The SETI Institute’s Seth Shostak has some things to say about the future of the universe in a recent posting on Space.com, referring to current observations suggesting that the rate of expansion of the cosmos is speeding up. That could make for a long, long night:

After all, most stars are older than the Sun, and the stellar population boom is definitely over. The Galaxy is graying (although the actual color change is to the red). The stars are going out. In about 100 billion years, the once-brightly spangled arms of the Galaxy will be riddled with Sun-sized carbon clinkers, black holes, and quiescent neutron stars – a hundred billion mute, stellar hulks.

The fun will be over, but the decay will go on. Chaotic encounters will eventually strip planets from the corpses of their erstwhile suns, and galaxies will slowly evaporate – spewing their dark and lifeless contents into the ever-expanding void. Even massive black holes will someday melt away, adding their mass to the inert and keenly cold fog that the universe will become.

A depressing, mind-numbing future indeed. Then assistant professor of physics Sean Carroll came along. Carroll and a University of Chicago graduate student named Jennifer Chen have posted a paper on ArXiv (available here in PDF) studying the so-called arrow of time. After all, “…for the most part the fundamental laws of physics don’t distinguish between past and future. They’re time-symmetric,” Carroll said.

A related issue is entropy, which measures the amount of disorder in the universe. Entropy increases with time, as Ludwig Boltzmann noted early in the 20th Century. Is entropy finite, or infinite? And why was entropy initially so small? “In our current universe,” says Carroll, “the entropy is growing and the universe is expanding and becoming emptier.” But is this true everywhere? And how does it tie into the theory of inflation, which says the universe underwent a period of massive expansion almost instantly after the Big Bang?

From a press release from the University of Chicago.

…even empty space has faint traces of energy that fluctuate on the subatomic scale. As suggested previously by Jaume Garriga of Universitat Autonoma de Barcelona and Alexander Vilenkin of Tufts University, these fluctuations can generate their own big bangs in tiny areas of the universe, widely separated in time and space. Carroll and Chen extend this idea in dramatic fashion, suggesting that inflation could start “in reverse” in the distant past of our universe, so that time could appear to run backwards (from our perspective) to observers far in our past.

Regardless of the direction they run in, the new universes created in these big bangs will continue the process of increasing entropy. In this never-ending cycle, the universe never achieves equilibrium. If it did achieve equilibrium, nothing would ever happen. There would be no arrow of time.

So we wind up with a universe that, as Carroll and Chen say in the ArXiv article, “…can be evolved both forward and backward in time,” with energy fluctuations leading to inflation in the distant past and the far future, and the ‘arrow of time’ reversing in each of these periods. Here again from the paper:

Observers in the very far past of our universe will also detect an arrow of time, but one that will be reversed from ours with respect to some (completely unobservable) global time coordinate throughout the entire spacetime. Both sets of observers will think of the others as living in their “past.”

This stuff makes Poul Anderson’s speculations in Tau Zero about driving a Bussard ramscoop into a new universe look tame. The ArXiv paper is tough reading but a fascinating contribution to current theory.