What's the ultimate fate of the universe? Man, you guys ask some pretty heavy questions. I mean, seriously, just a few weeks ago it was fun, light stuff about how shower curtains wave around. And now we're on to subjects spanning deep space and cosmic time. You don't mess around, do you?

The crazy thing about modern physics - and especially cosmology, the branch of science dealing with the study of the whole entire universe - is that we can actually begin to answer crazy, ridiculous questions like "How will the universe end?" The answer is, of course, "we're not exactly sure". But I can tell you what the answer isn't.

We live in an expanding universe, and the amount and type of stuff in the universe affects how quickly it expands. If there's too much stuff like stars and gas and bananas the expansion will eventually slow down, stop, and reverse. Instead of spreading out the galaxies will rush towards each other. Individual objects lose their identity as they crush into the same ever-increasing volume. Temperatures and pressures rise. Atoms turn to plasma. The forces of nature meld into more exotic forms. Everything you know - everything there is - compresses into an infinitely dense point.

The opposite of the Big Bang. Behold: the Big Crunch.

...is unlikely. Almost 20 years, observations of distant supernovae revealed something surprising. Not only is the universe expanding, but it's expanding at an ever-faster rate. We don't fully understand it, but that didn't stop us from giving it a cool name: Dark Energy. It started about 5 billion years ago and doesn't look like it's going to stop. Our long-term fate isn't a Crunch, but a Rip.

Pick a friend and head down to the equator. The specific country doesn't matter; just go. Stand a few feet apart and face north (pro-tip: pack a compass). Start walking, one baby step at a time, never deviating from a northern course.

Even though the two of you start perfectly parallel, you'll eventually end up bumping into each other. The exact place where this happens has a name: the north pole.

Congratulations, you've just proved that the Earth is curved.

That is, in fact, the very definition of a curved surface: lines that start out parallel end up not being parallel. Take out a globe and look at all the lines of longitude (the up-and-down ones): they're parallel at the equator but all intersect at the poles. If the Earth were flat, those parallel lines would stay parallel forever.

Our universe, as far as we can tell, is flat. Parallel lines racing through space will always stay parallel. But like I said: that's as far as we can tell. Within our little observable bubble, everything is flat. But the actual universe is much larger than we can see, and it's shape is...well, we don't know. It's like trying to figure out the curvature of the Earth by just drawing lines in your back yard.

Here's something to bend your mind before I go. The universe can be geometrically flat, but topologically curved. Find a glass and draw parallel lines. Even though they wrap back around on themselves, they stay parallel, don't they? Cylinders are technically flat! So maybe if you rocket off into space in one direction far enough, you'll end up back where you started.