That makes no sense though. Assuming exponential growth, then it wouldn't grow as a sphere going at the speed of light. The derivative would be a quadratic, not a exponential curve. So if anything that would only approximate the growth for a couple days, at which point exponential would take over. But this presupposes that the humans started on the earth at 5000 AD, where in reality we will have almost certainly taken over venus (aside: Venus is a much more likely target for human colonization than Mars) and probably have several space station states which could easily move at near the speed of light away from the earth.