A roiling debate about the origins of the universe has jumped from academic journals into the pages of Scientific American. The dispute revolves around what’s known as cosmic inflation, a widely accepted theory of how the universe expanded in the first moments of its history.
In the February issue of the magazine, theoretical physicists Anna Ijjas and Paul Steinhardt of Princeton, and Harvard’s Abraham Loeb, published a feature arguing that inflationary theory was probably wrong, and that the theory has become so open-ended that it is essentially unverifiable. The theory’s defenders, they write, have discarded “empirical testability” and started “promoting the idea of some kind of nonempirical science.”
That suggestion ruffled feathers among at least 33 prominent physicists, including Stephen Hawking at the University of Cambridge, four Nobel Prize laureates, and a Fields medalist — all of whom put their names to a response defending the empirical rigor of inflation models. The letter is scheduled to be published online at Scientific American this week, and in the July print issue.
David Kaiser, a physicist and historian of science at MIT who serves on Undark’s advisory board, sent an advance copy of the letter, which he co-authored and signed, to Undark on Monday. In an accompanying email message, Kaiser stated that the claims made by Ijjas et al had been “explicitly debunked several years ago,” and he questioned the judgment of the editors at Scientific American — among the oldest continually published magazines in the U.S. — for allowing the article’s authors to characterize the vast majority of researchers in the field “as if they were members of some cult.”
“I’m not here to say inflation is right,” Kaiser said in a follow-up phone call. “I’m just saying inflation gives us a testable set of questions and statements.”
Alan Guth, an MIT physicist who first proposed the inflation concept around 1980, told Undark that Ijjas and her colleagues were wrong to suggest that some scientists adhere to the theory of cosmic inflation even though they believe that it is untestable. “That’s totally outrageous,” Guth said. “It has no basis in fact whatever, as far as I know.”
Ijjas, Steinhardt, and Loeb all declined multiple requests for a telephone interview, but when asked via email if they could name any pro-inflation scientists who believe that the theory is nonetheless untestable, the trio pointed to a video of a 2014 panel during which Loeb asks Guth directly whether it’s possible to do an experiment that would falsify inflation.
The full exchange is below:
Ijjas and her colleagues also directed Undark to web page they’d prepared in response to the criticisms of their article. In a nutshell, the physicists stand by their argument, suggesting that the critics are misreading their work. “We have great respect for the scientists who signed the rebuttal to our article,” they wrote, “but we are disappointed by their response, which misses our key point: the differences between the inflationary theory once thought to be possible, and the theory as understood today.”
(Readers keen on digesting the full response are encouraged to visit the website for themselves.)
This is the third time in six years that Scientific American has published a challenge to inflationary cosmology. The magazine ran a print feature by Steinhardt in 2011, and a long Q&A with the physicist in 2014.
Still, the publication admits that it remains a fringe position. “It’s obvious that vastly more researchers favor the inflation idea than the alternative, and we’re certainly not pretending otherwise,” said Scientific American’s Clara Moskowitz, who edited the feature. “I don’t think that means that anyone who disagrees shouldn’t be allowed to have their say — in our pages or anywhere.”
This post has been updated to embed the full video exchange between Alan Guth and Abraham Loeb on the question of whether inflation theory is empirically testable.