After the Big Bang, our universe expanded at an exponential rate. According to this theory, known as cosmic inflation, the explosive growth caused tiny quantum fluctuations that later evolved into galaxies. Cosmic inflation clearly explains how our universe became so large and largely homogeneous, and that is why it has remained a strong theory in cosmology for decades.
But it is far from perfect. Cosmic inflation depends on certain theoretical assumptions that can be quite arbitrary – not ideal for a theory that is supposed to explain why our universe looks the way it does. It was this shortcoming that prompted theoretical physicist Raul Jimenez of the University of Barcelona in Spain to develop an alternative approach to deciphering the dynamics of the early Universe. His proposal, published earlier this month in the journal Physical Review Research, aims to eliminate the redundant, indirect parameters in traditional models that have prevented physicists from agreeing on a unified theory.
The proposal, developed by Jimenez and his colleagues, is a relatively simple paradigm based largely on well-understood principles of quantum mechanics and general relativity. It begins with the assumption that the very early universe existed in what is known as de Sitter space, which views the universe as a flat vacuum governed by general relativity. According to quantum mechanics, the application of certain energy to this space – namely, the Big Bang – generates quantum fluctuations that lead to the emergence of tensor modes, or gravitational waves. These waves organically seeded small patches of density throughout the universe, and these small patches, according to the theory, eventually evolved into galaxies, stars, and planets.
Critics of the traditional inflationary theory argue that it has too many adjustable parameters. One such parameter is inflation, the hypothetical scalar fields that physicists believe caused the rapid expansion in the early Universe. But the new theory removes inflation from the picture, replacing it with de Sitter space rocked by gravitational waves.
The fact that the new theory eliminates many of the adjustable parameters is a big bonus. “There’s no general principle that defines these things, so you basically have to input them manually,” Arthur Kosovsky, a cosmologist at the University of Pittsburgh who was not involved in the new work, explained in an email to Gizmodo. “Physicists always strive to create models and theories that are as simple as possible in some sense, which means that the number of arbitrary things that need to be manually input is as small as possible.”
In an ideal world, a robust theory or model should not require so many variables that can be adjusted. A similar problem exists with the all-encompassing Standard Model, which has 18 free parameters that need to be ordered every time. Physicists “spend a lot of blood, sweat, and tears (and money) because most people are convinced that there must be a better, more powerful model that has two or three parameters instead of 18,” Kosovsky says.
Indeed, finding a simple, convincing explanation for early cosmic inflation is what motivated the new work, Jimenez told Gizmodo during a video call. The strength of this theory is that it is “completely falsifiable” in the sense that it either can or cannot explain observational data, he said. However, this is also a weakness of the theory, which Jimenez acknowledged: “Nature may not have chosen this theory as the way things work.”
Of course, the most valuable thing about theories that can be falsified is that they tell us what doesn’t work, he added. (Although it may seem sketchy, physicists often use something like a process of elimination for unknown phenomena such as dark matter.) Regarding Jimenez’s newly proposed theory, it is fair to ask whether it will fit the observational data and whether it will stand up to further mathematical scrutiny.
