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Hawking's quantum universe

Cambridge physicist Stephen Hawking may speak out on global warming and space settlement as well as string theory, but his reputation really rests on his work on the frontiers of space, time and black holes. For scientists, then, the real buzz is over his latest research paper, which asserts that our universe is actually the result of fuzzy quantum interference involving a multitude of possibilities - including, just for instance, universes where Hawking rules the world.

Hawking doesn't mention his hegemony in the paper, titled "Populating the Landscape: A Top Down Approach" and accepted for publication in the journal Physical Review D (PDF file). I just made that up. But the idea is that the current state of the cosmos arises from the sum of all possible histories. Some of those possibilities might have Hawking (highly improbable) or Hitler (slightly less improbable) in charge. But the result would tend to follow the mostly likely branchings of cosmic cause and effect.

It all sounds pretty metaphysical, but Hawking and his co-author, Thomas Hertog of the European CERN research center, say that a detailed analysis of imprints from the universe's infancy - such as the cosmic microwave background or primordial gravitational waves - might turn up characteristic fluctuations that would support their "top-down" hypothesis.

The paper has generated notices in Physics News Update, on Nature's Web site and in New Scientist magazine (where the concept has been dubbed the "Flexiverse").

In an e-mail, I asked Hertog whether the current state of the art - the Wilkinson Microwave Anisotropy Probe (for the cosmic microwave background, or CMB) and the Laser Interferometry Gravitational-wave Observatory (for gravity waves) - could provide the evidence that he and Hawking were looking for. The alternative would be waiting for a space-based gravity-wave detector known as the Laser Interferometer Space Antenna, or LISA.

Here's Hertog's e-mailed response:

"The details of the fluctuation spectra will depend on the theory.

"First we calculate (in a given theory) what is the dominant history that leads to a universe like the one we observe. The observed CMB fluctuations, as well as the spectrum of primordial gravitational waves, arise from small quantum fluctuations around this dominant history.

"The precise shape of the fluctuation spectra can be computed, and depends on the history. This, as you say, provides a way to test the top down approach.

"However, to carry out these calculations in all their detail, we need a better understanding of the potential `landscape' of string theory.

"At present, therefore, we are only able to predict certain generic features of the fluctuations, some of them have been observed by WMAP.

"As for gravitational waves, LISA will be required."

The bottom line? Don't hold your breath, but keep your eye on the "landscape" debate.