The Big Rip or The Big Crunch? It doesn't matter because you'll be dead.

   

When we ask What is dark energy? the answer is tricky because no one really knows. Astronomers know it exists but don't know what it is. Einstein theorized that there was in fact energy in a vacuum absent of matter. His theory contained a cosmological constant, which produces a repulsive force in the universe. This repulsive force is accelerating the expansion of the universe. Because dark energy produces this same repulsive force, astronomers use the two terms interchangeably. Einstein's cosmological constant was a true constant, meaning it was constant with time. What if the cosmological constant was not constant with nature but could increase or decrease over time? Astronomers theorize a few possible outcomes possible outcomes.

The Big Crunch

If dark energy were to decrease rapidly then matter would be the dominate force, causing the expansion of the universe to slow down, stop, and reverse until the universe collapsed on itself.

The Big Rip

If dark energy were to increase rapidly then dark energy would be the dominate force, causing the universe to expand to the point where gravity would break down and the universe would fly apart.

The Big Chill

If the universe just slowed in its expansion but never stopped you have what astronomers refer to as the Big Chill. As dark energy took over as the dominate force. everything in the universe would drift farther and farther apart. The galaxies would be too far apart to be visible to each other and the stars would eventually die out until the universe was cold and dark.

Sources:

Kay, Laura. Palen, Stacy. Smith, Brad. Blumenthal, George. 21st Century Astronomy. New York. WW Norton. Print.  2013

HubbleSite staff. "Fate of the Universe." Hubblesite.org. 2016. Space Telescope Science Institute. Web. 28 Apr 16

http://hubblesite.org/hubble_discoveries/dark_energy/de-fate_of_the_universe.php