How to explain dark energy, which is pushing distant galaxies away at an accelerating rate? The cosmological constant that would account for the phenomenon — originally conceived but then rejected by Einstein — is far smaller than one would expect from conventional Big Bang scenarios. In fact, the observed vacuum energy (a possible explanation for the repulsive force) is smaller by a factor of 10120 than it would need to be to do the job. But if the universe were older than today’s estimate of 13.7 billion years, and I mean a lot older, then this tiny value might make sense.

So say Paul Steinhardt (Princeton) and Neil Turok (Cambridge, UK), who put forward a startling concept: there was indeed a time before the Big Bang. There is remarkable solace in this for all of us who grew up asking what happened before the Big Bang, only to be told that the question made no sense because it was unanswerable. So said a kindly astronomy professor in a long-ago college course, raising his shaggy eyebrow at me: How you can talk about something before the advent of spacetime?

But now these two researchers argue in Science that the universe may be at least a trillion years old. It may, in fact, be eternal. That sound you just heard is Fred Hoyle spinning in his grave — how I wish I could buy him a drink! But, of course, this new theory isn’t Hoyle’s ‘steady state’ notion, either; it’s a remodeling based on possibly testable gravity wave and vacuum energy discoveries, and it adjusts to quantum theory demands. Its eternal universe is anything but serene…

Couple an eternal universe with what we know of Big Bang physics and you get a universe that is cyclical. Each new Big Bang replenishes the universe and sets up the next collapse on a trillion-year cycle. To pull this off, Steinhardt and Turok argue that the vacuum energy responsible for the cosmological acceleration is mutable. It started big but has continued to decline to present values, with each change in values taking exponentially longer than the previous one. Each cycle of growth and collapse needs a trillion years or so, enough time for the cosmological constant to have decayed close to zero.

The abstract of “Why the Cosmological Constant Is Small and Positive” argues for this principle: “…a cyclic model of the universe can naturally incorporate a dynamical mechanism that automatically relaxes the value of the cosmological constant, taking account of contributions to the vacuum density at all energy scales. Because the relaxation time grows exponentially as the vacuum density decreases, nearly every volume of space spends an overwhelming majority of the time at the stage when the cosmological constant is small and positive, as observed today.”

Ingenious, and a solid reminder that we are far from a comprehensive theory about what got us here. The difference between a 13.7 billion year old universe and an eternal one is, to say the least, thought provoking. And if this post seems light-hearted, consider it a celebration of that universe, a place so inexplicably odd that it produces theories like these.

Addendum: Larry Klaes passes along the arXiv link for this paper. Also be aware of Alexander Vilenkin’s thoughts on the paper, published first in Science as a perspective essay for the Steinhardt/Turok discussion in the same issue.