My other universe is a Porsche

My other universe is a Porsche

MICHAEL SEXTON FOR NEW SCIENTIST

What if the universe we consider so special is just one of many habitable realities? Marcus Chown meets the adventurers charting the best of all possible worlds

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RONI HARNIK is an explorer who likes to think big. Foreign continents hold no sway over him. Not even the outer reaches of the solar system appeal. Harnik is a theoretical physicist at the Stanford Linear Accelerator Center in California and his expeditions are on a far grander scale: he goes in search of other universes. Harnik and his colleagues’ quest is not as futile as it sounds. Of late, many physicists and cosmologists have begun to believe that our universe is not alone, but instead is one among many. According to this “multiverse” idea, there are many different universes, with myriad possible laws of nature. Not all of them would allow stars and galaxies to form. Even if 38 | NewScientist | 7 October 2006

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they did, their twisted laws might not let stars forge the carbon believed to be so crucial for life. This makes the latest findings by Harnik and his colleagues all the more remarkable. They have found another universe complex enough to potentially support life. Even more startling is how different this new cosmos is from our own. The discovery cuts right to the heart of one of the most hotly contested issues in physics at the moment: the anthropic principle. According to this idea, the laws that characterise our universe must ultimately square with the observational fact that we exist. This topsy-turvy logic divides physicists with a passion. Most would like to believe that

Roni Harnik has discovered a habitable universe that is missing one of the fundamental forces of nature

there is some deep reason for why the laws of nature are the way they are that has nothing to do with our existence. The anthropic principle, say its fiercest opponents, is unscientific because it is impossible to disprove. And talk of a universe that is fine-tuned for life plays into the hands of creationists. Other physicists believe the idea that our universe is just one of many possible worlds sits well with the latest attempts to unite quantum mechanics and gravity. According to the multiverse view, it is unlikely that ours is the only universe www.newscientist.com

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complex enough to support sentient beings. Martin Rees of the University of Cambridge goes further and believes there may be an archipelago of “islands” in the multiverse, havens for life dotted in a vast sea of uninhabitable universes. The physical constants and parameters determining the basic physics and cosmology on these islands could be different to what we experience. They might have heavier electrons, for instance, or may have evolved from a cooler big bang. Light may run at slower speeds or the pull of gravity might be stronger. Although we cannot observe alternative universes through telescopes, theorists can explore them on paper or in computer models www.newscientist.com

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and see whether galaxies would still form and stars would still shine. Each island in the multiverse is actually a collection of hospitable universes, all with slightly different physical constants just like geographical coordinates on a terrestrial island. If Rees is right and we can show that our universe is part of the largest island in the multiverse, it would lend weight to the anthropic principle which asserts that our universe must be among the most common habitable universes and in no way special. Lately there has been an upsurge in interest in the anthropic principle. Part of the reason is that other theories have failed to explain the observed value of the cosmological constant, a number that describes the repulsive force speeding up the expansion of the universe. This is in stark contrast to the success of an anthropic argument put forward in the late 1980s by Nobel prizewinner Steven Weinberg at the University of Texas, Austin. He showed that if the cosmological constant were significantly larger than it is, its repulsive effect would have scattered matter far and wide before gravity could pull it together to make galaxies. Another reason is the buzz around string theory, which holds that all matter is composed of tiny strings of energy vibrating

the cosmological constant. Cosmologists using this approach tend to vary one property at a time while keeping everything else in their model constant. This may confirm that only a universe with roughly “our” value of that property will form stars, galaxies and the elements, but it doesn’t really give alternative universes a fair crack of the whip. “This can’t be right,” says Harnik. “It seems inconceivable that the multiverse would have vast numbers of universes all the same except for a difference in a single parameter.” It all comes down to numbers. Harnik argues that there will be countless more universes with myriad properties different from our own. By varying just one property, cosmologists have been too conservative. Harnik, Kribs and Perez decided to highlight this flaw in anthropic reasoning by taking a radical measure: they switched off the weak nuclear force, one of the four fundamental forces in nature. In practice, this means changing a multitude of parameters and constants simultaneously. The weak force is responsible for the radioactive beta decay of atomic nuclei and is considered essential for a complex universe like ours. Take it away, and you might expect the “weakless” universe to be wildly

in 10 dimensions of space-time. String theory is the most popular attempt to reconcile quantum mechanics and general relativity, yet it does not just describe one universe. It describes 10500 universes, each with different physical properties. “String theory appears to provide a theoretical basis for the multiverse,” says Harnik. The rising popularity of the anthropic principle worries Harnik and his colleagues Graham Kribs of the University of Oregon in Eugene and Gilad Perez of the University of California, Berkeley. What concerns them most is the methodology of researchers using anthropic arguments to explain the values of fundamental parameters such as

different from our own. Only Harnik, Kribs and Perez have discovered it isn’t. They considered what would happen to crucial processes in the history of the universe – the forging of elements in the big bang, the powering of stars and supernovae explosions. By examining the equations that describe these processes, they made an astonishing discovery: the weakless universe is still capable of supporting observers. How can this be? After all, without the weak force neutrons never decay into protons, and key nuclear reactions can never take place. Among them is the first step in the nuclear reaction chain that powers stars, in which hydrogen nuclei fuse together into helium 7 October 2006 | NewScientist | 39

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MICHAEL SEXTON FOR NEW SCIENTIST

nuclei. This relies on the weak force changing a proton into a neutron to make deuterium, or heavy hydrogen. “This process does not happen in the weakless universe,” says Harnik. Remarkably, however, Harnik and his colleagues have discovered that there is still a way for weakless stars to shine. It all depends on the relative proportions of ordinary matter and radiation created in the weakless big bang, a measure of the heat of the event. In our universe, the first few minutes were hot and dense enough for protons and neutrons – collectively known as baryons – to fuse to create deuterium, helium and lithium. Because neutrons cannot decay in the weakless universe, there would be many more of them free to fly around and fuse with protons to make deuterium. Harnik and his colleagues have calculated that with a slightly hotter big bang, 10 per cent of matter in the weakless universe would be deuterium, compared with just 0.001 per cent in ours. It is a key difference. “The extra deuterium in the weakless universe enables stars to leapfrog the first step in the reaction chain and derive heat from the second step, turning deuterium into helium-3,” says Harnik. He and his team have worked with Adam Burrows of the University of Arizona in Tucson to simulate such stars. They found that although stars of a wide range of masses should form in the weakless universe – just as in our universe – ones with about 2 per cent of the mass of the sun could shine for the billions of years necessary for intelligent life to evolve on their planets. Such small, deuteriumburning stars would be cool, feeble objects compared with the stars in our universe, so any life-bearing planets would have to orbit very close to the weakless stars. And the faintness of these stars means that the night sky would appear devoid of other stars to observers on a weakless planet. Another process that should be affected by the absence of the weak force helps drive supernovae. These explosions at the end of a star’s life are crucial for life because they blow all the heavy elements forged inside a star into space. When a massive star in our universe runs out of fuel, its core collapses under gravity to form a neutron star. Protons in the star fuse with electrons to create neutrons and neutrinos, which then surge through the star and turn the implosion into an explosion. This drives off the outer layers of the star in a supernova, causing heavy elements to sail into space and eventually end up in new stars and planets – and life. Switch off the weak force, though, and there are no neutrinos. It doesn’t spell the end of heavy elements, though. “Fortunately, there is a second type of supernova that does not require neutrinos to detonate,” says Harnik. Instead of the

Roni Harnik believes the approach of many cosmologists attempting to explain the nature of the universe is fundamentally flawed

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Why is our universe the way it is? As they stand, the accepted theories of particle physics and cosmology offer suprisingly few clues. They are stuffed with parameters, fixed by hand to explain the world we see. Change the wrong one and you alter the history of the cosmos. So maybe the fact that we’re here at all to ask such a profound question is the answer. That is the premise behind the anthropic principle. It is an idea that has divided physicists, partly because it comes in many shapes and guises that range from the obvious to the bizarre. The weak version takes the laws of nature and the physical constants as read and claims that we appeared at a time when conditions in the universe were ripe for life. For instance, the universe cannot be younger than the lifetime of stars, otherwise there would not have been enough time for stars to forge the chemical elements essential for life and then fling them far into space in supernova explosions. On the other hand, the universe cannot be too old, or all the stars would have fizzled out. So life can only exist when the universe is the age that we measure it to be. This is relatively uncontroversial. At the other extreme, the strong anthropic principle asserts that our emergence at a late date in the universe is what forced the constants of nature to be set as they were at the beginning. Many people are uncomfortable with this idea because it implies some sort of designer who fine-tuned the universe for life.

implosion of a massive star, this type of supernova happens when a white dwarf accumulates mass from a companion star. Eventually the growing pressure and temperature inside the white dwarf trigger an explosive plethora of nuclear reactions. Earlier this month, Louis Clavelli and Raymond White at the University of Alabama in Tuscaloosa reported that weakless supernovae are unlikely to produce enough oxygen to support life (www.arxiv.org/abs/ hep-ph/0609050). Even so, counter Harnik and Kribs, several generations of stars would eventually forge oxygen and enrich the interstellar medium with it. So what does the discovery of the weakless www.newscientist.com

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universe prove? For one thing, establishing if we live in a multiverse and if our existence determines many of the physical constants is going to be much harder than many people thought. Harnik and his colleagues have shown it is not as simple as varying one parameter at a time. By changing several parameters simultaneously they found that entirely new processes – such as a different mechanism for stellar burning – may come into play. “This is a complete surprise,” says Harnik. In fact, Harnik and his colleagues go much further than this. The fact that the weakless universe is viable at all suggests to them that the weak force may be fixed by some deeper theory, such as supersymmetry – a souped-up version of the standard model of particle physics – rather than by anthropic reasoning.

Islands in the multiverse It is not the only evidence to suggest that we need to broaden our horizons when it comes to testing the anthropic principle. In 2001, Anthony Aguirre of the University of California, Santa Cruz, found another island in the multiverse. Like Harnik, Aguirre was worried about the methodology of anthropic arguments, though for a different reason. “People tended to vary a parameter by a factor of 10 either way,” he says. “I wondered whether, if a parameter was varied by a much larger factor, new phenomena could come into play that would still permit complexity.” Aguirre knew a good parameter to vary. Before the triumph of the hot big bang model, researchers had explored so-called cold big bang models to see the effect on phenomena such as galaxy formation. The crucial parameter that determines whether the big bang is hot or cold is the number of photons per baryon. In our universe it is about a billion. Aguirre wondered what would happen if it was in the range 0.1 to 100 – much, much cooler. Aguirre’s universe started off quite unlike our own (Physical Review D, vol 64, p 083508) . After our hot big bang, the universe took tens of millions of years to cool to the point where matter could clump into stars. “But in the cold big bang universe, stars can begin to form within 100 years of the big bang,” says Aguirre. He even modelled an extreme cold big bang universe where the cosmological constant was 1017 times what it is in our universe. By rights, this strong repulsive force ought to fling matter apart, preventing the formation of galaxies. However, in the cold big bang universe, stars form so quickly that they are in place before this cosmological repulsion takes hold. “The stars then rush away from each other,” says Aguirre. “It’s a pretty dull universe with each star isolated in a vast ocean of space.

Nevertheless, there is nothing to prevent such stars having planets and observers.” Now that we have found two islands in the multiverse – three, including our own – the question is: are there more? “Undoubtedly,” says Harnik. “However, imagining what forms life may take and the finite time available to us to do this will always be an obstacle.” Rees always knew it was going to be like this: “What I would call the ‘anthropic programme’ involves doing just what Harnik’s group and Aguirre have done. We can then decide whether we are in a universe that is typical of the anthropically allowed subset.” Rees points out that we can do this only when – or if – we have an adequate theory for the multiverse, as string theory may one day provide. Equipped with such a theory, physicists hope to systematically survey the vast array of universes that it describes to see which ones support complexity. String theory might reveal a host of universes in which the cosmological constant is 10 per cent what it is in our universe, another horde where it is 50 per cent, and so on. Because we have no reason to expect that our universe is special, the majority of universes should have a cosmological constant very similar to our own. In other words, we should expect to be in a universe with the most probable value of the cosmological constant. String theory may one day tell us what these probable values are for every physical constant and cosmological parameter. “Unfortunately, we are a long way from being able to do this,” warns Harnik. Perhaps the biggest problem is defining what complexity is necessary for life. In assessing a universe’s habitability, physicists always use a proxy for observers such as the galaxies or stars. But the truth is we are simply fixating on a set-up we think is necessary for life from our own experience. Some physicists like Stephen Wolfram believe that the complexity of life can arise in a myriad of non-biological systems – from plasmas to subatomic particle collisions – and therefore the anthropic programme is fatally flawed. Even if Wolfram is wrong and the anthropic programme is not impossible but merely extremely hard, Aguirre and Harnik refuse to be depressed. “We have a superabundance of predictive power,” says Aguirre. “There are zillions of other universes. You’ve got to admit, it’s pretty interesting.” Harnik agrees. “Exploring imaginary universes is loads of fun,” he says. Perhaps that’s just as well. There is a lot of multiverse out there. ● Further reading: “A universe without weak interactions” by Roni Harnik, Graham Kribs and Gilad Perez, Physical Review D, vol 74, p 035006 7 October 2006 | NewScientist | 41

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