Narrow Results

This paper explores a cosmological reconstruction scheme in the background of $f(Q)$ gravity theory from a Holographic perspective. The basic motivation for this work is that the reconstruction is performed from a holographic origin, which has its roots in the black hole thermodynamics and quantum gravity. Dark energy models inspired by holographic prescription are used to reconstruct the $f(Q)$ gravity models. Two such models, namely, the Granda–Oliveros holographic dark energy model and its generalization, the Chen-Jing model, are considered for the study. Different scale factors are used and a thorough reconstruction scheme is set up using the dark energy models. The observationally constrained values of the free model parameters have been used to form the reconstructed models. Finally, a thorough investigation of the energy conditions has been performed to check the cosmological viability of the reconstructed $f(Q)$ models. As an outcome, we get some very promising and cosmologically viable $f(Q)$ models that present some interesting properties and demand further investigation. Finally, a method is discussed how the constructed $f(Q)$ models can be reconciled with a generalized holographic dark energy.

We establish the mathematical fundamentals for a unified description of curvature, torsion, and non-metricity 2-forms in the way extending the so-called Möbius representation of the affine group, which is the method to convert the semi-direct product into the ordinary matrix product, to revive the fertility of gauge theories of gravity. First of all, we illustrate the basic concepts for constructing the metric-affine geometry. Then the curvature and torsion 2-forms are described in a unified manner by using the Cartan connection of the Möbius representation of the affine group. In this unified-description, the curvature and torsion are derived by Cartan’s structure equation with respect to a common connection 1-form. After that, extending the Möbius representation, the dilation and shear 2-forms, or equivalently, the non-metricity 2-form, are introduced in the same unified manner. Based on the unified-description established in this paper, introducing a new group parametrization and applying the Inönü–Wigner group contraction to the full theory, the relationships among symmetries, geometric quantities, and geometries are investigated with respect to the three gauge groups: the metric-affine group and its extension, and an extension of the (anti)-de Sitter group in which the non-metricity exists. Finally, possible applications to theories of gravity are briefly discussed.

We consider the usual Einstein–Hilbert action in a Metric-Affine setup and in the presence of a Perfect Hyperfluid. In order to decode the role of shear hypermomentum, we impose vanishing spin and dilation parts on the sources and allow only for non-vanishing shear. We then consider an FLRW background and derive the generalized Friedmann equations in the presence of shear hypermomentum. By providing one equation of state among the shear variables we study the cases for which shear has an accelerating/decelerating effect on Universe’s expansion. In particular, we see that shear offers a possibility to prevent the initial singularity formation. We also provide some exact solutions in the shear-dominated era and discuss the physical significance of the shear current.

The Levi-Civita connection usually adopted on the non-commutative three-sphere is generalized to a connection possessing torsion and a non-metricity of the Weyl kind. This leads to geometrical properties that are qualitatively different from the ordinary ones and, in particular, provides an example of non-commutative “teleparallelism”. The comparison is made with the analogous case in classical (i.e. commutative) differential geometry. A general framework is developed, from which the unique connection having a given metric, torsion and non-metricity can be determined.

To show the important role that geometry may play in the problem of confining particles to our four-dimensional spacetime, we consider a five-dimensional model where fermions are confined in a hypersurface due to an interaction with geometric fields. We use two different fields which appear in non-Riemannian geometries, namely, the Weyl one-form and the torsion two-form. We show that for suitable choices of these fields one manages to confine fermions in our brane (our four-dimensional world). It turns out that this confinement is independent of the energy and the mass of the fermions.

In this manuscript, we will discuss the construction of covariant derivative operator in quantum gravity. We will find it is more perceptive to use affine connections more general than metric compatible connections in quantum gravity. We will demonstrate this using the canonical quantization procedure. This is valid irrespective of the presence and nature of sources. General affine connections can introduce new scalar fields in gravity.