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?
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
"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
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
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.