We consider four-dimensional spacetimes (M, g) which obey the Einstein equations G= T, and admit a global spacelike G_1= R isometry group. By means of dimensional reduction and local analyis on the reduced (2+1) spacetime, we obtain a sufficient condition on T which guarantees that (M, g) cannot contain apparent horizons. Given any (3+1) spacetime with spacelike translational isometry, the no-horizon condition can be readily tested without the need for dimensional reduction. This provides thus

more »

... useful and encompassing apparent horizon test for G_1-symmetric spacetimes. We argue that this adds further evidence towards the validity of the hoop conjecture, and signals possible violations of strong cosmic censorship.