stronomers have gazed out at the universe for centuries, asking why it is the way it is. But lately a growing number of them are dreaming of universes that never were and asking, why not?

Why, they ask, do we live in 3 dimensions of space and not 2, 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? Does it have to be that way, or are there places, other universes, where things are different?

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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, Dr. Max Tegmark, a University of Pennsylvania cosmologist, 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).

Dr. 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 fundamental and 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," Dr. 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 by Dr. Alan Guth, now at the Massachusetts Institute of Technology, in 1980. 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 fueled by an antigravitational 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, Dr. Guth likes to refer to the universe as "the ultimate free lunch." But Dr. Guth and various other theorists — including Dr. Andrei Linde of Stanford, Dr. Alexander Vilenkin of Tufts and Dr. 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 Dr. Linde called "eternal inflation."

"Once you've discovered it's easy to make a universe out of an ounce of vacuum, why not make a bunch of them?" asked Dr. Craig Hogan, a cosmologist at the University of Washington.

In fact, Dr. 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 nonuniformities in the primordial raw material would cause the cosmos to look different from place to place. If the universe is big enough, Dr. Tegmark and others say, 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," Dr. John Barrow, a physicist at the University of Cambridge, observed.