Modifications of Gravity Via Differential Transformations of Field Variables

Abstract

We discuss field theories appearing as a result of applying field transformations with derivatives (differential field transformations, DFT) to a known theory. We begin with some simple examples of DFTs to see the basic properties of the procedure. In this process the dynamics of the theory might either change or conserve. After that we concentrate on the theories of gravity which appear as a result of various DFT applied to general relativity, namely the mimetic gravity and Regge-Teitelboim embedding theory. We review main results related to the extension of dynamics in these theories, as well as the possibility to write down the action of a theory after DFT as the action of the original theory before DFT plus an additional term. Such a term usually contains some constraints with Lagrange multipliers and can be interpreted as an action of additional matter, which might be of use in cosmological applications, e.g. for the explanation of the effects of dark matter. action, which often turns out very helpful, and addresses many other issues in the theory of gravity, e.g., its quantization [3] and unification with other theories (for a historical survey, see [4] and the references therein; see also [5]).Another early attempt to reformulate GR in terms of alternate variables was made by Cartan [6] and later by Einstein himself (possibly independently, see [7]). They introduced the notion of the tetrad (or vierbein), which has been extensively used in the GR since then. In particular, it allowed V. A. Fock to generalize the Dirac equation to the case of curved spacetime [8]. Moreover, the tetrad approach provides a very convenient way to study torsion in the so-called Einstein-Cartan theory (see, e.g. [9]). Later, Newman and Penrose proposed another kind of tetrad approach to the GR, which was named after them and proved itself very powerful in the analysis of the geometrical structure of GR [10].Probably the most frequently used variables in GR besides the metric are canonical variables of some kind. Indeed, almost any attempt of quantization requires a Hamiltonian and the corresponding canonical formulation. Since the appearance of the pioneering work of Arnowitt, Deser and Misner [11], many other canonical formulations of gravity have been developed [12]. Of course, the canonical approach to GR can be combined with those above [13], leading, e.g., to the loop variables. Besides quantization, Hamiltonian formulation of GR can be of use, e.g., in the analysis of compact sources [14].It must be stressed that all these approaches (at least in their simplest forms) in general do not necessarily lead to the modification of dynamics. However, there are some redefinition procedures of the field variables in gravity, which are by default altering the dynamics of the theory. Many of these are characterized by the presence of additional derivatives of new field variables in the relation between old and new ones.The main purpose of this paper is to put together the results and methods related to the extended dynamics of Lagran...

Canonical Description for Formulation of Embedding Gravity as a Field Theory in a Flat Spacetime

“…This similarity between embedding gravity and mimetic gravity is due to that fact that in both theories the modification of theory occurs as a result of substitution of the expression for the metric, which contains derivatives of some new variables, into EH action. After such modification of the theory, which is a particular case of differential transformations of field variables, the extension of dynamics occurs in most cases [43].…”

“…The simplest example is a brane (and, in particular, a string) action, for which R in ( 23) needs to be replaced by an arbitrary constant and L m needs to be excluded. It is worth mentioning that another modification of the original embedding approach also provides a way to alternative description of strings and branes, see [43].…”

“…the quantity n A which is connected toż A by relations (44) and (45). Transforming the integration variables in the path integral (47) from π A to n A and using (43) and (46), we can rewrite it in the form…”

Energy–Momentum Pseudotensor and Superpotential for Generally Covariant Theories of Gravity of General Form

“…The action of such theory remains unchanged in terms of dependence on the old variables. As discussed in the works [15,26], such modification could lead to richer dynamics in comparison to the original theory. Namely, a new theory has all the solutions from the original one and also has some extra solutions, that may appear useful for the explanations of certain observable effects that are absent in the original theory.…”

Non-Relativistic Limit of Embedding Gravity as General Relativity with Dark Matter

“…This model appears as a result of the change of variables, which isolates the conformal mode of the metric. Since this change of variables involves differentiation (being a particular case of the differential field transformations in GR [16]), it leads to the extension of dynamics of the theory, and some additional degrees of freedom w.r.t. GR appear.…”