Introduction to loop quantum gravity. The Holst’s action and the covariant formalism

Abstract

We review Holst formalism and dynamical equivalence with standard GR (in dimension [Formula: see text]). Holst formalism is written for a spin coframe field [Formula: see text] and a [Formula: see text]-connection [Formula: see text] on spacetime [Formula: see text] and it depends on the Holst parameter [Formula: see text]. We show the model is dynamically equivalent to standard GR, in the sense that up to a pointwise [Formula: see text]-gauge transformation acting on (uppercase Latin) frame indices, solutions of the two models are in one-to-one correspondence. Hence, the two models are classically equivalent. One can also introduce new variables by splitting the spin connection into a pair of a [Formula: see text]-connection [Formula: see text] and a [Formula: see text]-valued 1-form [Formula: see text]. The construction of these new variables relies on a particular algebraic structure, called a reductive splitting. A weaker structure than requiring that the gauge group splits as the products of two sub-groups, as it happens in Euclidean signature in the selfdual formulation originally introduced in this context by Ashtekar, and it still allows to deal with the Lorentzian signature without resorting to complexifications. The reductive splitting of [Formula: see text] is not unique and it is parameterized by a real parameter [Formula: see text] which is called the Immirzi parameter. The splitting is here done on spacetime, not on space as it is usually done in the literature, to obtain a [Formula: see text]-connection [Formula: see text], which is called the Barbero–Immirzi connection on spacetime. One obtains a covariant model depending on the fields [Formula: see text] which is again dynamically equivalent to standard GR. Usually in the literature one sets [Formula: see text] for the sake of simplicity. Here, we keep the Holst and Immirzi parameters distinct to show that eventually only [Formula: see text] will survive in boundary field equations.