Photo: James Duncan Davidson
Superstring theorist and physicist and the co-founder of the World Science Festival, Brian Greene splits his visually rich, action-packed talk into three distinct sections, all in the name of convincing us of the existence of the multiverse, the possibility that way beyond the earth, the milky way, we’ll find that our universe is part of a vast complex of universes we call the multiverse.
Part One: The history and mystery
In 1929, the astronomer Edwin Hubble discovered that distant galaxies were rushing away from us; that space was stretching and expanding. This was a revolutionary idea; the prevailing wisdom to that point had been that the universe was static. One thing everyone agreed on: the expansion of the universe must be slowing down over time.
Fast forward to the 1990s, when two teams of Nobel Prize-winning astronomers aimed to measure the rate at which the expansion of the universe is slowing. Instead, they discovered the opposite: it’s actually accelerating. This, says Greene drily, was a surprise. The next obvious question: what force is driving this behavior? Well, gravity can push as well as pull. According to Einstein’s math, if space is filled with a uniform, invisible mist, the gravity generated by that mist would also be repulsive. And repulsive gravity, now also known as dark energy, is exactly what we need in order to explain the concept that as galaxies push away from each other, they cause the expansion to speed up, not slow down.
Now here’s Greene’s mystery. When the astronomers tried to work out precisely how much dark energy must be infusing space to account for cosmic speedup they found a number that is, well, “spectacularly small”. So the mystery became to explain this number. “We want it to emerge from laws of physics. But so far no one has found a way to do that,” says Greene. Should we care? Yes, it might be a technical detail, but he says, “some details really matter.” And, let’s face it, a detail that might evoke the meaning of new universes surely matters at least a little.
Part Two: The solution to the mystery
Now, Greene gives a quick recap of his own topic of research: string theory, an approach to reaching Einstein’s dream of a unified theory of physics. “The central idea of string theory is quite straightforward,” he says. “If you examine any piece of matter ever more finely, at first you’ll find molecules, atoms, sub-atomic particles. Probe the smaller particles, you’ll find something else, a tiny vibrating filament of energy, a little tiny vibrating string.” It’s these strings that vibrate to create all the different particles that create the “cosmic symphony of all the richness we see in the world around us.”
One problem of this elegant unification, confesses Greene: “the math of string theory doesn’t quite work.” That is, unless we allow for extra dimensions of space, to think beyond the three dimensions of height, width and depth with which we are familiar. String theory says there are additional dimensions crumpled to such a tiny size they remain undetected. But while they are hidden, they have profound effect: the shape of those extra dimensions constrains how the strings can vibrate. And naturally, the amount of dark energy within each universe is also determined by the shape of those extra dimensions. So now we just need to figure out the shape of those dimensions.
Another problem: we don’t actually know how to do that. And, where once there were five candidate shapes for these dimensions, now there are billions of them–10 to the power 500, in fact. “Some researchers lost heart,” confesses Greene, who then proposes his solution: turn the problem on its head. Maybe, in a multiverse, each of the shapes is on an equal footing. Maybe, they’re all real, each with a different shape. And if other universes have different shapes, their physical features will be different and their amount of dark energy will be different. Instead of focusing on answering the wrong questions, we need to rethink the problem. Perhaps the right question to ask would be why humans find themselves in a universe with that particular amount of dark energy instead of in any of the other possibilities? After all, in those universes with more dark energy than here, they blow apart immediately and galaxies don’t form. Those universes with less energy implode so quickly that again, galaxies don’t form. And without galaxies, there’s no chance for stars or planets and therefore no chance for our form of life to exist in those other universes.
Part Three: The cosmological theory that pulls the story together
We often envision a cosmic explosion that created our universe and sent space rushing outward, says Greene, who points out a simple truth: the Big Bang leaves out something important: “the bang tells us how our universe evolved after the bang, but gives us no insight into what would have powered the bang itself.”
Inflationary cosmology describes a particular kind of fuel that would naturally generate a natural outrushing of space. This fuel is so efficient, it’s virtually impossible to use it all up, which means that it would not only have generated our big bang, but countless other big bangs, too, each one giving rise to its own separate universe with ours becoming just one. Meld this idea with string theory, and we can imagine other universes with extra dimensions in a wide variety of different shapes. “It’s only in our universe that the physical features–like the amount of dark energy–were right for our form of life to take hold.”
Of course, questions remain. For one thing: could we ever confirm the existence of other universes? Again, Greene allows, it’s hard to imagine, but it is possible. Inflationary theory has observational support; the Big Bang would have been so intense that as space stretched, tiny quantum jitters would have stretched from the micro to the macro world, creating a distinctive fingerprint across space which powerful telescopes have now observed. Similarly, we might be able to detect if one universe collided with another, we might one day detect those temperature differences.
Greene concludes with the implication of these ideas for the very far future. If we accept these ideas, then this means that “in the far future galaxies will rush away so fast we won’t be able to see them, not because of technological limitations but because the light will never traverse the ever-widening gap between us.” And that means that future astronomers will see nothing but “an endless stretch of static, inky black stillness.” Those astronomers might look back at the ancient records we left them, but they will likely conclude “that the universe is static and unchanging and populated by the single oasis of matter they habit; a picture of cosmos we definitely know to be wrong.” And why will they believe the records of earlier astronomers? Unlikely. This, he concludes, means that we are living through a remarkably privileged era.