What are you guys talking about? There is no equivalence here! The twin taking the space flight has to first accelerate to near the speed of light and then change acceleration to head back towards the earth. And then again he changes acceleration to slow down so he doesn't collide with the earth. His acceleration is not constant at all. The twin on the earth, on the other hand, experiences constant acceleration of 1g the entire time. There is no equivalence here and hence no problem.