Dimensionally restricted causal set quantum gravity: examples in two and three dimensions

We study dimensionally restricted non-perturbative causal set quantum dynamics in 2 and 3 spacetime dimensions with non-trivial global spatial topology. The causal set sample space is generated from causal embeddings into spacetime lattices with global spatial topology S-1 and T-2 in 2 and 3 dimensions, respectively. The quantum gravity partition function over these sample spaces is studied using Markov chain Monte Carlo (MCMC) simulations after analytic continuation. In both 2 and 3 dimensions we find a phase transition that separates the dominance of the action from that of the entropy. The action dominated phase is characterised by 'layered' posets with a high degree of connectivity, while the causal sets in the entropy dominated phase are manifold-like. This phase transition is similar in character to that seen for the sample space of 2-orders, which are topologically trivial, hence suggesting that this is a generic feature of dimensionally restricted sample spaces. The simulations use a newly developed framework for causal set MCMC calculations. Ours is the first implementation of a causal set dynamics restricted to 3 dimensions.