s the universe all that is the case? Or are there worlds beyond our own? Many cosmologists these days contemplate a so-called multiverse of an endless series of big bangs blooming from one another, creating universes without limit, but they are not alone. A variety of modern theories predict a plethora of universes. Here are a few.

Advertisement

One such multiverse comes from quantum mechanics, the strange rules that govern nature at the subatomic scale. In quantum theory, objects like electrons or cats have no definite position or properties before they are observed or measured, but are rather spread out over all space in a cloud of possibility known as a wave function.

Once an observation is made, the wave function "collapses," according to the standard interpretation championed by the Danish physicist Neils Bohr in the 1920's, freezing the cat or electron into the place or form in which it is actually seen.

But what happens if there is no observer, as there was certainly not at the beginning of the universe?

In 1957 Hugh Everett III, then a graduate student at Princeton, and his adviser, Dr. John Wheeler, proposed a variant of quantum theory that came to be known as the many worlds interpretation. The mathematics is the same as ordinary quantum theory, but in this version, instead of the wave function's collapsing with each measurement, the universe splits into parallel universes, one for every possible outcome of every possible contingency: universes in which the Boston Red Sox are just now wrapping up their 10th World Series championship, universes in which dinosaurs still stride the earth.

Alas, the other universes, if they exist, would be invisible and unreachable.

Dr. Lee Smolin of the Perimeter Institute for Theoretical Physics in Waterloo, Ontario, has speculated that the universe can reproduce itself through black holes, the bottomless gravitational pits in which stars disappear, giving birth to baby universes with slightly altered laws of physics.

Under his theory, termed cosmological natural selection, universes with more black holes will produce more progeny and so the laws of physics will evolve in a direction favorable to black hole production.

But only the largest, densest stars can collapse to make black holes, and those are the same stars that make the elements necessary for life, like carbon, oxygen and iron, Dr. Smolin has pointed out in his book "The Life of the Cosmos." So a multiverse emerging from such black holes will necessarily be biofriendly.

Dr. Smolin's conjecture can be tested, he says, by trying to imagine a universe with laws that generate black holes even more efficiently than our own, which, he said, should be near the maximum efficiency.

"I personally think Smolin's prediction is unlikely to be borne out, but he deserves our thanks for presenting an example that illustrates how a specific multiverse theory can be vulnerable to disproof," Dr. Martin Rees of Cambridge University wrote in his own book, "Our Cosmic Habitat."

Multiple universes of a different sort pop up in the most recent versions of string theory, which holds that the world can be envisioned as a 3-dimensional membrane (or "brane" in the jargon) floating in 10-dimensional space, like a leaf in a fish tank.

While most of the particles and forces of our universe are confined to this brane, gravity is not, and other branes can interact and even collide with us, according to some theories, relegating some of the features of the universe to the effects of a kind of inter-cosmic weather.

For example, in a model developed by Dr. Lisa Randall of Harvard and Dr. Raman Sundrum of Johns Hopkins, the laws of physics would depend on how far away in a warped fifth dimension our own brane is from another brane.

Outlandish as it sounds, such notions are testable, say string theorists.

"There is an experiment you can do," said Dr. Joseph Lykken, a theorist at the Fermi National Accelerator Laboratory, where physicists have been searching in particle collisions for evidence that some particles have been escaping our brane into the fifth dimension — so far with no luck.