Image that shows how an object's time curve can loop around the cylinder and meet up with itself.
John D. Norton
<a rel="nofollow" class="external free" href="http://www.pitt.edu/~jdnorton/teaching/HPS_0410/2012_Spring/assignments/10_cosmology/cylinder_U.gif">http://www.pitt.edu/~jdnorton/teaching/HPS_0410/2012_Spring/assignments/10_cosmology/cylinder_U.gif</a>