Closed time like curve and the energy condition in 2+1 dimensional   gravity
  We consider gravity in 2+1 dimensions in presence of extended stationary sources with rotational symmetry. We prove by direct use of Einstein's equations that if i) the energy momentum tensor satisfies the weak energy condition, ii) the universe is open (conical at space infinity), iii) there are no CTC at space infinity, then there are no CTC at all. 
P. Menotti and D. Seminara