Geoff Brumfiel, "Our Universe: Outrageous fortune" (2006)

Our Universe: Outrageous fortune
> <font>Geoff Brumfiel

     Geoff Brumfiel is Nature's physical sciences Washington correspondent.

Why are we here? It's a question that has troubled philosophers, theologians and those who've had one drink too many. But theoretical physicists have a more essentialist way of asking the question: why is there anything here at all?

For two decades now, theorists in the think-big field of cosmology have been stymied by a mathematical quirk in their equations. If the number controlling the growth of the Universe since the Big Bang is just slightly too high, the Universe expands so rapidly that protons and neutrons never come close enough to bond into atoms. If it is just ever-so-slightly too small, it never expands enough, and everything remains too hot for even a single nucleus to form. Similar problems afflict the observed masses of elementary particles and the strengths of fundamental forces.

In other words, if you believe the equations of the world's leading cosmologists, the probability that the Universe would turn out this way by chance are infinitesimal - one in a very large number. "It's like you're throwing darts, and the bullseye is just one part in 10120 of the dart board," says Leonard Susskind, a string theorist based at Stanford University in California. "It's just stupid."

One in a zillion
> Physicists have historically approached this predicament with the attitude that it's not just dumb luck. In their view, there must be something underlying and yet-to-be-discovered setting the value of these variables. &quot;The idea is that we have got to work harder because some principle is missing,&quot; says David Gross, a Nobel-prizewinning theorist and director of the Kavli Institute for Theoretical Physics in Santa Barbara, California.<p>

But things have changed in the past few years, says astronomer Bernard Carr of Queen Mary, University of London, UK. String theorists and cosmologists are increasingly turning to dumb luck as an explanation. If their ideas stand up, it would mean the constants of nature are meaningless. "In the past, many people were almost violently opposed to that idea because it wasn't seen as proper science," Carr says. "But there's been a change of attitude."

Much of that change stems from work showing that our Universe may not be unique. Since the early 1980s, some cosmologists have argued that multiple universes could have formed during a period of cosmic inflation that preceded the Big Bang. More recently, string theorists have calculated that there could be 10⁵⁰⁰ universes, which is more than the number of atoms in our observable Universe. Under these circumstances, it becomes more reasonable to assume that several would turn out like ours. It's like getting zillions and zillions of darts to throw at the dart board, Susskind says. "Surely, a large number of them are going to wind up in the target zone." And of course, we exist in our particular Universe because we couldn't exist anywhere else.

It's an intriguing idea with just one problem, says Gross: "It's impossible to disprove." Because our Universe is, almost by definition, everything we can observe, there are no apparent measurements that would confirm whether we exist within a cosmic landscape of multiple universes, or if ours is the only one. And because we can't falsify the idea, Gross says, it isn't science. Or at least, it isn't science in any conventional sense of the word. "I think Gross sees this as science taking on some of the traits of religion," says Carr. "In a sense he's correct, because things like faith and beauty are becoming a component of the discussion."

And yet in the overlapping circles of cosmology and string theory, the concept of a landscape of universes is becoming the dominant view. "I really hope we have a better idea in the future," says Juan Maldacena, a string theorist at the Institute for Advanced Study in Princeton, New Jersey, summing up the views of many in the field. "But this idea of a landscape is the best we have today." The stakes are high: string theorists know that pursuing an unverifiable theory could look like desperation, but they fear that looking for meaning in a meaningless set of numbers may be equally fruitless.

Kepler's error
> At the core of this dilemma is a concept known as the anthropic principle: the idea that things appear the way they do because we live at a certain spot in the Universe. It's not a new concept, and has previously been regarded more as philosophy than science.<p>

But some scientists say that it offers a useful change of perspective. "It's very important to take into account stuff like this, or you can come to completely incorrect conclusions about the Universe," argues Max Tegmark, a cosmologist at the Massachusetts Institute of Technology, Cambridge. "For example, you might assume our Solar System is typical, but a typical point in space is some intergalactic void where you can't see a single star."

Failing to consider our observational location has burned scientists in the past. The sixteenth-century German astronomer Johannes Kepler spent years trying to understand what seemed to be the even, geometrical spacing of our planets from the Sun. Kepler searched for meaning in the planets because he thought our Solar System was unique; today's scientists understand that our Solar System is but one of probably billions in the Galaxy. Under such circumstances it seems reasonable to assume the planets are spaced according to little more than random chance.

In much the same way as Kepler worried about planetary orbits, cosmologists now puzzle over numbers such as the cosmological constant, which describes how quickly the Universe expands. The observed value is so much smaller than existing theories suggest, and yet so precisely constrained by observations, that theorists are left trying to figure out a deeper meaning for why the cosmological constant has the value it does.

Many are still searching for some great unifying theory that would explain these variables. But others have started to believe that, like Kepler, today's physicists are looking for meaning where there is none. "In recent years, it was looking more and more to me like the laws of nature were environmental," says Susskind, who has just written a book making this argument (L. Susskind, The Cosmic Landscape: String Theory and the Illusion of Intelligent Design. Little Brown, 2005). He suspects that there are many universes, all with different values for these variables. Just as human life had to evolve on a planet with water, he says, perhaps we also had to evolve in a Universe where atoms could form.

Leap of faith
> Still, many scientists distrust the concept and continue to seek alternative explanations. Among them is Lisa Randall, also at Harvard. Randall suspects that multiple universes are a mirage resulting from the unrefined equations of string theory. &quot;You really need to explore alternatives before taking such radical leaps of faith,&quot; she contends. And with no foreseeable way to detect other universes, Gross feels that such leaps of faith should not be taken. &quot;I feel that it's a rather extreme conclusion to reach at this point,&quot; he says.<p>

Susskind, too, finds it "deeply, deeply troubling" that there's no way to test the principle. But he is not yet ready to rule it out completely. "It would be very foolish to throw away the right answer on the basis that it doesn't conform to some criteria for what is or isn't science," he says.

Gross believes that the emergence of multiple universes in science has its origins in theorists' 20-year struggle to explain the finely tuned numbers of the cosmos. "People in string theory are very frustrated, as am I, by our inability to be more predictive after all these years," he says. But that's no excuse for using such "bizarre science", he warns. "It is a dangerous business."
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