ONE night in June 2007, I got to watch astronomer Sandra Faber put the 10-meter Keck II telescope through its paces. She was observing galaxies in a region of the sky called the Extended Groth Strip, in the direction of the constellation Ursa Major. We sat in the cozy confines of the telescope control room, far below the telescope’s perch near the 13,796-foot-high summit of the Mauna Kea volcano in Hawaii.
Around midnight, Faber wrapped up her observations and we stepped out for a few minutes under the night sky. “I take comfort in the fact that it is a beautiful universe, and we belong here and that we fit,” Faber mused. “This is our home.”
Faber, a professor at the University of California, Santa Cruz, was referring to the idea that there is something uncannily perfect about our universe. The laws of physics and the values of physical constants seem, as Goldilocks said, “just right.” If even one of a host of physical properties of the universe had been different, stars, planets, and galaxies would never have formed. Life would have been all but impossible.
Take, for instance, the neutron. It is 1.00137841870 times heavier than the proton, which is what allows it to decay into a proton, electron and neutrino—a process that determined the relative abundances of hydrogen and helium after the big bang and gave us a universe dominated by hydrogen. If the neutron-to-proton mass ratio were even slightly different, we would be living in a very different universe: one, perhaps, with far too much helium, in which stars would have burned out too quickly for life to evolve, or one in which protons decayed into neutrons rather than the other way around, leaving the universe without atoms. So, in fact, we wouldn’t be living here at all—we wouldn’t exist.
Examples of such “fine-tuning” abound. Tweak the charge on an electron, for instance, or change the strength of the gravitational force or the strong nuclear force just a smidgen, and the universe would look very different, and likely be lifeless. The challenge for physicists is explaining why such physical parameters are what they are.
This challenge became even tougher in the late 1990s when astronomers discovered dark energy, the little-understood energy thought to be driving the accelerating expansion of our universe. All attempts to use known laws of physics to calculate the expected value of this energy lead to answers that are 10120 times too high, causing some to label it the worst prediction in physics.
“The great mystery is not why there is dark energy. The great mystery is why there is so little of it,” said Leonard Susskind of Stanford University, at a 2007 meeting of the American Association for the Advancement of Science. “The fact that we are just on the knife edge of existence, [that] if dark energy were very much bigger we wouldn’t be here, that’s the mystery.” Even a slightly larger value of dark energy would have caused spacetime to expand so fast that galaxies wouldn’t have formed.
That night in Hawaii, Faber declared that there were only two possible explanations for fine-tuning. “One is that there is a God and that God made it that way,” she said. But for Faber, an atheist, divine intervention is not the answer.
“The only other approach that makes any sense is to argue that there really is an infinite, or a very big, ensemble of universes out there and we are in one,” she said.
This ensemble would be the multiverse. In a multiverse, the laws of physics and the values of physical parameters like dark energy would be different in each universe, each the outcome of some random pull on the cosmic slot machine. We just happened to luck into a universe that is conducive to life. After all, if our corner of the multiverse were hostile to life, Faber and I wouldn’t be around to ponder these questions under stars.
This “anthropic principle” infuriates many physicists, for it implies that we cannot really explain our universe from first principles. “It’s an argument that sometimes I find distasteful, from a personal perspective,” says Lawrence Krauss of Arizona State University in Tempe, Arizona, author of A Universe From Nothing. “I’d like to be able to understand why the universe is the way it is, without resorting to this randomness.”
And he’s not the only one who feels this way. Nobel laureate Steven Weinberg of the University of Texas at Austin once told me, “I would, and most physicists would, prefer not to have to rely on anything like the anthropic principle, but actually to be able to calculate things.”
Nonetheless, there is growing and grudging acceptance of the multiverse, especially because it is predicted by a theory that was developed to solve one of the most frustrating of fine-tuning problems of all—the flatness of our universe.
Spacetime today is flat, not curved—meaning that two rays of light that start out parallel stay parallel, neither converging nor diverging. This has been confirmed to exquisite precision by measurements of the cosmic microwave background, the radiation left over from the big bang. That means that a cosmological parameter called Omega, which dials in the curvature of spacetime, is very close to one. But for today’s universe to have an Omega anywhere near one, its value just one second after the big bang had to be exactly one to precision of about fourteen decimal places. This smacked of fine-tuning.
But in 1979, the physicist Alan Guth, now of MIT, discovered a way to get that value of Omega without fine-tuning. Guth showed that in the instants after the big bang, the universe would have undergone a period of exponential expansion. This sudden expansion, which Guth called “inflation,” would have rendered our observable universe flat regardless of the value of Omega before inflation began.
Imagine starting with a small balloon whose surface is curved and blowing it up some forty orders of magnitude. Any small piece of the balloon’s surface will now look flat. In the inflationary view, that’s what happened to our universe—our local patch of spacetime looks flat regardless of the curvature of spacetime before inflation began.
Some physicists believe that inflation continues today in distant pockets of spacetime, generating one new universe after another, each with different physical properties. Inflation, therefore, walks both sides of the fine-tuning line: It lends credence to the anthropic principle by predicting a multiverse, but it also reminds us that parameters we once thought were fine-tuned, like Omega, can be explained by a more fundamental theory. “The history of physics has had that a lot,” says Krauss. “Certain quantities have seemed inexplicable and fine-tuned, and once we understand them, they don’t seem to so fine-tuned. We have to have some historical perspective.”
We’ll gain such perspective only after we have a fundamental theory of everything—or perhaps when we detect signs of other universes. The urge to understand our universe from first principles and not ascribe it to some divine force compels us to seek scientific explanations for what seems to be an incredible stroke of luck.
Editor's picks for further reading
FQXi: The Patchwork Multiverse
Raphael Buosso examines links between string theory, dark energy, and the multiverse.
FQXi: Testing the Multiverse
Hiranya Peiris looks for evidence of other universes in the cosmic microwave background radiation.
Skeptical Inquirer: Anthropic Design: Does the Cosmos Show Evidence of Purpose?
Victor Stenger provides a critical analysis of the "so-called anthropic coincidences."
TED: Why Is Our Universe Fine-Tuned for Life?
In this video, Brian Greene asks why our universe appears so exquisitely tuned for life.