CERN-TH.6236/91 DISCRETE QUANTUM GRAVITY: THE REGGE CALCULUS APPROACH 1 Ruth M. Williams 2 Theory Division, CERN, 1211 Geneva 23, Switzerland Abstract After a brief introduction to Regge calculus, some examples of its application in quantum gravity are described. In particular, the earliest such application, by Ponzano and Regge, is discussed in some detail and it is shown how this leads naturally to current work on invariants of three-manifolds. CERN-TH.6236/91 September 1991 1Invited talk delivered at the Research Conference on Advanced Quantum Field Theory and Critical Phe- nomena, Como, Italy, 17{21 June 1991. 2Permanent addresses: Girton College, Cambridge CB3 0JG, and D.A.M.T.P., Silver Street, Cambridge CB3 9EW, U.K. 1 REGGE CALCULUS The essential idea of the discrete approach to general relativity known as Regge calculus [Regge 1961] is the use of piecewise linear spaces, in which the curvature is restricted to subspaces of codimension two, the \hinges". For example, a two-dimensional curved surface can be approxi- mated arbitrarily closely by a network of at triangles and the curvature resides at the vertices. The de cit angle at a vertex, de ned by X " =2 vertex angles ; (1) triangles meeting at vertex gives a measure of the curvature there. In three dimensions, the lattice consists of at tetrahe- dra, with curvature restricted to the one-simplices or edges. Similarly in four dimensions, the curvature is distributed over the triangles. Regge showed that the discrete form of the Einstein action 1 Z p I = R gdnx (2) 2 is given by X i IR = j j"i ; (3) hinges i i i where j j is the volume of a hinge and "i is the de cit angle there. The edge lengths, which completely specify a simplicial space, are the variables of the theory, and so the requirements for such a space to be a classical solution of Einstein's equations may be obtained by varying the Regge action with respect to these lengths. Regge [1961] showed that the term involving the variation of the de cit angle vanishes, and so the Regge calculus analogue of Einstein's equations is X @jij "i =0: (4) i @lj Simplicial spaces are of great interest in their own right, especially in the light of theories that the underlying structure of space{time may be discrete. If, on the other hand, they are regarded as approximations to spaces where the curvature is distributed continuously, it is 1 important to study their convergence properties in the continuum limit. Cheeger, Muller and Schrader [1984] have shown that the Regge action converges to the continuum action in the sense of measures, provided that certain conditions on the fatness of the simplices are satis ed. Barrett, in a series of papers (see, e.g., Barrett 1988 and references therein) has studied the relationship between solutions of the Regge equations and continuum solutions of Einstein's equation, and has formulated a convergence criterion in the linearized case. There are many mathematical results on piecewise linear spaces which have potential use in a theory of quantum gravity based on Regge calculus. Let me mention some work with Martin Rocek, in which we have obtained some new formulae for the Euler characteristic of a simplicial manifold of dimension 2n, rstly in terms of the numbers of even-dimensional subsimplices and secondly in terms of even-dimensional de cit angles [Rocek and Williams 1991]. The starting point is the well-known formula for the Euler characteristic X2n (2n) i (2n) = (1) Ni (5) i=0 (2n) where Ni is the number of subsimplices of dimension i. For a given simplicial complex, (2n) the numbers Ni are not independent but satisfy a set of linear equations called the Dehn{ (2n) Sommerville equations. These can be used to eliminate the Ni 's for odd i from (5) to give Xn (2n) (2n) = ai+1N2i ; (6) i=0 with 2 a = (22i 1)B ; (7) i i 2i where fB2ig are the even Bernoulli numbers. Now a set of equations exactly equivalent to the (2n) Dehn{Sommerville ones, is also satis ed by the angle sums Si de ned by (2n) X Si = (i; 2n) ; (8) i2n where (i; j) is the normalized interior dihedral angle at the subsimplex j in the simplex i, and we have de ned (2n) (2n) S1 0 ;N1 : (9) Therefore the result for the angle sums analogous to (6) is n X (2n) 0= ai+1 S2i : (10) i=0 We can then use (7) and (10) to show that nX1 X (2n) = a "(2n) ; (11) i+1 2i i=0 2i where the normalized k-dimensional de cit angle at a simplex j is de ned by (k) X "j =1 (j; k) : (12) kj 2 For example, in four dimensions, (12) gives X 1 X = "(4) "(4) ; 0 2 2 0 2 (4) 0 (4) where "0 is the \solid" de cit angle at a vertex , and "2 is the de cit angle at a triangle (the usual de cit angle appearing in four-dimensional Regge calculus). The appearance of the Bernoulli numbers in these formulae is intriguing; characteristically they appear in expressions that are in some sense approximations (for example in nite di erence approximations to deriva- tives) and their presence here may re ect the idea that a piecewise linear space is in a sense an approximation to a space with continuously distributed curvature. 2 QUANTUM APPLICATIONS Rather than producing an exhaustive list of work on quantum gravity using Regge calculus (which can be found in Williams and Tuckey, 1991), I shall just mention a few typical applica- tions in di erent categories. 2.1 Perturbative Calculations Analytic calculations in four dimensions using Regge calculus are very complicated, so most work of this type has involved the weak eld limit. Rocek and Williams [1984] calculated the graviton propagator by considering small uctuations about a at-space background and showed that it agreed with the continuum propagator in the long wavelength limit. 2.2 Numerical Simulations Recent progress in the understanding of functional integral methods for simplicial quantum gravity, and the need for a non-perturbative approach, have led to some large-scale numerical studies, using ideas developed in lattice gauge theories. The basic idea is to start with, say, a at lattice and then to allow the system to evolve, using a Monte Carlo algorithm, to an equilib- rium con guration, about which the edge lengths make quantum uctuations. The expectation values of certain operators (e.g. volume, curvature) can then be measured. The inclusion of a curvature-squared term to ensure positivity of the action and hence convergence of the functional integral, was considered by Hamber and Williams [1984, 1986a, 1986b] in their calculations in two and four dimensions. For a lattice with the topology of a four-torus, it was found that at strong coupling the system developed an average negative curvature. Hamber [1990, 1991] has carried out further investigation of the phase diagram and critical exponents for pure gravity in four dimensions, using lattices of size up to 164. Berg [1986] has also performed numerical simulations of gravity in four dimensions, keeping the total volume constant. The two-dimensional Regge calculus numerical simulations I have just mentioned are not the same in method as the large number of recent two-dimensional simulations using equila- teral triangles and summing over triangulations. These calculations use the Regge action but there is no integration over edge lengths. However it seems that the results from the two 3 approaches are compatible (see, e.g., Gross and Hamber 1991). Work is currently in progress on three-dimensional simulations using both the standard Regge calculus approach with xed triangulation and integration over edge lengths, and the dynamical triangulation method with equilateral tetrahedra, and it will be very interesting to see how the results compare. 2.3 Quantum Cosmology In quantum cosmology, it has been proposed that the wave function of the Universe, (G(3)), for a given three-geometry G(3), can be calculated from the functional integral, Z (G(3))= D[G(4)] exp[I(G(4)] (13) where the integral is over all four-geometries G(4) having the required three-geometry as bound- ary, I[G(4)] is the Einstein action with cosmological constant and D[G(4)] is some measure on the space of four-geometries. The integral can be approximated by a sum over discrete ge- ometries described by a nite number of parameters, and Regge calculus provides the natural way of specifying such geometries. In a series of papers on simplicial minisuperspace, Hartle [1985a, 1986, 1989] has described how this idea may be implemented and has performed a sam- ple calculation using a ve-simplex model. Hartle [1985b] has also discussed the need to sum over topologies as well as over geometries, and this means that triangulations are needed for many di erent four-manifolds. Hartle, Sorkin and Williams [1991] have studied triangulations on product manifolds. 2.4 The work of Ponzano and Regge Long before any of the applications described so far, Ponzano and Regge [1968] had pointed out an extremely interesting connection between a sum involving the asymptotic values of 6j-symbols associated with a triangulated three-manifold, and the Feynman path integral for three-dimensional simplicial quantum gravity with the Regge action. This was discussed further by Hasslacher and Perry [1981] and Lewis [1983]. Let us look at the Ponzano and Regge result in detail to see how it anticipates current work on invariants of three-manifolds, which will be described in the next section.