The primordial universe underwent a phase of swift expansion, referred to as inflation. For many years, cosmologists believed that this growth was fueled by a new component in the universe known as the inflaton. However, recent studies indicate that it might have been feasible to expand the universe without any additional source driving that inflation.
In the 1970s, physicist Alan Guth developed a groundbreaking concept of the very early universe. Initially aiming to address certain problematic traits exhibited by high-energy physics in the young, compact, hot universe, he envisioned a framework wherein a novel quantum field, termed the inflaton, facilitated a brief but intense period of extraordinarily accelerated expansion, enlarging the universe many orders of magnitude in size in less than a second.
Inflation has endured as a compelling theory of the early universe due to its ability to resolve numerous issues simultaneously. For instance, it elucidates why the universe seems geometrically flat: It’s so vast that, notwithstanding its overall curvature, any region of the universe will appear flat. It also clarifies why areas of the universe separated by immense distances are approximately the same: They established a connection before inflation separated them.
Most importantly, inflation accounts for the formation of our large-scale structures. The process of inflation took the quantum foam of space-time and expanded it to larger scales, establishing the gravitational seeds that would eventually evolve into stars, galaxies, and the cosmic web.
Nonetheless, enigmas persist. We remain unaware of the identity of the inflaton, its source of power, or why it ceased when it did. Moreover, we lack definitive evidence that inflation truly occurred.
Given these obstacles, there might be a method to replicate the observed traits of the universe without necessitating an inflaton. In a recent study, astrophysicists propose a model where inflation occurs, leading to the universe’s large-scale structure, without any new driving force.
This model commences with an explanation of space that is expanding due to a cosmological constant, similar to the dark energy observed in today’s universe. Against this backdrop, the quantum foam continues its usual behavior, which involves shaking space-time at submicroscopic scales.
These fluctuations generate gravitational waves, which are ripples that travel outward through space. Gravitational waves alone cannot form the seeds of contemporary structures, as they exert the incorrect type of influence on space-time.
Related: How did inflation occur — and why is it significant?
However, the researchers discovered that under appropriate conditions, the gravitational waves triggered by the quantum foam can sometimes induce exactly the right type of distortions in space. Specifically, they sought out distortions that were approximately uniform across various length scales. We understand that the seeds of structure had to exhibit this type of pattern, as that’s precisely what we observe in the cosmic microwave background, the leftover light pattern from when the universe was merely 380,000 years old. That afterglow retains a record of those early structures, enabling us to study models of inflation.
There exist slight variances between the types of structures produced in this inflation-without-inflaton scenario and traditional inflation. In this paper, the researchers did not assess how significant those differences are, but a crucial next step is to investigate the observational implications of this model.
The model is not flawless. It still assumes something about the nascent universe — specifically, that the cosmological constant is sufficiently strong to induce a rapidly expanding cosmos. Furthermore, it does not clarify the flatness problem or why remote areas of the universe are generally consistent. Nevertheless, it represents a fascinating avenue of research, as it may unveil potentially valuable alternative perspectives that do not rely on an inflaton to drive the transformations in the early universe.
The youthful cosmos continues to be a substantial enigma in contemporary cosmology. While we believe we are heading in the right direction — it indeed seems that the universe experienced a phase of rapid expansion — there is still much we have yet to comprehend.