During a 2015 conference on theoretical cosmology at Princeton University, Roger Penrose, a pioneer in the field of mathematical physics, was asked to speak on a panel about the origin of the universe. For decades, the leading theory had been that, during roughly the first trillionth of a trillionth of a nanosecond following the Big Bang, there was a single period of extremely rapid expansion, known as inflation, that formed the universe we observe today.
“I was pointing out major flaws with the theory of inflation. Nobody commented on that at all.”
When it was Penrose’s turn to speak, however, he wanted no part of that dogma. Instead, he reiterated his belief that the theory of inflation was false, and he proposed that the universe could instead be better described by an alternative theory, conformal cyclic cosmology, which posits that our universe continually alternates between periods of expansion and contraction. In Penrose’s formulation, the universe as we know it began not so much with a bang but with a bounce.
As Penrose recalls, few people in the audience seemed to pay him any attention, and those who did shot back with ridicule, murmuring their disbelief. “I was pointing out major flaws with the theory of inflation,” he says. “Nobody commented on that at all.” Inflation had secured such a strong foothold in the physics zeitgeist, it seemed, that even one of the world’s most accomplished theorists couldn’t chink its armor.
Today, Penrose and other physicists who seek to rewrite the narrative of how the universe began continue to face an uphill battle. To many of them, the dismissals and rejections feel more personal than scientific, driven by an academic job culture that penalizes risk taking. They worry that — for young professionals especially — the quest to unravel the deepest mysteries of the early universe will take a backseat to a far more mundane pursuit: career survival.
By many accounts, inflation is a hugely successful theory. Conceived nearly 40 years ago by Alan Guth, a cosmologist at the Massachusetts Institute of Technology (MIT), it provided some of the first plausible answers to longstanding riddles about the universe. It explained the so-called flatness problem (how the universe ended up just dense enough to keep from flying apart, but not so dense that it collapsed under its own gravity) and why the universe looks uniform instead of patchy. According to University of Oxford astrophysicist Jamie Farnes, “you can kind of think of inflation as smoothing out the universe in the same way that blowing up a balloon smooths out all the creases in the rubber.”
According to its critics, however, not everything about inflation is so smooth. In early 2017, physicists Abraham Loeb, Anna Ijjas, and Paul J. Steinhardt argued in Scientific American that proponents of inflation were essentially gaming the system: Whenever an astronomy observation disagreed with one of the theory’s predictions, theorists simply added new wrinkles to their models to make them fit the data. With every new wrinkle, the theory became more complicated and, in the eyes of Loeb and company, less plausible. “Inflation is such a flexible idea that any outcome is possible,” they wrote, concluding by calling it an “empty theory.”
A few months later, Guth and 32 other prominent physicists, including the late Stephen Hawking, published a rebuttal, defending inflation as a testable, evidence-based theory that had produced numerous successful predictions. But many critics weren’t convinced.
Among those critics is Sabine Hossenfelder, a theoretical physicist at the Frankfurt Institute for Advanced Studies in Germany. In her view, a lot of people certainly want inflation to be the answer, and it’s gotten to the point where “people keep repeating statements that are obviously wrong.”
In Penrose’s formulation, the universe as we know it began not so much with a bang but with a bounce.
One example, she says, is the idea that inflation solves the flatness problem. Hossenfelder points out that there is no mathematical reason why the universe needs to be flat, so in a way, inflation is solving a problem that never existed. She says there are other explanations and that “the people who understand inflation know this perfectly well.” However, Hossenfelder says that doesn’t stop the “disturbing” pattern of promoting inflation, saying, “if you talk to them about it, they will admit that [this and other assertions] are wrong, but they keep repeating them anyway.”
But not every supporter of inflation is so philosophically entrenched. Xingang Chen, an early universe cosmologist at Harvard University, began his career at MIT working under Guth, the father of inflation himself. But lately, he’s begun to seriously explore the alternative theories. Among them are a class of cyclic universe theories — Big Bounce models, if you will — including the one that Penrose proposed at Princeton. Introduced in the 1950s, Big Bounce theories predate inflation and are championed today by prominent physicists like Steinhardt and Neil Turok. Another contender is string gas cosmology, proposed in the 1980s by cosmologists Robert Brandenberger and Cumrun Vafa, which attempts to explain the expansion of the primordial universe using the equations of string theory.
Ask Chen, and he’ll tell you those alternative theories aren’t perfect either. For one, they are more complicated and less elegant than inflation theory. And in physics, Occam’s razor — the idea that simpler answers are more likely to be correct — generally holds sway. Chen still believes that inflation is the most plausible theory of how the universe came to be. But at the same time, he recognizes that alternative theories like the Big Bounce and string gas models can explain all the same observations that inflation can. Strictly speaking, there’s nothing special about inflation.
And, yet, inflation reigns. On ArXiv, the leading repository for physics research papers, articles mentioning cosmological inflation outnumber those mentioning cyclic cosmology and string gas cosmology by approximately 14 to 1. A search of a National Science Foundation grants website finds that, of the roughly 2,000 projects actively funded under the agency’s physics and astronomy programs, three dozen mention cosmological inflation in their project descriptions. None mention string-gas or cyclic cosmology. (In an email, Keith Dienes, the NSF’s program director for theoretical astrophysics and cosmology, said the agency funds “a lot of string cosmology as well as ‘alternative cosmological theories,’” though three of the four examples he cited do not appear to have early universe cosmology as a primary focus.)
As Hossenfelder sees it, this ideological convergence around inflation is indicative of a culture that’s become overly risk-averse in its publishing, hiring, and funding practices. She’s critical of this because “you get a lot of people who start producing [inflation] models that really don’t help you with anything,” and they do it because “you can get it published.”
“It has become very politicized,” Hossenfelder contends. “This is particularly pronounced in the United States, where people are really worried about their funding.” The anxiety is evident, she says, in the way scientists talk about securing money for their research.
Penrose agrees. “The competition at the universities is horrendous,” he says. “You’ve got to get a job.” But, he adds, the people doing the hiring are the ones who believe in these fashionable ideas.
At the end of the day, Ijjas says, challenging the mainstream “shouldn’t be impossible — but it’s okay if it’s hard. Because it should be hard.”
Indeed, today’s theoretical physicists are fighting for crumbs from an increasingly shrinking pie. NSF program director Dienes says there’s one combined budget for theoretical particle physics, cosmology, and high-energy physics, approximately $13 million dollars a year, which amounts to less than 1 percent of the total allocation for research in mathematical and physical sciences. Only a fraction of that $13 million goes to theoretical cosmology.
Ijjas, a theoretical physicist at Princeton and one of the coauthors of the controversial 2017 Scientific American article, is one of a handful of young theorists who have won funding to study an alternative theory of the early universe. But her funding came from the privately-run Simons Foundation, and even then only after the ideas — pertaining to Big Bounce models — were mature enough to be supported with high-level computations.
Ijjas says that, as a young researcher, there has to be a balance between risk-taking and conservatism. At the end of the day, she says, challenging the mainstream “shouldn’t be impossible — but it’s okay if it’s hard. Because it should be hard.”
When Penrose thinks back to that 2015 conference at Princeton, he recalls that at the end of the panel, a colleague stood up to comment on the field of cosmology as a whole — where it was headed and what was left to learn. As Penrose remembers it, the moderator’s message was that “there’s nothing new,” and “we really know fairly well everything … There’s a few things we can look at, but we’ve got the general picture.” It was the projection of certainty in the statement that bothered Penrose, who continues to argue that inflation is “a hugely artificial theory” — and that the physics community doesn’t afford the space, professionally or personally, to explore alternative ideas.
Hossenfelder captures the sentiment in a thought experiment. “Imagine you would want to come up with an entirely new explanation for the physics of the early universe,” she says. “This would mean that you would have to sit down for like, say five years, you know, do a lot of thinking, do a lot of reading, do a lot of calculations, and in these five years, there would probably not be much coming out of this.”
And then comes the rub, because “if you tried to do this,” she says, “you would be unemployed after two years, and that’s the end of that.”
Jessica Romeo is a freelance science writer based in New York City. She is currently an editorial intern at Popular Science. Her work has appeared in Scholastic Science World, Scientific American, and Smithsonian Magazine.