A Big Crunch?

A wealth of astronomical observations indicates that the expansion of our Universe is accelerating. If this trend continues, our Universe is destined to expand forever. In the far distant future, the Universe will be a cold, desolate place with almost no prospects for life. All the stars will burn out, protons will decay into their subatomic constituents, and the cosmic microwave background will have cooled to a temperature a tiny fraction of a degree above absolute zero.

Is there a way around this less-than-appealing "Big Chill" scenario? Is there any hope that our Universe may somehow turn around its expansion and collapse back upon itself, ending up in a Big Crunch that may trigger a cosmic rebirth? Most scientists would say "no," because the available evidence strongly supports the Big Chill. But our Universe may be holding a wild card up its sleeve that offers a glimmer of hope for an eventual cosmic renewal.

Most physicists think the dark energy is the result of quantum fluctuations, in which empty space itself exerts an outward pressure due to the constant appearance and annihilation of virtual particles. This is analogous to Einstein's famous cosmological constant, in which vacuum energy is an inherent property of space. But when physicists calculate the effect the cosmological constant should have on our Universe, they find a value 10¹²⁰ greater than what we observe. This is the most spectacular disagreement between theory and observation in the history of science. Physicists try to get around this problem by invoking some unknown mechanism that almost but not completely cancels this effect, but so far they have not been able to reach a consensus of how this could happen.

Still, many scientists think dark energy is somehow related to vacuum energy. If this is true, the more volume of space that is created by cosmic expansion, the stronger the pressure becomes, and the faster the Universe expands. If the dark energy is indeed due to Einstein’s cosmological constant, as most physicists think, the Universe's expansion will continue to accelerate forever and there is no hope of avoiding a Big Chill.

But a few physicists have suggested that the dark energy might be due to some kind of "quintessence" field that pervades our Universe. And this field might be dynamical, meaning it changes over time. If such an evolving field is responsible for the current acceleration, it's conceivable that the current outward pressure may reverse itself, causing our Universe to collapse upon itself. This scenario is not ruled out by any observational evidence or laboratory experiment, but most scientists consider it to be extremely unlikely. The poet Robert Frost once wrote that the world is destined to end in fire or ice. Right now, most bets are on the ice.