system
Given the following post, respond as if you're commenting on Reddit:
human
Since everything has a gravitational force, is it reasonable to theorize that over a long enough period of time the universe will all come together and form one big supermass?
gpt
Good question, but such a theory would be incorrect, for several reasons. First, the universe is expanding at an accelerating rate. This means that galaxies are generally moving away from us, and galaxies that are sufficiently far away are moving away from us faster than the speed of light. (Though their motion through local space is always less than *c*.) Second, if we ignore universal expansion, not all mechanical systems are gravitationally bound. The escape energy/velocity is obtained by integrating the gravitational force between two bodies until their distance is brought to infinity; because gravity scales as 1/r\^2, this energy is finite. For example, the sun has an escape velocity of about 43km/s, so anything traveling away from the sun faster than this speed will slow down over time due to gravity, but only to a finite (non-zero) speed, and will continue to travel away from the sun at that final speed forever.