I hope I was wrong about inflation. For decades, I've been bashing this theory of cosmic creation, lumping it together with strings, multiverses (which inflation has helped popularize) and other highly speculative propositions sprung from theorists' fecund minds. [See Addendum below for an exchange between me and multiverse popularizer "Mad Max" Tegmark.]
Proposed more than 30 years ago, inflation holds that an instant—10 -43 seconds, according to one estimate—after the Big Bang, gravity flipped inside out, briefly becoming a repulsive rather than attractive force. As a result the cosmos underwent an almost unimaginably rapid growth spurt, which had a profound impact on its evolution, before slowing down to a more leisurely rate of expansion.
Many cosmologists fell in love with inflation, because it seemed to solve riddles posed by the basic Big Bang theory. Why, for example, does the universe appear so uniform in all directions? The answer is that inflation would have smoothed out lumps in spacetime, just as blowing up a balloon smooths out its wrinkles.
But inflation has always been more a product of imagination than empirical evidence. There has never been more than circumstantial, hand-wavy support for its core mechanism, the reversal of gravity. Worse, the theory came in many different forms. My favorite was the eternally self-reproducing chaotic inflationary multiverse model proposed by Andrei Linde, who along with Alan Guth and Paul Steinhardt is credited with inventing inflation. [*For more on Linde and his inflation theorizing, see my 1992 profile of him, which I just posted on this blog.]
Indeed, inflation, like string theory, has always suffered from what is sometimes called the "Alice's Restaurant Problem." Like the diner eulogized in the iconic Arlo Guthrie song, inflation comes in so many different versions that it can give you "anything you want." In other words, it cannot be falsified, and so--like psychoanalysis, Marxism and other overly flexible hypotheses--it is not really a scientific theory.
Inflation enthusiasts have claimed vindication before—for example, in 1992, when the COBE satellite produced a detailed map of the cosmic microwave background, the afterglow of the Big Bang; and in the late 1990s, when astrophysicists discovered that the expansion of the universe is accelerating. But neither of these supposed confirmations of inflation held up.
Just two months ago, inflation pioneer Paul Steinhardt wrote on the website Edge.org: "I think a priority for theorists today is to determine if inflation and string theory can be saved from devolving into a Theory of Anything and, if not, seek new ideas to replace them. Because an unfalsifiable Theory of Anything creates unfair competition for real scientific theories, leaders in the field can play an important role by speaking out—making it clear that Anything is not acceptable—to encourage talented young scientists to rise up and meet the challenge." (See also Steinhardt's April 2011 Scientific American article: "Is the theory at the heart of modern cosmology deeply flawed?")
I'm intrigued by today's news that observations of gravitational waves provide "direct proof of the theory of inflation," as my colleague Clara Moskowitz puts it in a terrific, information-packed post. "The Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2) experiment in the South Pole," she continues, "found a pattern called primordial B-mode polarization in the light left over from just after the Big Bang, known as the cosmic microwave background (CMB). This pattern, basically a curling in the polarization, or orientation, of the light, can be created only by gravitational waves produced by inflation."
"If corroborated," Dennis Overbye writes in The New York Times, the BICEP2 study "will stand as a landmark in science comparable to the recent discovery of dark energy pushing the universe apart, or of the Big Bang itself. It would open vast realms of time and space and energy to science and speculation."
I hope that turns out to be the case, because cosmology and physics desperately need a jolt of energy (which the anti-climactic discovery of the Higgs boson did not provide). But here is what I'd like to see: First, corroboration of the BICEP2 findings by other groups and observatories. Second, experiments from high-energy physics that provide some sort of corroborating evidence of the driving mechanism of inflation. Third, an explanation of why the Alice's Restaurant Problem isn't still a problem. Fourth, an explanation of why only inflation, and not other more conventional physical phenomena, can account for the gravity-wave findings.
When these conditions are met, I'll be happy to admit I was wrong about inflation. But multiverses? Never!
Addendum: I want to draw attention—and respond—to a comment below from physicist and multiverse popularizer "Mad Max" (as he calls himself) Tegmark: "Thanks John for this thought-provoking post! When you say 'But multiverses? Never!', do you really feel that this is a scientifically defensible stance, or is it more of an emotional conviction? It gives me flashbacks from the recent Nye-Ham creationism debate, where Ham was asked whether there was any evidence that would even make him admit that he was wrong about Earth being 6000 years old, and he basically said 'no'. A key point I make in chapter 6 of my book (http://mathematicaluniverse.org) is that parallel universes aren’t theories, but *predictions* of certain theories, and that’s it's unscientific to accept a theory while rejecting some of its predictions: https://blogs.scientificamerican.com/guest-blog/2014/02/04/are-parallel-universes-unscientific-nonsense-insider-tips-for-criticizing-the-multiverse/."
My response: "Max, so you equate my rejection of multiverses, which are totally imaginary and by definition can never be observed, with a young-earth creationist's rejection of evolution, which has been confirmed by a virtually infinite number of observations? Really? That’s funny. Here’s my attempt at multiverse humor, 'Is speculation in multiverses as immoral as speculation in subprime mortgages?', which I posted in 2011 in response to Brian Greene's book The Hidden Reality. The core of that column goes: 'Multiverse theories aren’t theories—they’re science fictions, theologies, works of the imagination unconstrained by evidence. At their best, science fiction and theology can leave us awestruck before the unutterable strangeness and vastness of the cosmos. Multiverse theories used to arouse these emotions in me. When the Russian physicist Andrei Linde—one of the inventors of the inflation theory of cosmic creation—first explained his chaotic, self-reproducing, fractal, inflationary multiverse theory to me 20 years ago, my reaction was, 'Wow! That’s so cool!' Multiverse theories don’t turn me on anymore. Perhaps it’s because of 9/11 and all its bloody consequences, especially the U.S. wars in Afghanistan and Iraq… Now, multiverse theories strike me as not only unscientific but also immoral, for two basic reasons: First, at a time when we desperately need science to help us solve our problems, it’s irresponsible for scientists as prominent as Greene to show such a blithe disregard for basic standards of evidence. Second, like religious visions of paradise, multiverses represent an escapist distraction from our world.'"
Photo of BICEP2 observatory at South Pole by Keith Vanderlinde.