In the slimmest fractions of the very first second, the universe grew, and grew, and grew. By the time it slowed down, what had been a tiny, quivering quantum realm was stretched out until it looked smooth and flat, save for speckles of denser matter that later became galaxies, stars, and planets.
This is the origin story of cosmic inflation, a school of thought developed in the 1980s that has itself grown into the dominant way cosmologists think about the beginning of time.
Since it was first worked out on paper, pioneers of inflation like Alan Guth of MIT and Stanford’s Andrei Linde have continued to refine the theory and to advocate for it in scientific and public circles, celebrating when astronomical observations of the early universe appear to match what inflationary models would predict. But another of inflation’s initial architects, Princeton cosmologist Paul Steinhardt, went rogue. For the last 15 or so, Steinhardt has been a persistent critic of inflationary theory. He has even gone so far as to question whether it’s a scientific idea at all.
Now what has been a largely technical dispute between some of cosmology’s heavy hitters has prompted the establishment to strike back publicly—and hard.
In January, Steinhardt, and fellow Princeton physicist Anna Ijjas, and Harvard astronomer Avi Loeb published a feature in Scientific American criticizing inflation. They concluded by characterizing it as an idea outside of empirical science altogether. The myriad ways inflation could have played out would lead to so many possible outcomes that no astronomical observation can ever rule the general idea out, they say—and moreover, some advocates for inflation know it. This would go against a basic, popular framing of science suggested by philosopher Karl Popper, in which a theory becomes scientific when it takes the risk of making predictions that nature could then uphold or disprove.
“They really made the accusation that the inflationary community understands that the theory is not testable,” Guth, one of the idea’s founding fathers, says. “Those words angered me.” In response, Guth and his colleagues have taken the unusual step of replying with their own letter in Scientific American that insists they are doing science. They even went to the trouble of circulating their response, in order to collect signatures from many of the world’s most prominent cosmologists. “What’s the point of just making it look like it’s three people disagreeing with three people?” says David Kaiser, another author of the letter.
The 29-person list of other experts who signed on includes four Nobel Prize winners, a Fields Medal winner, Steven Hawking, and leading figures from the cosmology experiments COBE, WMAP, and Planck. (Also, twenty-five members of the list are men.) In turn, Ijjas, Steinhardt, and Loeb have published their own response-to-the-response.
For both sides, the core of the issue is whether inflation as a general approach makes specific predictions that can be checked against the sky, and the extent to which these comparisons count as empirical tests. If the universe did inflate, some kind of mysterious, short-lived field must have pushed everything apart. But theorists have wiggle room as to how exactly that field behaved, with a wide array of consequences that can both match and contradict reality, the critics note.
One battleground is the cosmic microwave background, a dim afterglow from the Big Bang itself, which is visible in all directions. Before that glow had been mapped out in detail, inflation—or at least a few flavors of inflation among others—suggested that its dappled spots would follow simple mathematical regularities. “All of those things have now been observed with high accuracy,” Guth says. The critics, meanwhile, point out that other theories can produce the same effect.
What hasn’t yet been observed, though, are additional swirling patterns, evidence of primordial gravitational waves, which many models of inflation predict should be there. Cosmologists thought they had spotted them a few years ago, prompting some inflationary theorists to pop champagne bottles—literally—when the results where announced by the BICEP2 collaboration in 2014. But that signal turned out to be caused closer to home, by dust grains in our own galaxy.
Another sticking point is the multiverse, an idea ushered in by inflation that helped drive away Steinhardt in the first place. In many models, once inflation starts it never quite stops, because the field that causes it varies in strength in different locations. Sure, inflation halted in the pocket of space we can see, but there was likely a little corner of the cosmos somewhere with an unusually high endowment of inflationary juice that will balloon into its own universe with its own different rules of physics, and a little corner inside that that will then do the same, and so on and so on, forever.
This prospect, which Ijjas, Steinhardt, and Loeb prefer to call the “multimess,” suggests that inflation doesn’t really predict any one thing—it predicts everything, because every possible combination of physical laws will exist in such a system. The inflationists, by contrast, argue this doesn’t change the work that scientists would do to investigate the physics of our own slice of space.
Instead of deciding these arguments, the current back and forth could have a stifling effect, says Andreas Albrecht at UC Davis. In the 1980s, Albrecht was Steinhardt’s graduate student; their work together helped build inflation up from the ground floor. “It’s a little disappointing to see things slip into such a polarized state,” says Albrecht. “The whole thing is not pretty.”
Ijjas, Steinhardt, and Loeb declined to be interviewed, referencing their published answer in which they say they are “disappointed” with Guth’s letter. Over email, though, they defended the choice to describe inflation and its proponents as beyond the bounds of what is empirically testable. In 2014, for example, Loeb asked Guth during a panel discussion if inflation was falsifiable—whether you could design an experiment to disprove it. Guth called that a “silly question,” suggesting that inflation was an umbrella over many theories, making it very hard to knock them all out at once. The hope right now, he says, is to use observation and further theory to winnow inflation down to just one specific version.
“Our point is that this kind of reasoning is inconsistent with normal science and cannot be resolved by invoking authority,” Ijjas, Steinhardt, and Loeb wrote to The Atlantic. They argue their intention is to direct a new generation of cosmologists to look for opportunities away from the established orthodoxy. “When a prevalent scientific theory runs into trouble, it creates a tremendous opportunity for young people to have an impact by identifying ways to avoid the trouble or inventing new approaches altogether,” they write. One such approach, favored by the team, is the idea that the big bang was not a beginning but a “bounce” where the universe shrunk and then rebounded back outwards.
Although feelings may be strained, both sides stress that they don’t want it to be personal. Kaiser, for his part, describes Loeb as a gracious colleague and recalls staying up all night in college to study Steinhardt’s work. “The frustration comes some times where I think we may be talking past each other,” he says.