'Big Rip'

Cosmology estimates that the cosmos did in fact begin acceleration about 5 billion years ago, give or take a billion years - before that, it was thought that dark matter and baryons where decelerating the universe. However, the density of dark matter disappears quickly in an expanding universe and will inexorably disappear, leaving dark energy to become the dominant matter in the universe.

When the volume of the universe doubles, the density of dark matter is halved, but the density of dark matter is, as i am sure you could guess, halved, and according to the cosmological constant, dark energy is almost unchanged. Though, here is where Armageddon raises its ugly head, and informs us, if acceleration continues, then our universe will probably face a 'Big Rip'. The hypothesis focuses on the quintessence, given the appropraite title of phantom energy, and a steady balance in the equation of state (w), which resembles the ratio between the dark energy pressure and its energy density.

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Tensed Spacetime

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When we reach in mathematics w<-1, the universe will be violently ripped apart. In a universe that expands at this alarming rate would mean that the observable universe is shrinking (which is around 14 billion light years away - note, that it is an estimated 40 billion light years away to the edge of the universe). This means that all of those galaxies and stars out on the observable edge of our universe are being whisked away at the speed of light - dragged by space and time itself, (this is not in violation of relativity). You can imagine, as the observable universe (a matter dominated region) produces a lot of gravity - and as they are pulled away from each other, interaction of the forces will be strained - and eventually when it reaches a certain critical value, absolutely no interaction will occur - no gravitational pulls, no electromagnetic, no influence of strong or weak forces... the result is a catastrophic rip. Not one particle will survive the rip.

The authors of this doomsday vision say that the first evidences would be that the galaxies are separated from each other - some scientists argue hauntingly that this is what is happening right now. 60 million years before the end, gravity will not be able to support our galaxy, the Milky Way, and other individual galaxies. In the very last three minutes, our planet will be ripped apart by the destructive forces of acceleration. Not to worry though - by then our sun will have turned into a Red Giant, and we will have all been cindered alive - then all that would be left to rip apart would be a barren earth and a star that was now a black dwarf; and i don't like the thought of that. Perhaps this 'civilized' race will blow itself up first using nuclear weapons... Alternatively, our solar system might suffer heat death or (Big Chill theory), with the local superclusters moving beyond the observable horizon, preventing any communication between them causing all useable energy negligible. This was the preferred fate of the universe before big rip theory... however, dark energy is not impervious - and thus can dissipate with time. This means that the main proponents that might cause the big rip, might not happen.

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Other Suspects

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The Omega Point may come about via another cause. Gravity... that weak force that keeps us fixed to this planet might even be responsible for the contraction of the universe. If the gravitational attraction of all the matter in the observable universe where strong enough, the universe would slow down, and eventually reverse on itself. It is believed, that if big crunch was to happen, it would surely contract as fast as it did expand. This theory is caused through the hypothetical function of (LQG) - Loop Quantum Gravity. A lot of speculation has been made concerning when this might happen; one idea is that contraction will begin in about 25 billion years from now... according to physicist Frank Tipler, the universe will begin to contract 10 billion billion years from now - so no immediate worry eh? Though, let us work with the smaller estimation here. Eventually, after 42 billion years of contraction, everything will be shoved back into the gravitational singularity whence it came from, and space and time will end. For this to happen though, there needs to be a fine tuning in the density of matter in the overall spatial curvature of our universe making it positive, just like the surface of a sphere - if it is less than this hypothetical critical value, curvature is thus negative, and is usually depicted as being a saddle shape, called the 'hyperbolic surface,' and here gravity is far too weak to have any effect to counter the inertia completely... in this picture, the universe will slow down, but never contract... but this, as we can see, using standard white dwarf candles, mentioned in part one, shows us that the universe is not slowing down, but is in fact speeding up in its uncontrolled expansion - thus a big rip might be more appropriate.

Of course, there is also the problem of galactic collisions. Recently, the Hubble Telescope captured images of the 'Antennae galaxies,' colliding into each other, giving off vibrant orange colors. Our own galaxy is in fact rushing towards Andromeda galaxy. If my memory serves me right, Andromeda is coming towards us at speeds around 300,000 miles per hour, and in around seven billion years, our galaxy will crash to its death. Thus, we will not die of a Big Rip or a Big Crunch, though we might die by an intergalactic collision... that is if our Sun has just enough fuel...