Inflation without inflaton: A Spanish scientist proposes a revolutionary theory about the origin of the Universe.

It was gravitational waves , not hypothetical particles called inflatons, that were responsible for the sudden inflation that occurred in the very beginnings of the Universe. Just as ocean waves shape coastlines, these ripples in the fabric of space-time set the newborn cosmos on the path that led it to become what we see today. A truly revolutionary idea, captured in an article titled "Inflation without an Inflaton," recently published in Physical Review Letters by a team of researchers led by Spaniard Raúl Jiménez, an ICREA researcher at the Institute of Cosmos Sciences at the University of Barcelona, in close collaboration with scientists from the University of Padua in Italy.

For decades, the theory of cosmic inflation has been the cornerstone that allows us to understand, at least in part, the mystery of the origin of the Universe. It is a model that, in essence, tells us that the Universe, in its earliest moments, grew explosively , expanding at an unimaginable speed in just a fraction of a second. Without this sudden expansion, it would be impossible to explain, for example, why, despite the incredible distances, the average temperature of the Universe and the way matter is distributed within it are uniform—that is, the same everywhere. These regions would never have been able to 'share' these properties if they had not previously been in contact. That is, if a small region that was already uniform and in thermal equilibrium from the beginning had not rapidly expanded to encompass the entire Universe. This kind of cosmic 'stretch,' the theory says, was what paved the way for the formation of everything we see today.

However, despite its success, the theory of cosmic inflation has an "Achilles' heel" that physicists are well aware of: its excessive dependence on adjustable parameters. In other words, inflation, in its most widespread formulation, requires the existence of a hypothetical field, the inflaton, for which we have no experimental evidence. Furthermore, for the model to fit observations, a number of parameters must be "tweaked" and adjusted, which for many scientists is a problem.

In the world of science, a model is considered more robust the less it requires manipulation to work. Conversely, a model that fits the data perfectly, but only after adjusting a thousand and one variables, is more like a custom-made suit than a universal law of nature.

And this is where the bold new idea proposed by Raúl Jiménez and his colleagues comes in. Their theory doesn't need to resort to the inflaton or any other exotic ingredient to work. Instead, it suggests that the quantum fluctuations of space-time itself, in the form of gravitational waves, were sufficient to sow the seeds of the cosmic structures we see today. It's an elegant and minimalist idea, using only two essential ingredients: gravity and quantum mechanics.

In a rapidly expanding universe—a concept that fits perfectly with what we see today due to the action of dark energy—those tiny oscillations of "nothingness" occur continuously and are natural. These fluctuations manifest themselves as gravitational waves that propagate, collide, and interact.

Through complex calculations, Jiménez's team was able to demonstrate in their study that these tiny gravitational waves, as they interact with each other, can generate the density variations necessary for gravity to do its work over time and give rise to the structures we know. The end result is a model that generates a spectrum of perturbations (density differences) consistent with what we observe in the Cosmic Microwave Background, the echo of the Big Bang. And it does so without the need to postulate new particles. As Jiménez himself points out, "we are demonstrating that gravity and quantum mechanics may be sufficient to explain how the structure of the cosmos emerged."

The proposal by Jiménez and his team, although not the only alternative (there are, among others, "string cosmology," the "Big Bounce," and "Loop Quantum Gravity"), stands out for its simplicity and, above all, because it is verifiable. As Jiménez himself points out, "science, at its best, is the ability to make clear predictions that future observations can confirm or reject."

Of course, the 'classical' theory of inflation also makes predictions, but it is difficult to verify, as its own flexibility and adaptability work against it. The new proposal, however, is much more restrictive. And if it is correct, we should be able to test it with the next generations of telescopes and experiments currently under construction. The key lies in being able to detect the so-called 'primordial' gravitational waves, something that has not yet been achieved.

In fact, current gravitational wave detectors, such as LIGO and Virgo, detect very "coarse" waves, those that come from extremely violent events, such as the collision of black holes or the explosion of stars. But the "fingerprint" of the primordial gravitational waves on which Jiménez's model is based would be much more subtle, of a much lower frequency, and beyond the reach of current instruments. This is something that future projects, such as the LISA space telescope or the search for gravitational waves through the observation of pulsars, could resolve, and thus be able to confirm or rule out the Spanish scientist's fascinating idea.

If Jiménez's theory is ultimately confirmed, it would not only give us a more elegant view of the origin of the cosmos, but would also open a new chapter in our understanding of its earliest moments of existence. It would show us that, sometimes, the answer lies not in the exotic and the unexplored, but in the depths of physics we already know. A true lesson in humility and a testament to the intrinsic beauty of the Universe.