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Take black holes. In the 1960s, Princeton physicist John Wheeler coined the term to describe a region where matter is so dense and gravity so intense that even light can't escape. At the core of a black hole is a singularity, a spot where density and gravity appear to become infinitely great-- unleashing forces that could rip a hole in the very fabric of space-time and send a brand-new universe expanding in a direction undetectable and imperceptible to us. Since giant black holes lurk at the cores of many billions of galaxies and smaller holes are left behind by many billions of individual exploding stars, that could mean our cosmos has given birth to a staggering number of baby universes. And each of those could give birth in turn to billions more.
Then there is inflation theory, which came along in the 1980s as a kind of amendment to the original Big Bang. Its basic premise is that when the universe was less than a billionth of a billionth of a billionth of a second old, it briefly went through a period of supercharged expansion, ballooning from the size of a proton to the size of a grapefruit (and thus expanding at many, many times the speed of light). Then the expansion slowed to a much more stately pace. Improbable as the theory sounds, it has held up in every observation astronomers have managed to make.
And inflation, it turns out, leads once again to multiple universes. The inflationary period in our own region of space ran out of steam early on, but theorists, including Stanford University's Andrei Linde and Tufts University's Alexander Vilenkin, have shown that it should continue in others. Our own part of the cosmos took a sort of off ramp to evolve into the universe we see today, but the rest kept going, at breakneck speed--and that part is still going, spawning universes along the way, beyond our comprehension. In some, says Linde, the laws of physics could easily be so different that our sort of life would be impossible.
Multiple universes emerge from so-called superstring theory as well. This still evolving theory is based on the notion that, matter is made, not of particles, but of tiny, vibrating loops of energy called strings. The strings exist in a world of up to 10 spatial dimensions, all but three of which are too minute for us to perceive. Strange though it sounds, most physicists agree that it is the most likely candidate for the long-sought theory of everything that could finally unite relativity and quantum mechanics, the two great but mutually incompatible ideas of 20th century physics.
Superstring theory, which has lately been renamed Mtheory for reasons that interest only theoretical physicists, is so dauntingly complex that the smartest scientists in the world are still trying to nail it down. But among other things, it provides for multiple universes.