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.
Here is the link to the paper they just released:
http://arXiv.org/abs/astro-ph/0605173
Thanks, Larry. I’m going to move that link up into the post to make sure everyone sees it.
“..the observed vacuum energy (a possible explanation for the repulsive force) is smaller by a factor of 10^120 than it would need to be to do the job.”
isn’t that backwards? i thought the calculated vacuum energy is 10^120 greater than what is needed for the observed acceleration. and that one proposal for the discrepency is supersymmetry cancelling out some of the energy to the tune of 10^60.
What aabout the notion that spacetime is flat? I thought it’s been determined that there is no “Big Crunch” in our universe’s future.
Hehe now now be carefull with these blogs, if you make the braners and the stringers gnash their teeth to much they’ll have terrible dental bills :)
All jesting aside I seem to recall attempts at using brane theory along these lines. I can no more than the next person visualize infinities yet the concept that there was nothing before and is nothing beyond our observed universe fails sanity checks. I’m glad to see theories developing again that include rather than exclude possibilities.
Re sineral’s comment above:
“isn’t that backwards? i thought the calculated vacuum energy is 10^120 greater than what is needed for the observed acceleration. and that one proposal for the discrepency is supersymmetry cancelling out some of the energy to the tune of 10^60.”
I think we’re saying the same thing from different angles. As I read Steinhardt and Turok, the *calculated* vacuum energy is much greater than what would be needed to account for what we see. The *observed* vacuum energy is thus far smaller that our calculations. Here’s a snip from the paper:
“One of the greatest challenges in physics to day is to explain the small positive value of the cosmological constant or, equivalently, the energy density of the vacuum. The observed value, 7 × 10^?30 g/cm^3 , is over one hundred twenty orders of magnitude smaller than the Planck density, 10^93 g/cm^3 , as the universe emerges from the big bang, yet its value is thought to be set at that time…”
Re Eric James’ comment:
“What about the notion that spacetime is flat? I thought it’s been determined that there is no “Big Crunch” in our universe’s future.”
It seems to me that this is the crux of the problem; to get to a crunch, you have to invoke brane theory. Thus from the paper:
“According to the cyclic picture, the big bang is collision between orbifold planes (branes) along an extra dimension of space, as might occur in heterotic M-theory. A weak, spring-like force draws the branes together at regular intervals, resulting in periodic collisions that ?ll the universe with new matter and radiation. After each collision, the branes separate and
start to re-expand, causing the matter and radiation to cool and spread out. Eventually, the matter and radiation become so dilute that the potential energy associated with the inter-brane force takes over.”
All this sends me back to Greene’s The Fabric of the Universe, and reinstills a yearning for hard, experimental evidence!
Abalone… er, I mean ah baloney. I’ll believe in branes when I see one.
Forget the Big Bang Theory
On Point Radio May 31, 2007
*************************
Renegade physicists are making a
new case that there is no beginning
and no end to the Universe….
http://www.kurzweilai.net/email/newsRedirect.html?newsID=6867&m=25748
From Big Crunch to Big Bang with AdS/CFT
Authors: Neil Turok, Ben Craps, Thomas Hertog
(Submitted on 12 Nov 2007 (v1), last revised 27 Dec 2007 (this version, v3))
Abstract: The AdS/CFT correspondence is used to describe five-dimensional cosmology with a big crunch singularity in terms of super-Yang-Mills theory on R times S^3 deformed by a potential which is unbounded below.
Classically, a Higgs field in the dual theory rolls to infinity in finite time. But since the S^3 is finite, the unstable mode spreads quantum mechanically and the singularity is resolved when self-adjoint boundary conditions are imposed at infinity. Asymptotic freedom of the coupling governing the instability gives us computational control and the quantum spreading provides a UV cutoff on particle creation.
The bulk interpretation of our result is a quantum transition from a big crunch to a big bang. An intriguing consequence of the near scale-invariance of the dual theory is that a nearly scale-invariant spectrum of stress-energy perturbations is automatically generated in the boundary theory. We comment on implications for more realistic cosmologies.
Comments: 4 pages; discussion of backreaction improved, incorporating dependence on width of initial wavepacket
Subjects: High Energy Physics – Theory (hep-th); Astrophysics (astro-ph)
Cite as: arXiv:0711.1824v3 [hep-th]
Submission history
From: Neil Turok [view email]
[v1] Mon, 12 Nov 2007 19:32:07 GMT (12kb)
[v2] Tue, 13 Nov 2007 13:47:04 GMT (12kb)
[v3] Thu, 27 Dec 2007 02:58:56 GMT (12kb)
http://arxiv.org/abs/0711.1824