Introduction to Loop Quantum Gravity. The Holst's action and the covariant formalism
We review Holst formalism and we discuss dynamical equivalence with standard GR (in dimension 4). Holst formalism is written for a spin coframe field $e^I_\mu$ and a $Spin(3,1)$-connection $\omega^{IJ}_\mu$ on spacetime $M$ and it depends on the Holst parameter $\gamma\in \mathbb{R}-\{0\}$. We show the model is dynamically equivalent to standard GR, in the sense that up to a pointwise $Spin(3,1)$-gauge transformation acting on 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 $Spin(3)$-connection $A^i_\mu$ and a $Spin(3)$-valued 1-form $k^i_\mu$. The construction of these new variables relies on a particular algebraic structure, called a reductive splitting. A reductive splitting is 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 $SL(2, \mathbb{C})$ is not unique and it is parameterized by a real parameter $\beta$, called the Immirzi parameter. The splitting is here done on spacetime, not on space, to obtain a $Spin(3)$-connection $A^i_\mu$, which is called the Barbero-Immirzi connection on spacetime. One obtains a covariant model depending on the fields $(e^I_\mu, A^i_\mu, k^i_\mu)$ which is again dynamically equivalent to standard GR (as well as the Holst action). Usually, in the literature one sets $\beta=\gamma$ for the sake of simplicity. Here we keep the Holst and Immirzi parameters distinct to show that eventually, only $\beta$ will survive in boundary field equations.