The BRST transformations for gravity in Ashtekar's variables are obtained by using the MaurerCartan horizontality conditions. The BRST cohomology in Ashtekar's variables is calculated with the help of an operator S introduced by Sorella [Commun. Math. Phys. 157, 231 (1993)], which allows one to decompose the exterior derivative as a BRST commutator. The BRST cohomology leads to the differential invariants for four-dimensional manifolds.PACS number(s): 04.6O.Ds, 02.40.Re, 11.30.Ly
I. I N T R O D U C T I O NA great amount of work has been done recently in the reformulation of general relativity in terms of a new set of variables that replace the spacetime metric [2]. These new variables, called Ashtekar's variables, have been intensively used in a large number of problems in gravitational physics. Ashtekar's main task was the quantum gravity issue and these new variables, indeed, have opened a novel line of approach to it.In any quantum field theory, one of the most important questions is the existence and t,he form of the anomalies. The anomalies, as well as the Schwinger terms and the invariant Lagrangians, could be calculated in a purely algebraic way by solving the Wess-Zumino consistency condition [3], which is equivalent to a tower of descent equations, involving the nilpotent Becchi-Rouet-StoraTyutin (BRST) operator s, as well as the exterior spacetime derivative d.The usual procedure for solving the descent equations is based on the formula of Baulieu and co-workers and the transgression equation [4] (see also . However, for the gravity in Ashtekar's variables it is difficult to write down these equations and it is necessary to follow a different scheme.First, for the Ashtekar variables, the BRST transformations [14] are different from those obtained in the Yang-Mills case. In addition, the formula of Baulieu and co-workers in the form given for the Yang-Mills case 151, does not hold and we have to find a new method for obtaining a generalization of it in this case. On the other hand, for the Ashtekar variables, as well as for the gravity with torsion [15], it is difficult to write down a transgression equation and to obtain the anomalies, the Schwinger *Permanent address: Dept. Theor. Phys., University of Cluj, Romania. terms, and the invariant Lagrangians. All these quantities are BRST invariants modulo d-exact terms and they can be obtained by using an operator 6 which allows one to express the exterior derivative d as a BRST commutator [I]:Once the decomposition (1.1) has been found, successive applications of the operator 6 on a polynomial Q which is a nontrivial solution of the equationgive an explicit nontrivial member of the BRST cohomology group modulo d-closed terms.The solving of Eq. (1.2) is a problem of local BRST cohomology instead of a modulo d one. Therefore we see that, due to the operator 6, the study of the cohomology of s modulo d can be reduced to the study of the local cohomology of s which, in turn, could be analyzed by using the spectral sequences method [16].In this paper we prove t...