Post by Bozur on May 4, 2009 22:52:09 GMT -5
How to map the multiverse
newscientist.com — If our universe really is just one of zillions, what do the rest look like? Welcome to the ultimate mapping expedition.
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How to map the multiverse
* 04 May 2009 by Anil Ananthaswamy
* Magazine issue 2706. Subscribe and get 4 free issues.
* For similar stories, visit the Cosmology Topic Guide
BRIAN GREENE spent a good part of the last decade extolling the virtues of string theory. He dreamed that one day it would provide physicists with a theory of everything that would describe our universe - ours and ours alone. His bestselling book The Elegant Universe eloquently captured the quest for this ultimate theory.
"But the fly in the ointment was that string theory allowed for, in principle, many universes," says Greene, who is a theoretical physicist at Columbia University in New York. In other words, string theory seems equally capable of describing universes very different from ours. Greene hoped that something in the theory would eventually rule out most of the possibilities and single out one of these universes as the real one: ours.
So far, it hasn't - though not for any lack of trying. As a result, string theorists are beginning to accept that their ambitions for the theory may have been misguided. Perhaps our universe is not the only one after all. Maybe string theory has been right all along.
Greene, certainly, has had a change of heart. "You walk along a number of pathways in physics far enough and you bang into the possibility that we are one universe of many," he says. "So what do you do? You smack yourself in the head and say, 'Ah, maybe the universe is trying to tell me something.' I have personally undergone a sort of transformation, where I am very warm to this possibility of there being many universes, and that we are in the one where we can survive."We keep banging into the possibility that we are one universe of many. Maybe that's telling us something
Greene's transformation is emblematic of a profound change among the majority of physicists. Until recently, many were reluctant to accept this idea of the "multiverse", or were even belligerent towards it. However, recent progress in both cosmology and string theory is bringing about a major shift in thinking. Gone is the grudging acceptance or outright loathing of the multiverse. Instead, physicists are starting to look at ways of working with it, and maybe even trying to prove its existence.
If such ventures succeed, our universe will go the way of Earth - from seeming to be the centre of everything to being exposed as just a backwater in a far vaster cosmos. And just as we are unable to deduce certain aspects of Earth from first principles - such as its radius or distance from the sun - we will have to accept that some things about our universe are a random accident, inexplicable except in the context of the multiverse.
One of the first to argue for a multiverse was Russian physicist Andrei Linde, now at Stanford University in California. In the 1980s, Linde extended and improved upon an idea called inflation, which suggests that the universe underwent a period of exponential expansion in the first fractions of a second after the big bang. Inflation successfully explains why the universe looks pretty much the same in all directions, and why space-time is "flat", despite Einstein showing that it can just as easily be curved.
Linde realised that inflation could be ongoing or "eternal", in the sense that once space-time starts inflating, it can stop in some parts (such as ours) yet take off with renewed vigour elsewhere. This process continues ad infinitum, giving rise to a patchwork of regions of space, each with different properties. When and how inflation ceases in a particular patch dictates the exact nature and types of fundamental particles there and the laws of physics that govern their behaviour. Over time, eternal inflation gives rise to just about every possible type of universe predicted by string theory. Our universe, argues Linde, is a part of this multiverse.
It wasn't until 1998, however, that the multiverse gained any traction, when astronomers studying distant supernovae announced that the expansion of the universe is accelerating. They put this down to the vacuum of space having a small energy density, which exerts a repulsive force to counteract gravity as the universe ages. This became known as dark energy, or the cosmological constant.
Its discovery was a huge blow. Up till then, physicists had hoped that some ultimate theory would deduce the values of fundamental constants of nature from first principles, including the cosmological constant, and explain why the laws of physics are as they are, just right for the formation of stars and galaxies and possibly the emergence of life. This seems not to be the case. Nothing in string theory, or indeed any other theory in physics, can predict the observed value of the cosmological constant.
However, if our universe is part of a multiverse then we can ascribe the value of the cosmological constant to an accident. The same goes for other aspects of our universe, such as the mass of the electron. The idea is simply that each universe's laws of physics and fundamental constants are randomly determined, and we just happen to live in one where these are suited for life. "If not for the multiverse, you would have these unsolved problems at every corner," says Linde.
The other compelling argument for a multiverse comes from string theory. This maintains that all fundamental particles of matter and forces of nature arise from the vibration of tiny strings in 10 dimensions. For us not to notice the extra six dimensions of space, they must be curled up, or compacted, so small as to be undetectable. For decades, mathematicians toiled over what different forms this compaction could take, and they found myriad ways of scrunching up space-time - a staggering 10500 or more.
Each form gives rise to a different vacuum of space-time, and hence a different universe - with its own vacuum energy, fundamental particles and laws of physics. The hope, nurtured by Greene and others, was that there was some kind of uniqueness principle that would pick out the particular form of space-time that produces our universe.
That hope has since receded dramatically. In 2004, Michael Douglas of the State University of New York in Stony Brook, and Leonard Susskind of Stanford University surveyed the developments in string theory to date and concluded that all these theoretical varieties of space-time should be taken seriously as physical realities - that is, they point to a multiverse. Susskind coined the term "the landscape of string theory" to describe the 10500 or more different universes. Nothing in string theory suggests that any one of these universes is preferred over others. Rather, it appears all are equally likely.
Together, dark energy and string theory are making physicists see the multiverse anew. "Just about everybody is convinced that the idea of uniqueness has gone down the drain," says Susskind. So what are we to do? Throw up our hands and admit that we will never be able to explain why our universe is the way it is?
Exploring the landscape
Not a bit of it. Susskind argues that we can still ask meaningful questions within the context of the multiverse, just not the ones we'd ask if ours were the only universe. Questions such as: can we identify the exact point in the landscape that corresponds to our universe, or at least the parts of the landscape that most closely resemble our universe? Is it possible to tell which of our universe's properties can be derived from first principles and which ones are random?We can still ask meaningful questions about the universe, just not the ones we'd ask if it were unique
Also, can we find parts of the landscape with the right conditions for eternal inflation to take place? After all, the landscape and eternal inflation are independent concepts. Confirming that they are compatible would lend more credence to the multiverse idea.
These are not trivial questions to answer, but string theorists are rising to the challenge by feverishly exploring the landscape. Investigating a collection of 10500 universes is not a matter of enumerating the properties of each of them, however. "We just can't make a list of 10500 things," says Nobel laureate Steven Weinberg of the University of Texas at Austin. "That's more than the number of atoms in the observable universe."
The first line of attack has been to develop mathematical models of the landscape. These describe the landscape as a terrain of hills and valleys, where each valley represents a place with its own parameters (such as the mass of the electron) and fields (such as gravity).
How does a universe develop according to this scenario, and what can it tell us about ours? Imagine the universe as it starts off as a speck of space-time. This baby universe is filled with fields, whose properties change due to quantum fluctuations. If the conditions are ripe for inflation, the speck will grow and this will alter its nature. Depending on the changing environment inside the emerging universe, the inflationary process could grind to a halt, continue apace or even spawn other specks of space-time.
According to the landscape picture, the baby universe starts off in one valley. Quantum fluctuations can then cause the entire universe to "tunnel" through an adjoining hill, eventually ending up in another valley with different properties. This process continues, with the universe tunnelling from valley to valley, until it reaches a place stable enough for inflation to run its full course.
Given this scenario, one of the most important tasks is reconciling eternal inflation with the landscape. "The whole picture can be boiled down to one issue: is there eternal inflation in the landscape?" says Henry Tye of Cornell University in Ithaca, New York. In Linde's model of eternal inflation, the speck of space-time starts off with high energy density. The energy density slowly falls as space-time inflates. The quest is to find configurations of space-time among the 10500 that match Linde's requirements for eternal inflation.
Until recently, this had seemed impossible. Then, last year, Eva Silverstein and Alexander Westphal of Stanford University identified two places within the landscape for Linde's version of eternal inflation to take place (Physical Review D, vol 78, p 106003).
It's a promising start, but Tye argues that eternal inflation within string theory is not a done deal. Physicists could just as well start with string theory models of the universe with entirely different initial conditions that would lead to inflation, though not eternal inflation.
Experiments are the key to answering such concerns, by testing the predictions of the various alternative theories. For instance, the energy density in the model proposed by Silverstein is high enough to create strong gravitational waves, ripples in space-time generated by the rapid expansion of the universe. Such waves could have polarised the photons of the cosmic microwave background, the radiation left over from the big bang, and such an imprint would still be detectable today. The European Space Agency's Planck satellite, due to launch soon, will look for any polarisation.
If Planck sees it, then it will lend support to Silverstein's models and eternal inflation. But even if experiments like Planck do lend support for eternal inflation, theorists will need independent confirmation for the ideas of string theory. Unfortunately no specific predictions of string theory are yet within experimental reach, but there is one key general property that could be confirmed soon. String theory requires that the universe has a property known as supersymmetry, which posits that every particle known to physicists has a heavier and as yet unseen superpartner. Physicists will be looking for some of these superpartners at the Large Hadron Collider, the new particle accelerator at CERN, near Geneva, Switzerland.
The scenario of a universe tunnelling through the landscape also makes a unique prediction. If our universe emerged after tunnelling in this way, then the theory predicts that space-time today will be ever so slightly curved. That's because in this scenario, inflation does not last long enough to make the universe totally flat.
Today's measurements show the universe to be flat, but the uncertainty in those measurements still leaves room for space-time to be slightly curved - either like a saddle (negatively curved) or like a sphere (positively curved). "If we originated from a tunnelling event from an ancestor vacuum, the bet would be that the universe is negatively curved," says Susskind. "If it turns out to be positively curved, we'd be very confused. That would be a setback for these ideas, no question about it."
Until any such setback the smart money will remain with the multiverse and string theory. "It has the best chance of anything we know to be right," Weinberg says of string theory. "There's an old joke about a gambler playing a game of poker," he adds. "His friend says, 'Don't you know this game is crooked, and you are bound to lose?' The gambler says, 'Yes, but what can I do, it's the only game in town.' We don't know if we are bound to lose, but even if we suspect we may, it is the only game in town."
Anil Ananthaswamy is a consulting editor for New Scientist
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