Re-imagining the galaxies
By Dennis OverbyeASTRONOMERS have gazed out at the universe for centuries, asking why it is the way it is. But a growing number of them are now dreaming of universes that never were and asking why not.
Why, they ask, do we live in three dimensions of space and not two, 10 or 25? Why is a light ray so fast and a whisper so slow? Why are atoms so tiny and stars so big? Why is the universe so old and so vast?
Once upon a time (only a century ago), a few billion stars and gas clouds smeared along the Milky Way were thought to encompass all of existence, and the notion of understanding it was daunting – and hubristic – enough. Now astronomers know that galaxies are scattered like dust across the cosmos. And understanding them might require recourse to an even broader canvas, what they sometimes call a “multiverse.”
For some cosmologists, that means universes sprouting from one another in an endless geometric progression, like mushrooms upon mushrooms upon mushrooms, or baby universes hatched inside black holes. Others imagine island universes floating and even colliding in a fifth dimension.
For example, Max Tegmark, a cosmologist at the University of Pennsylvania, has posited at least four different levels of universes, ranging from the familiar (impossibly distant zones of our own universe) to the strange (space-times in which the fundamental laws of physics are different from our own).
Martin Rees, a University of Cambridge cosmologist and the astronomer royal, said contemplating these alternate universes could help scientists distinguish which features of our own universe are necessary and which are accidents of cosmic history.
“It’s all science, but science for the 21st century, to seek the answers to these questions,” Rees said, adding that he is often accused of believing in other universes.
“I don’t believe,” he said, “but I think it’s part of science to find out.”
Some cosmologists now say the realm we call the observable universe – roughly 14 billion light-years deep of galaxies and stars – could be only a small patch of a vast bubble or “pocket” in a much vaster ensemble bred endlessly in a chain of big bangs.
The idea, they say, is a natural extension of the theory of inflation, introduced in 1980 by Alan Guth, now at the Massachusetts Institute of Technology.
That theory asserts that when the universe was less than a trillionth of a trillionth of a second old it underwent a brief hyperexplosive growth spurt fuelled by an anti-gravitational force embedded in space itself, a possibility suggested by theories of modern particle physics.
Because inflation can grow a whole universe from about an ounce of primordial stuff, Guth likes to refer to the universe as “the ultimate free lunch.”
But Guth and various other theorists – including Andrei Linde of Stanford, Alexander Vilenkin of Tufts and Paul Steinhardt of Princeton – have suggested that it may be an endless one as well. Once inflation starts anywhere, it will keep happening over and over again, they say, spawning a chain of universes, bubbles within bubbles, in a scheme that Linde called “eternal inflation.”
Craig Hogan, a cosmologist at the University of Washington, asked: “Once you’ve discovered it’s easy to make a universe out of an ounce of vacuum, why not make a bunch?”
In fact, Guth said, “Inflation pretty much forces the idea of multiple universes upon us.”
Moreover, there is no reason to expect that these universes will be identical. Even within our own bubble, tiny random non-uniformities in the primordial raw material would cause the cosmos to look different from place to place. If the universe is big enough, Tegmark and others said, everything that can happen will happen, so that if we could look out far enough we would eventually discover an exact replica of ourselves.
Moreover, cosmologists say, the laws of physics themselves, as experienced by creatures like ourselves, confined to four dimensions and the energy scales of ordinary life, could evolve differently in different bubble universes.
“Geography is a now a much more interesting subject than you thought,” said John Barrow, a physicist at the University of Cambridge.
Inflation has gained much credit with cosmologists, despite its strangeness, Guth said, because it plays a vital role in calculations of the Big Bang that have been vindicated by the detection of the radio waves it produced. The prediction of other universes must therefore be taken seriously, he said.
The prospect of this plethora of universes has brought new attention to a philosophical debate that has lurked on the edges of science for the last few decades, a debate over the role of life in the universe and whether its physical laws are unique – or, as Einstein once put it, “whether God had any choice.”
Sprinkled through the Standard Model, the suite of equations that describe all natural phenomena, are various mysterious constants, like the speed of light or the masses of the elementary particles, whose value is not specified by any theory now known.
In effect, the knobs on nature’s console have been set to these numbers. Scientists can imagine twiddling them, but it turns out that nature is surprisingly finicky, they say, and only a narrow range of settings is suitable for the evolution of complexity or Life as We Know It. – IHT
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