What caused the Big Bang? Did it have a cause? Or did it begin time, creating itself by its bootstraps? These philosophical puzzles have bothered astronomers for decades. Now two cosmologists are proposing a radical new theory in which the Big Bang resulted from the death of a previous era of the cosmos, part of an infinitely old cycle of collapses and rebirths.
The idea of a cyclic universe has been around ever since the Big Bang was first proposed in the 1930s. But no one could find a way to make the "big crunch" that ends one cycle of the universe "bounce" to become the big bang of the next.
Now, using tools from the outer reaches of theoretical physics known as M theory, along with concepts of spacetime "branes" moving through higher dimensions, Paul J. Steinhardt (Princeton University) and Neil Turok (Cambridge University) say they have finally done the trick. Their paper (published April 25th in the online edition of Science) "is extraordinarily exciting and represents the first big new idea in cosmology in over two decades," comments Princeton astrophysicist Jeremiah Ostriker.
Steinhardt and Turok picture two flat, parallel, four-dimensional spacetimes — our familiar universe and a separate "mirror" version — lying close together in a larger, fifth dimension. This outer dimension is inaccessible to us, the same way that two-dimensional creatures living in a sheet of paper could not see into the third dimension above and below their plane. Mathematicians have a word for spaces embedded in higher dimensions: "branes," analogous to 2-D membranes in our 3-D world.
The two branes in Steinhardt and Turok's theory move together and apart in cycles perhaps many trillions of years long. When they come together and intersect, the energy of their collision fills each with the matter and energy of a new Big Bang. As they move apart, their internal contents disperse over billions of years, as observed. Eventually they become very nearly empty and their motion apart stagnates, setting the stage for them to recollide and set off a new Big Bang creation.
All this is less arbitrary than it sounds. The physics should work, Steinhardt and Turok claim, and their model explains all the observed phenomena of the cosmos in the same detail as today's standard version of the Big Bang.
In particular, the cyclic model does away with the need for "inflation" very early in the Big Bang. Inflation, introduced around 1980, was cosmology's biggest triumph since the confirmation of the Big Bang itself in the mid-1960s. Inflation explained things that were otherwise inexplicable, such as why space is so nearly flat (extreme expansion very early stretched out any curvature), and why parts of the cosmos on opposite sides of the sky, which should never have shared any common influence due to the speed of light, can be so much alike.
Inflation even explains how structure — lumpiness — arose in the smooth early cosmos to condense into galaxies and galaxy clusters. The large-scale clumping that astronomers observe corresponds exactly to the microscopic, random quantum fluctuations that would have occurred when the hot, dense inflationary universe was some 10–32 second old.
Steinhardt and Turok say their model does all of this just as well as inflation and goes it one better — by naturally incorporating the recently discovered "dark energy," which is making the expansion of the universe speed up. Their theory replaces inflation and dark energy with a single energy field that oscillates in such a way as to sometimes cause expansion and sometimes a recollapse.
As abstract and mind-bending as it is, the cyclic-universe theory may be testable fairly soon. Inflation predicts that the cosmic microwave background radiation should show certain, specific patterns of polarization. The cyclic-universe version of the Big Bang would leave different patterns. The European Space Agency's Planck satellite, due for launch around 2007, is designed to make just such measurements. If it proves sensitive enough, Planck may declare a clear winner in the showdown of inflation versus the cyclic universe before the decade is out.