“It’s no longer completely crazy to ask what happened before the Big Bang,” says Caltech’s Marc Kamionkowski. A good thing, too, for this is an absorbing subject, one I’ve been interested in ever since reading Poul Anderson’s 1971 novel Tau Zero, in which the crew of the runaway starship Leonora Christine punches through into another universe. That novel assumed a cyclic universe, a collapse and a rebound, naturally making one ask whether a universe hadn’t existed before our own. If so, could we learn anything about it?
I would always have assumed the answer is no, but Kamionkowski’s work, and that of collaborators Adrienne Erickcek and Sean Carroll, at least opens the possibility that we might see an ‘imprint’ of that earlier universe in data we can collect today. The work grows out of measurements of the cosmic microwave background (CMB), as examined by the Wilkinson Microwave Anisotropy Probe. Temperature differences in the CMB can be used to study the theory of inflation, the idea that the universe went through a dramatic expansion immediately after the Big Bang, which would explain why it appears identical in all directions.
The problem is that the CMB isn’t as uniform as once thought. The Big Bang’s afterglow is more mottled in one half of the sky than the other. Exploring an energy field called the curvaton, which had been proposed to explain CMB fluctuations, Kamionkowski’s team tweaked the field so that its effects would more adequately explain the temperature variations. This theoretical tweak is, fortunately, subject to testing by the Planck satellite, which will launch in 2009. Says Erickcek:
“Inflation is a description of how the universe expanded. Its predictions have been verified, but what drove it and how long did it last? This is a way to look at what happened during inflation, which has a lot of blanks waiting to be filled in.”
Looking at the inflation era is itself extraordinary, but there is also the possibility that the perturbation the scientists introduced into the inflation picture is an imprint from whatever came before inflation. This is heady stuff, but it’s clear that Kamionkowski isn’t alone in thinking that we can perhaps glimpse some of these early mechanisms.
For a new paper by Jean-Luc Lehners and Paul Steinhardt (both at Princeton) looks first at cyclic universe models in which the universe undergoes periods of expansion and contraction, with a big crunch followed by a big bang marking the transition between the two, and then goes on to posit a ‘phoenix universe’ in which a tiny part of the universe survives the cataclysmic cycling but manages to become the basis for everything in the next universe. Steinhardt has been a major player in so-called ‘ekpyrotic universe’ models (a term meaning ‘out of the fire’), which offer alternatives to standard inflation.
Intriguingly, Lehners and Steinhardt see a role for dark energy in managing the survival of at least some of the earlier universe. This is dense and challenging reading, but the paper is well worth your time in its examination of this transformative process. A snippet:
…without adding some mechanism to force the universe to begin very close to the classical track, an overwhelming fraction of the universe fails to make it all the down the classical trajectory simply due to quantum ﬂuctuations. This fraction is transformed into highly inhomogeneous remnants and black holes that do not cycle or grow in the post-big bang phase. However, …something curious happens if the dark energy expansion phase preceding the ekpyrotic contraction phase lasts at least 600 billion years. Then, a sufficiently large patch of space makes it all the way down the classical trajectory and through the big bang such that, fourteen billion years later, it comprises the overwhelming majority of space. This surviving volume, which grows in absolute size from cycle to cycle, consists of a smooth, ﬂat, expanding space with nearly scale-invariant curvature perturbations, in accordance with what is observed today. As with the mythical phoenix, a new habitable universe grows from the ashes of the old.
A third way of poking into the early universe is Abhay Ashtekar’s work on a recycled universe that can be explained through loop quantum cosmology (LQC), a universe that works its way through an eternal series of expansions and contractions. New Scientist wrote this up in a recent article, examining the notion that space itself comes in the form of indivisible quanta 10-35 square meters in size. Martin Bojowald, working with Ashtekar at Penn State, used loop quantum gravity to create a model of the universe that has been the subject of much modification ever since. A singularity-free universe results, one in which universal collapse is reversed and the infinitely dense singularity disappears:
If LQC turns out to be right, our universe emerged from a pre-existing universe that had been expanding before contracting due to gravity. As all the matter squeezed into a microscopic volume, this universe approached the so-called Planck density, 5.1 × 1096 kilograms per cubic metre. At this stage, it stopped contracting and rebounded, giving us our universe.
The Planck density itself cannot be reached, as the New Scientist story goes on to explain:
According to Bojowald, that is because an extraordinary repulsive force develops in the fabric of space-time at densities equivalent to compressing a trillion solar masses down to the size of a proton. At this point, the quanta of space-time cannot be squeezed any further. The compressed space-time reacts by exerting an outward force strong enough to repulse gravity. This momentary act of repulsion causes the universe to rebound. From then on, the universe keeps expanding because of the inertia of the big bounce. Nothing can slow it down – except gravity.
How far we have to go in all this, but what fascination in the attempt! The mind sometimes boggles, but Anil Ananthaswamy’s article in New Scientist is a major help at untangling what Ashtekar is doing. Caltech offers a helpful news release on Kamionkowski’s work on asymmetry in the early universe; the paper itself (abstract here) is Kamionkowski et al., “A hemispherical power asymmetry from inflation,” Physical Review D Vol. 78, Issue 12 (16 December 2008). Lehners’ and Steinhardt’s paper is “Dark Energy and the Return of the Phoenix Universe,” available online.