except for the inclusion of a new term, proportional both to the CS coupling parameter and the curl of the PPN vector potentials. This new term encodes the key physical effect of CS gravity in the weak-field limit, leading to a modification of frame dragging and, thus, the Lense-Thirring contribution to gyroscopic precession. We provide a physical interpretation for the new term, as well as an estimate of the size of this effect relative to the general relativistic Lense-Thirring prediction. This correction to frame dragging might be used in experiments, such as Gravity Probe B and lunar ranging, to place bounds on the CS coupling parameter, as well as other intrinsic parameters of string theory.
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