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We develop a symmetric teleparallel gravity model in a space–time with only the nonmetricity as nonzero, in terms of a Lagrangian quadratic in the nonmetricity tensor. We present a detailed discussion of the variations that may be used for any gravitational formulation. We seek Schwarzschild-type solutions because of its observational significance and obtain a class of solutions that includes Schwarzschild-type, Schwarzschild–de Sitter-type, and Reissner–Nordström-type solutions for certain values of the parameters. We also discuss the physical relevance of these solutions.

Within the framework of metric-affine gravity (MAG, metric and an independent linear connection constitute space–time), we find, for a specific gravitational Lagrangian and by using prolongation techniques, a stationary axially symmetric exact solution of the vacuum field equations. This black hole solution embodies a Kerr–de Sitter metric and the post-Riemannian structures of torsion and nonmetricity. The solution is characterized by mass, angular momentum, and shear charge, the latter of which is a measure for violating Lorentz invariance.

Inter-relation between 1-form of nonmetricity and change of entropy in the course of time is considered in the study. It is shown that change of entropy in expanding universe will be always negative. The obtained result contravenes the second law of thermodynamics, however it explains available ordered macrostructures in the universe.

This paper concerns the relationship between the nonmetricity 1-form and the change in entropy. Motion equations have been obtained for test bodies in a gravitational field created by a massive body with entropy varying over time. It has been shown that increasing entropy of the gravitational source will bring about an increase in the acceleration of the test body. Applied to the theory of gravitation with nonmetricity, black hole dynamics equations based on foundations laid by S. Hawking and J. Beckenstein, enabled identification of changes in black holes event horizon surface area as a putative source of nonmetricity field. The implication is that changes in event horizon area will affect test body motion. The latter property makes it possible to contemplate a completely new method for discovering short-lived microscopic black holes.

The physical meaning and mathematical meaning of nonmetricity on Weyl manifold and the application of dual affine connection for it are discussed. In viewpoint from affine connection, Weyl 1-form in Weyl manifold is recognized as nonmetricity and causes scale change with conformal invariance under parallel transport. On manifolds expressing spacetime and material space, Weyl 1-form is equivalent to the extra coupling field to field expressed as Riemannian manifold, protruding from Riemannian manifold. The scale change with conformal invariance corresponds to the similar change in the magnitude of the field with the volumetric distortion of each manifold. Weyl 1-form is the change rate of volume element (scale) in Riemannian manifold. Therefore, nonmetricity in Weyl manifold is defined as expansion or contraction rate as similar volumetric distortion due to the extra coupled field. The volumetric distortion as nonmetricity on Riemannian manifold can be canceled out by setting dual affine connection on Riemann–Cartan–Weyl manifold, which is used in statistical manifold. Moreover, the role of nonmetricity in modified gravity theory in the framework of symmetric teleparallel manifold and the meaning of nonmetricity on statistical manifold are discussed based on this concept.

This work interprets the quantum terms in a Lagrangian, and consequently of the wave equation and momentum tensor, in terms of a modified spacetime metric. Part I interprets the quantum terms in the Lagrangian of a Klein–Gordon field as scalar curvature of conformal dilation covector nm that is proportional to $\hslash$ times the gradient of wave amplitude R. Part II replaces conformal dilation with a conformal factor $\rho$ that defines a modified spacetime metric $g^{\prime }$= exp$(\rho )g$, where g is the gravitational metric. Quantum terms appear only in metric $g^{\prime }$ and its metric connection coefficients. Metric $g^{\prime }$ preserves lengths and angles in classical physics and in the domain of the quantum field itself. $g^{\prime }$ combines concepts of quantum theory and spacetime geometry in one structure. The conformal factor can be interpreted as the limit of a distribution of inclusions and voids in a lattice that cause the metric to bulge or contract. All components of all free quantum fields satisfy the Klein–Gordon equation, so this interpretation extends to all quantum fields. Measurement operations, and elements of quantum field theory are not considered.

In the conventional formulation of general relativity, gravity is represented by the metric curvature of Riemannian geometry. There are also alternative formulations in flat affine geometries, wherein the gravitational dynamics is instead described by torsion and nonmetricity. These so-called general teleparallel geometries may also have applications in material physics, such as the study of crystal defects. In this work, we explore the general teleparallel geometry in the language of differential forms. We discuss the special cases of metric and symmetric teleparallelisms, clarify the relations between formulations with different gauge fixings and without gauge fixing, and develop a method of recasting Riemannian into teleparallel geometries. As illustrations of the method, exact solutions are presented for the generic quadratic theory in 2, 3 and 4 dimensions.

We present a novel theory of gravity, namely, an extension of symmetric teleparallel gravity. This is done by introducing a new class of theories where the nonmetricity Q is coupled nonminimally to the matter Lagrangian. This nonminimal coupling entails the nonconservation of the energy-momentum tensor, and consequently the appearance of an extra force. We also present several cosmological applications.

In this manuscript, we will discuss the construction of a theory of gravity with nonmetricity and general affine connections. We will consider a simple potential formalism with symmetric affine connections and symmetric Ricci tensor. Corresponding affine connections introduce two massless scalar fields. One of these fields contributes a stress-tensor with opposite sign to the sources of Einstein’s equation when we state the equation using the Levi-Civita connections. This means we have a massless scalar field with negative stress-tensor in the familiar Einstein equation. These scalar fields can be useful to explain dark energy and inflation. These fields bring us beyond strict local Minkowski geometries.