Time theory

--Linear vs Cyclical Time Theory--

It is implied in The Late Phillip J. Fry that time follows a sort of cycle, that the remaining particles of the previous universe come together to create a new universe by way of a second big bang. Within the second Cyclic Universe model, the second big bang is simply one of many and the crew that travel through time are not the only ones that do so. They don't ever return to their own universe, merely a post-Big Bang copy of it, which would explain the Professor's remark about the new universe being "three feet lower than the old one". This is two universes "further down" the "line" of linked big bang events and subsequent universal decay.

This Cyclic Universe model does not match up with the crew's previous treks through time, since in those, it was always the same universe, never a new one, it's an equal version created after the original is destroyed. It is not a second universe, but an exact recreation of the previous one where all living things are reborn and all events happen again(unless, of course, time travelers alter it). In short, any parallel/alternate universe must always contain a version of its own universe. This follows a highly deterministic model where any event is "set" and in theory any single "random" event could be predicted, and possibly, foreseen by looking at the interactions of all particles during and/or since the original event(the big bang) that set the universe in motion.

The Cyclical Time theory allows for random and non-deterministic fluctuations within events and fits in with previously established methods of time travel, whether by machine or time code. (Let's not get started on the convoluted mess that is Bender's age and the time code, that's another can of worms)