From: "Al G"
Date: 16 Nov 2006

Q What is the difference between "the big Rip" vs "the big Crunch"?

A "Big Rip" and "Big Crunch" are colorful descriptions for possible ultimate fates for our universe.

We've known since Hubble's work in the 1920s that our universe is expanding, since we see the galaxies within it rushing apart from one another. One of the great questions of cosmology is whether this expansion will continue on forever or if the universe will eventually recollapse upon itself under its own gravity - a "Big Crunch" to match the "Big Bang".

To answer this question we need to know the amount (and kinds) of stuff there is in the universe. If the universe has enough matter in it the gravitational pull of that matter will eventually halt the expansion and draw space back in on itself. If not, the expansion will continue. Recent measurements of the microwave background and of distant supernovae have led us to believe that this will not happen - there is not enough matter in the universe to cause a recollapse, and the universe will indeed expand forever. Surprisingly, recent observations have also taught us that the universe's expansion is actually speeding up due to a repulsive force we call "dark energy". So the "Big Crunch" has mostly been ruled out as a fate for our universe.

The "Big Rip" is a much more speculative fate. The question here is just how strong the repulsive force of dark energy is, characterized by an "equation of state" parameter called w. If w is greater than or equal to -1, the universe's expansion will continue to accelerate but bound objects within the universe will remain bound. The stars in our own galaxy, for example, are bound to one another by their own gravity and so orbit the galactic center. As the expansion accelerates in the distant future, distant galaxies will zoom away from us but our own galaxy will always hang together. This is what happens in nearly all mainstream theories.

Some theorists have suggested the idea of "phantom energy", a sort of pumped-up dark energy with w < -1. In this case the repulsive force is stronger, and eventually bound objects are ripped apart by the universe's expansion. In these models, our own galaxy will eventually be broken up, and eventually even atoms themselves will be ripped apart by the relentless expansion. This dismal fate is called the "Big Rip".

The Big Rip is an intriguing thought, but it's very speculative and there is no evidence that it will actually take place. Current observations suggest that w is fairly close to -1, the value expected from a cosmological constant. Cosmologists continue to work on measuring w more accurately (it's extremely difficult), and hopefully we can someday get a handle on how things will finally play out.

Thanks for the question,

Jeff Filippini
UC Berkeley Cosmology Group