Ways the world COULD end:
Human scale:
Obviously, this is the largest group. We are just one pathetic species among many, with very few things to brag about (like aposable thumbs and the computer). Barely any species on Earth has lasted a few million years (yes, even dinosaurs don't serve as an excuse [yes, the Planet is really old, 4.6 billion years]), so saying "we will last forever" is total bull ****. We are fragile, and we can become even more fragile.
Things that would work:
outbreak
Meteor
Gamma ray burst
Super volcanic eruption
rapid climate change (global warming to the idiots)
and more...
Life as we know it scale:
Face it, life AS WE KNOW IT is pathetically fragile, It doesn't take much to destroy it. We had a bunch of near misses with Mass extinctions, one wiping out over 90% of earth's species 270 million years ago (yes, there was something worse than the Dino killing asteroid). There is a key reason why I didn't say Life in general, and that I will explain later...
Really big asteroid
Gamma ray burst
Ozone depletion
Life in general:
The reason I split Life is because Life as we know it involves fragile plants and Animals, while life in general includes Bacteria, which are ****ing hard to kill (even with Antibiotics), it would take something truly devastating to eliminate life altogether
Sun's aging
Earth:
It is hard enough to get rid of the bacteria on Earth, try the planet itself. Unfortunately, such things do exist...
Black holes
Sun's aging (in theory)
Galactic collision
The Universe:
There are two ways the universe will die, the fast way and the slow way. The fast way is also known as the Big rip. see there's something called Dark energy (dark energy is something we don't know. If it sounds scary, we don't know what it is. If it sounds confusing, we do). Essentially, this is the exact opposite of gravity (gravity pulls stuff together, dark energy pulls stuff apart), and since the very start, the two were in a tug of war. Until around 5 billion years ago, gravity was winning, but was losing it's grip. 5 BYA, and now Dark energy has gone amok, accelerating the universe's expansion. The degree is up to question. The Big rip is the fast way, which, in lame man's terms, says that everything in the entire universe, explodes. The galaxy explodes, the solar system (or what's left of it, this thing isn't expected for 50 billion years) explodes, the planet Earth (if it's still around) explodes, humans explode, and finally atoms explode. The long way, does not have an official term (let me put it in lame mans terms, we're all gonna die the Nordic "by ice" method, instead of the Christian "by fire" method). Essentially, we are living on the universes High point, or for the pessimist/Xbox360 owner, on the borrowed time point. According to the second law of thermodynamics, every thing has a rate of decay (goodbye dreams of mortality), so there will be a point where every star in the sky, dies. The era of the universe after that is called the degenerate era. What's left of galaxies fall apart, and become prey of black holes. What matter doesn't get destroyed this method would essentially dissolve into nothing. Essentially, the atom will fall apart (and so would Protons) and there is only one thing left recognizable in the galaxy, Black holes. Hello black hole era. Unfortunately, even these things have a time limit on their life. Due to hawking radiation, and lack of fuel, Black holes have only each other to eat, and when they don't, MORE EXPLODING. Once those are gone, the final era of the universe starts, the Dark era. essentially, nothing except Microscopic particles in the cosmic wind exists. To end on a bright note, Empty space does count as something according to quantum Mechanics, and things that could happen given an infinite amount of time (which the Dark era essentially is) is the formation of, a new universe. We live in a multiverse reality, so there are many, many universes out there, each with it's own laws of physics. If we can find a way to them, we can escape the end. Then again... METROID UNIVERSE, HERE I COME!