Following the Big Bang, our universe expanded at an exponential rate. According to this theory, known as cosmic inflation, the explosive growth produced tiny quantum fluctuations that later evolved into galaxies. Cosmic inflation neatly explains how our universe got so large and mostly homogenous, and that’s why it’s remained a strong theory in cosmology for decades.
But it’s far from perfect. Cosmic inflation depends on certain theoretical assumptions that can get rather arbitrary—not ideal for a theory that’s supposed to explain why our universe appears the way it does. It’s this shortcoming that motivated theoretical physicist Raúl Jiménez from the University of Barcelona in Spain to devise an alternative approach to decoding the dynamics of the very early universe. The resulting proposal, published earlier this month in Physical Review Researchseeks to eliminate the excessive, circumstantial parameters in traditional models that have made it difficult for physicists to agree on a single theory.
The proposal, developed by Jiménez and colleagues, is a relatively simple paradigm founded mostly on well-understood principles of quantum mechanics and general relativity. It starts with the assumption that the very early universe existed in what’s called a Sitter Spacewhich sees the universe as a flat-shaped vacuum governed by general relativity. According to quantum mechanics, applying some energy to this—namely the Big Bang—generates quantum fluctuations that give rise to tensor modes, or gravitational waves. These waves organically seeded small bits of density throughout the universe, and those little bits eventually evolved into galaxies, stars, and planets, according to the theory.
Critics of traditional inflationary theory argue that it has too many adjustable parameters. One such parameter is the inflaton—hypothetical scalar fields that physicists believe drove rapid expansion in the early universe. But the new theory removes the inflaton from the picture, substituting it with a de Sitter space rocked by gravitational waves.
That the new theory removes many adjustable parameters is a big bonus. “There is no general principle that determines these things, so basically you need to put them in by hand,” explained Arthur Kosowskya cosmologist at the University of Pittsburgh not involved in the new work, in an email to Gizmodo. “Physicists always strive to make models and theories which are in some sense as simple as possible, meaning that the number of arbitrary things you need to put in by hand is as small as possible.”
In an ideal world, a solid theory or model shouldn’t require so many adjustable variables. A similar problem exists with the all-encompassing Standard Model, which features a whopping 18 free parameters that need to be sorted out every single time. Physicists “expend lots of blood, sweat, and tears (and money) because most people are convinced that there must be a better, more powerful model which has two or three parameters instead of 18,” Kosowsky said.
And indeed, finding a simple, compelling explanation for early cosmic inflation is what motivated the new work, Jiménez told Gizmodo during a video call. The strength of this theory is that it is “fully falsifiable” in the sense that it either can or cannot explain observational data, he said. However, this is also the theory’s weakness, which Jiménez acknowledged: “Maybe nature didn’t choose this theory as the way things work.”
Of course, the most valuable thing about falsifiable theories is that they tell us what doesn’t work, he added. (While this might seem sketchy, physicists often employ something akin to a process of elimination for unknown phenomena, such as dark matter.) As for Jiménez’s newly proposed theory, it’s fair to ask whether it will hold up to observational data and survive further mathematical scrutiny.
“I like the overall philosophy driving this paper, [which is] ‘let’s see if we can come up with a situation where inflation arises naturally out of some basic physics,’” Kosowsky said. “If we can, this is both more elegant than adding some speculative and, in some sense, arbitrary physical elements and also is likely to make more specific predictions, which can then hopefully be compared with observations.”
“I believe it’s an interesting and novel proposal—it’s something that’s well worth a closer look,” commented Andrew Liddlea theoretical cosmologist at the Institute of Astrophysics and Space Sciences (IA) at the University of Lisbon in Portugal, during a video call with Gizmodo. At the same time, its simplicity could also be its biggest flaw, but only time will tell if more mathematically minded cosmologists take a liking to it, he said.
“There have always been cosmologists who are uncomfortable with inflation [theory]. I’m one of them—and I work on it,” said CORTE MARINAalso with the IA, in the same call. “One of the most uncomfortable things about inflation is that physicists understand everything from the Big Bang onwards, but not the Big Bang and the earliest stages.”
Liddle and Cortês, both uninvolved in the new work, said that while cosmologists (including themselves) often disagree on how to best interpret cosmic inflation, the evidence seems to support the notion that inflation did in fact take place. Many physicists have devised alternative explanations, but practically everything has ended up in a “dustbin” of discarded ideas, Liddle explained.
“But there’s no limit to people’s imagination,” Liddle said. And the next few decades should see no shortage of new ideas and models—just like this one, according to the two cosmologists.
“Cosmology right now is mostly about these things called tensions, or hints that things are not quite well aligned with the standard cosmological model,” Liddle said. Several questions threatening to usurp what we know about the physical universe—dark energy, the Hubble tension—appear to be coming together in one paradoxical package for scientists, and inflation could be a part of that, Cortês added.
No matter what happens, it goes without saying that we’re witnessing a time of excitement, chaos, and discovery for cosmology—a sentiment that all the scientists agreed on.
“Not only is the data growing at exponential amounts, but the quality of the analysis is also growing at an exponential quality,” Jiménez said. “I think that we are living a golden age of cosmology.”
“When we are thinking about inflation, we are trying to take the next step and answer the question of why the universe looks the way it does, and not just describe how it looks,” Kosowsky said. “Is this due to some deep physics principle yet undiscovered? It could be, and this is what keeps us working hard to push back the boundaries of our understanding.”
