In this computer generated sequence, the universe evolves, inflating and expanding its terrain. The gentle valleys represent quiescent cosmic zones where all is stable. The jutting hills and soaring peaks symbolize the inflationary engine of universe creation, where new cosmic realms embody alternate physics and strange life — or none at all.
Courtesy Andre Linde
MIT physicist Alan Guth found a viable, but flawed, solution to the puzzle in 1981. Linde shored up that work shortly thereafter, making improvements to overcome those flaws. In a nutshell, Guth and Linde proposed that the universe underwent a colossal growth spasm in the first instants of its existence, a phenomenon called inflation. Today widely accepted as the standard version of the Big Bang theory, inflation holds that regions of the universe that are currently separated by many billions of light-years were once close enough to each other that they could exchange heat and reach the same temperature before they were wildly super-sized. Problem solved.
By the mid-1980s Linde and Tufts University physicist Alex Vilenkin had come up with a dramatic new twist that remains nearly as controversial now as it was then. They argued that inflation was not a one-off event but an ongoing process throughout the universe, where even now different regions of the cosmos are budding off, undergoing inflation, and evolving into essentially separate universes. The same process will occur in each of those new universes in turn, a process Linde calls eternal chaotic inflation.
Linde has spent much of the past 20 years refining that idea, showing that each new universe is likely to have laws of physics that are completely different from our own. The latest iteration of his theory provides a natural explanation for the anthropic principle. If there are vast numbers of other universes, all with different properties, by pure odds at least one of them ought to have the right combination of conditions to bring forth stars, planets, and living things.
“In some other universe, people there will see different laws of physics,” Linde says. “They will not see our universe. They will see only theirs. They will look around and say, ‘Here is our universe, and we must construct a theory that uniquely predicts that our universe must be the way we see it, because otherwise it is not a complete physics.’ Well, this would be a wrong track because they are in that universe by chance.”
Most physicists demurred. There wasn’t any good reason to believe in the reality of other universes—at least not until near the beginning of the new millennium, when astronomers made one of the most remarkable discoveries in the history of science.
The Accelerating Universe
In 1998 two teams of researchers observing distant supernovas—exploding stars—found that the expansion of the universe is accelerating. The discovery was baffling. Just about everyone had expected that the cosmic expansion, which started with the Big Bang, must be gradually slowing down, braked by the collective gravitational pull of all the galaxies and other matter out there. But built into the very fabric of space, it seems, is some unknown form of energy—physicists call it simply dark energy—that is pushing everything apart. Many cosmologists were skeptical at first, but follow-up observations with the Hubble Space Telescope, along with independent studies of radiation left over from the time of the Big Bang, have powerfully confirmed the reality of dark energy.
Dark energy appears calibrated for stars, galaxies, and us.
The idea that empty space might contain energy was not the part that surprised physicists. Ever since the birth of quantum mechanics in the 1920s, they have known that innumerable “virtual” particles pop into and out of existence all around us, a sort of quantum white noise, always there but forever beneath our notice. What astonished them was the peculiar specificity of the amount: exactly enough to accelerate expansion, yet not so much that the universe would rapidly rip itself apart. The observable amount of dark energy appears to be another one of those strange anthropic properties, calibrated to allow planets, stars, and us.
“If [dark energy] had been any bigger, there would have been enough repulsion from it to overwhelm the gravity that drew the galaxies together, drew the stars together, and drew Earth together,” Stanford physicist Leonard Susskind says. “It’s one of the greatest mysteries in physics. All we know is that if it were much bigger we wouldn’t be here to ask about it.”
Nobel laureate Steven Weinberg, a physicist at the University of Texas, agrees. “This is the one fine-tuning that seems to be extreme, far beyond what you could imagine just having to accept as a mere accident,” he says.
The Multiverse on a String
Dark energy makes it impossible to ignore the multiverse theory.Another branch of physics—string theory—lends support as well. Although experimental evidence for string theory is still lacking, many physicists believe it to be their best candidate for a theory of everything, a comprehensive description of the universe, from quarks to quasars. According to string theory, the ultimate constituents of physical reality are not particles but minuscule vibrating strings whose different oscillations give rise to all the particles and forces in the universe. Although string theory is enormously complex, requiring a total of 11 dimensions to work correctly, it is a mathematically convincing way to knit together all the known laws of physics.
In 2000, however, new theoretical work threatened to unravel string theory. Joe Polchinski at the University of California at Santa Barbara and Raphael Bousso at the University of California at Berkeley calculated that the basic equations of string theory have an astronomical number of different possible solutions, perhaps as many as 101,000*. Each solution represents a unique way to describe the universe. This meant that almost any experimental result would be consistent with string theory; the theory could never be proved right or wrong.
Some critics say this realization dooms string theory as a scientific enterprise. Others insist it is yet another clue that the multiverse is real. Susskind, a leading proponent of that interpretation, thinks the various versions of string theory may describe different universes that are all real. He believes the anthropic principle, the multiverse, and string theory are converging to produce a coherent, if exceedingly strange, new view in which our universe is just one of a multitude—one that happened to be born with the right kind of physics for our kind of life.
“Some people would call this the great disaster of string theory, that instead of giving rise to a single theory, it gave rise to something that is so diverse we can never make any sense out of it,” Susskind says. “Others would say, ‘Ah, this is exactly what we need for eternal inflation, for the multiverse, for anthropic thinking, and so forth.’”
* Correction: Due to a formatting error, this number was originally rendered as “101,000.” Return to the corrected sentence.
