Nonminimal Coupling Of Matter Research Articles

Non-minimal coupling of scalar and gauge fields with gravity: an entropy current and linearized second law

This work extends the proof of a local version of the linearized second law involving an entropy current with non-negative divergence by including the arbitrary non-minimal coupling of scalar and U(1) gauge fields with gravity. In recent works, the construction of entropy current to prove the linearized second law rested on an important assumption about the possible matter couplings to gravity: the corresponding matter stress tensor was assumed to satisfy the null energy conditions. However, the null energy condition can be violated, even classically, when the non-minimal coupling of matter fields to gravity is considered. Considering small dynamical perturbations around stationary black holes in diffeomorphism invariant theories of gravity with non-minimal coupling to scalar or gauge fields, we prove that an entropy current with non-negative divergence can still be constructed. The additional non-minimal couplings that we have incorporated contribute to the entropy current, which may even survive in the equilibrium limit. We also obtain a spatial current on the horizon apart from the entropy density in out-of-equilibrium situations. We achieve this by using a boost symmetry of the near horizon geometry, which constraints the off-shell structure of a specific component of the equations of motion with newer terms due to the non-minimal couplings. The final expression for the entropy current is U(1) gauge-invariant for gauge fields coupled to gravity. We explicitly check that the entropy current obtained from our abstract arguments is consistent with the expressions already available in the literature for specific model theories involving non-minimal coupling of matter with higher derivative theories of gravity. Finally, we also argue that the physical process version of the first law holds for these theories with arbitrary non-minimal matter couplings.

Cosmology in Rastall theory with non-minimal matter coupling

In this paper we will consider cosmological implications of the Rastall theory with a non-minimal coupling with baryonic matter fields. In Rastall theory, the matter energy-momentum tensor is not conserved and the non-conservation is related to the curvature. We will generalize this relation to become dependent on both curvature and matter fields. Cosmology of the model shows more matter abundance compared to the model. We will show that the dynamical system of the model is the same as with an additional degree of freedom.

Deflation of vacuum energy during inflation due to Bulk-Brane interaction

We consider a brane world inflationary model in which inflation is driven by dynamics of a self-interacting scalar field living in the five-dimensional bulk. The scalar field is non-minimally coupled to matter fields on the brane and acts as an inflaton which induces a slow-roll inflation. We show that although the Friedmann equation is modified at early times due to effects of the extra dimension, the slow-roll condition is the same as that of the four-dimensional case. Due to the non-minimal coupling of matter with the bulk scalar, there is an energy transfer between the two components. We investigate the conditions under which the direction of this energy transfer can be from matter onto the bulk scalar. There are at least two advantages in this case. 1) It establishes a mechanism by which a large effective cosmological term on the brane is deflated during the inflation period. 2) The energy flow onto the bulk inflaton gives a more strongly damped evolution of the scalar field in the slow-roll region for a given potential. We then show that our results are supported by numerical estimations with quadratic and exponential potentials.

General relativity as a special case of Poincaré gauge gravity

We demonstrate that Einstein's general relativity theory arises as a special case in the framework of the Poincar\'e gauge theory of gravity under the assumption of a suitable nonminimal coupling of matter to the Riemann-Cartan geometry of spacetime.

Irreversible thermodynamic perception of particle creation in generalized non-minimally coupled gravity

In this article we are interested to deal with non-minimally coupled F(R, T, Q) theory of gravity where R, T, Q=RμνTμν are Ricci scalar, trace of energy momentum tensor and interaction of Ricci tensor and energy momentum tensor, respectively. Such theories preserve the property that divergence of energy momentum tensor is non-zero. Here, we interpret the generic conservation equation of F(R, T, Q) theory from thermodynamic point of view, as describing irreversible matter creation processes. We formulate some explicit results of particle creation rate, pressure of created particle and entropy with respect to time variation in the context of FRW space-time. We presented our results for three different models of F(R, T, Q) gravity in the background of power law cosmology. It is concluded that non-minimal coupling of matter and geometry can provide a phenomenological description for the role of particle creation or decay on cosmic evolution of matter contents.

Phenomenology in type-I minimally modified gravity

We study cosmology in a class of minimally modified gravity (MMG) with two local gravitational degrees of freedom. We classify modified gravity theories into type-I and type-II: theories of type-I have an Einstein frame and can be recast by change of variables as general relativity (GR) with a non-minimal matter coupling, while theories of type-II have no Einstein frame. Considering a canonical transformation of the lapse, the 3-dimensional induced metric and their conjugate momenta we generate type-I MMG. We then show that phenomenological deviations from GR, such as the speed of gravitational waves cT and the effective gravitational constant for scalar perturbations Geff, are characterized by two functions of an auxiliary variable. We study the phenomenology of several models all having cT=1. We obtain a scenario with cT=1 in which the effective equation-of-state parameter of dark energy is different from −1 even though the cosmic acceleration is caused by a bare cosmological constant, and we find that it is possible to reconstruct the theory on choosing a selected time-evolution for the effective dark energy component.

Note on ghost-free matter couplings in massive gravity and multigravity

We consider a recently proposed non-minimal matter coupling in massive gravity and multi-gravity. We argue that, when formulated in terms of unconstrained vielbeins, this matter coupling contains the primary constraints necessary to remove the Boulware-Deser ghost to all orders away from the decoupling limit.

On the uniqueness of the non-minimal matter coupling in massive gravity and bigravity

In de Rham–Gabadadze–Tolley (dRGT) massive gravity and bi-gravity, a non-minimal matter coupling involving both metrics generically reintroduces the Boulware–Deser (BD) ghost. A non-minimal matter coupling via a simple, yet specific composite metric has been proposed, which eliminates the BD ghost below the strong coupling scale. Working explicitly in the metric formulation and for arbitrary spacetime dimensions, we show that this composite metric is the unique consistent non-minimal matter coupling below the strong coupling scale, which emerges out of two diagnostics, namely, the absence of Ostrogradski ghosts in the decoupling limit and the absence of the BD ghost from matter quantum loop corrections.

Conservation laws in gravity: A unified framework

We study general metric-affine theories of gravity in which the metric and connection are the two independent fundamental variables. In this framework, we use Lagrange--Noether methods to derive the identities and the conservation laws that correspond to the invariance of the action under general coordinate transformations. The results obtained are applied to generalized models with nonminimal coupling of matter and gravity, with a coupling function that depends arbitrarily on the covariant gravitational field variables.

Conformal Transformations and Nordström's Scalar Theory of Gravity

As we shall briefly recall, Nordstr\"om's theory of gravity is observationally ruled out. It is however an interesting example of non-minimal coupling of matter to gravity and of the role of conformal transformations. We show in particular that they could be useful to extend manifolds through curvature singularities.

Alternative to the Palatini method: A new variational principle

A variational principle is suggested within Riemannian geometry, in which an auxiliary metric and the Levi Civita connection are varied independently. The auxiliary metric plays the role of a Lagrange multiplier and introduces nonminimal coupling of matter to the curvature scalar. The field equations are 2nd order PDEs and easier to handle than those following from the so-called Palatini method. Moreover, in contrast to the latter method, no gradients of the matter variables appear. In cosmological modeling, the physics resulting from the alternative variational principle will differ from the modeling using the standard Palatini method.

Junction Conditions in f(R) Theories of Gravity

Taking advantage of the conformal equivalence of f(R) theories of gravity with General Relativity coupled to a scalar field we generalize the Israel junction conditions for this class of theories by direct integration of the field equations. We suggest a specific non-minimal coupling of matter to gravity which opens the possibility of a new class of braneworld scenarios.

Cosmology with nonminimal derivative couplings

We study a theory which generalizes the nonminimal coupling of matter to gravity by including derivative couplings. This leads to several interesting new dynamical phenomena in cosmology. In particular, the range of parameters in which inflationary attractors exist is greatly expanded. We also numerically integrate the field equations and draw the phase space of the model in second order approximation. The model introduced here may display different inflationary epochs, generating a non-scale-invariant fluctuation spectrum without the need of two or more fields. Finally, we comment on the bubble spectrum arising during a first-order phase transition occuring in our model.

Behavior of effective charges in a quantum-field-theory model in an external gravitational field with torsion

Renormalization group equations are obtained for the parameters of nonminimal coupling of matter with an external gravitational field with torsion. The asymptotical behavior of these parameters in the short-distance limit is studied. The asymptotical conformal invariance of the theory is established.

On the renormalization of models of quantum field theory in an external gravitational field with torsion

Renormalization of two asymptotically free models of quantum field theory in curved space-time with torsion is considered. It is shown that the principle of multiplicative renormalizability requires nonminimal coupling of matter with gravitation. Using the equations of a renormalized group it is stated that the models under consideration are asymptotically conformally invariant.

Theories of gravitation with nonminimal coupling of matter and the gravitational field

The foundations of a theory of nonminimal coupling of matter and the gravitational field in the framework of Riemannian (or Riemann-Cartan) geometry are presented. In the absence of matter, the Einstein vacuum field equations hold. In order to allow for a Newtonian limit, the theory contains a new parameter l0 of dimension length. For systems with finite total mass, l0 is set equal to the Schwarzschild radius.