One is "Many Worlds," Hugh Everett IIIs idea that quantum events whose wave functions seem to "collapse" in our world to yield specific measurements actually never collapse but realize all other characteristics implicit in their wave functions in "other universes." Every time something happens here--of all the very large number of possibilities--all the rest of them happen in other universes.
The second such approach comes from string theory, where the equations make mathematical "sense" in more than 4--possible 10 or 11--dimensions, so along these "curled-up" dimensions that string theorists talk about, other universes flourish.
Photo Credit: NASA
One could question both theories on many grounds. It's true that when Paul Dirac solved his famous equation for the electron in 1928 and noticed positive and negative energy levels, he boldly interpreted the negative ones as belonging to an anti-electron, or positron--thus predicting the existence of particles nobody could even imagine before. But does this mean that mathematical solutions always result in new and real discoveries?
According to Brian Greene, a string theorist, the order of infinity of the multiverse is that of the points on the real line (he said this during my interview with him on C-Span). I find such a statement very hard to accept. The points on the real line are infinitely "dense," in the sense that between any two points on the line, no matter how close, there is always another point. If universes were so infinitely tightly packed, we should be experiencing these other universes with every step we make!
So to paraphrase Enrico Fermi (who said this about the question of aliens): "Where is everybody?" An infinitely dense multiverse seems very hard to believe--given that we have no evidence at all for it. And "many worlds" is equally maligned: its size is immense, and where would all these other universes that we must use as "dumps" for all our non-occurring quantum events in our own world be?
But the multiverse from cosmic inflation tells a completely different story. Guth's talk was titled "Inflationary Cosmology, The Origin of Density Perturbations, and the Door to the Multiverse," and he showed directly how the process of inflation believed to have followed the Big Bang most likely leads to other universes. The way in which this happens is fascinating. Inflation is a field with certain characteristics--for example, it is a scalar field, just like the Higgs field, and some physicists believe that the Higgs boson therefore may have played a key role in inflation.
The field intensity starts at an unstable point, and then "rolls off" that point, triggering the exponential growth of our universe--which "irons out" all the kinks in the early universe, producing the very nearly "flat," or mathematically Euclidean, space we see around us today. The quantum imperfections in this process are what caused the formations of the seeds for all the galaxies in the universe--and we see these "flaws" on maps of the microwave background radiation measured in space (through the WMAP and other satellites). Both the uniformity of the satellite picture of the microwave data emanating from the very early universe (to a lever of 1/100,000), and the "flaws" in it--leading to the formation of galaxies--agree to a stunningly high degree with the predictions of Guth's inflationary universe cosmology.
So here is where the other universes come in. The inflationary field is a quantum field, and therefore it is given to quantum fluctuation. It inflates our universe, and then stops. But it can't really stop, because of these quantum fluctuations in the field intensity--so the field, where its intensity is still high--goes elsewhere. Quantum uncertainty, quantum fluctuations, make it impossible for the inflationary field to die. It moves to another part of the universe, and inflates another universe, and so on forever.
I asked Guth what is the cardinality of the multiverse created by inflation, and he answered: "Aleph-zero." This was a far better answer than the one given me by Brian Greene. Aleph-zero is the order of infinity of the integers and the rational numbers. It is a far more "pedestrian" kind of infinity than that of the real line. You can think of these universes as the integers: 1, 2, 3, 4,... Every time inflation finishes doing its trick on one universe, it moves on to the next in line. And since there is no stopping it, inflation continues this way, so that as time goes to infinity, the number of universes gets the infinite cardinality of the integers. From this kind of very reasonable infinity to the immensely complicated, infinitely-dense real line of universes there is a long, long way to go.
Amir D. Aczel's book on the Higgs discovery, Present at the Creation: Discovering the Higgs Boson, is reissued in paperback this month by Broadway Books.