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.