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.