Taken to its absolute limit, space travel is limited by the speed of light. So when you get to that limit, you are limited by space, as you can only expand in a

Taken to its absolute limit, space travel is limited by the speed of light. So when you get to that limit, you are limited by space, as you can only expand in a sphere--i.e. O(n^3). Space is so enormous that even the tiniest exponential will overrun O(n^3) before even getting out of our galaxy. Result. That exponential is going to flatten out and look more like a sigmoid.