Based on the first data collected by the Dark Energy Spectroscopic Instrument (DESI), an international collaboration, made up of approximately 900 scientists belonging to more than 70 institutions, including various Spanish research centers, has created map 3- D most extensive and precise of the universe to date, in a project whose main objective is to validate the current cosmological model that explains what our universe is like and how it evolves.
The observations made by DESI in just one year provide information from an impressive record made up of more than 5.7 million objects distributed over a wide range of distances, including galaxies that emitted their light more than 11 billion ago. of years (by the time the project ends, up to 40 million galaxies will have been observed).
The first results derived from these observations support the prevalent model in current cosmology, and according to which the accelerated expansion of the universe, discovered at the end of the last century, is fueled by an energy that is called dark and that remains with a constant density over time. However, when the DESI data is combined with data from other studies, the analyzes reveal clues that suggest that dark energy could be variable, which would open the door to profoundly rethinking the existing theoretical framework.
The study of dark energy and its relationship with the expansion of space has become, in recent years, one of the frontiers of knowledge about the cosmos. Science still does not know what the nature of this energy is and what role it can play in the future development of the universe.
When, at the beginning of the 20th century, Albert Einstein proposed the theory of general relativity, the equations he derived to describe the behavior of the universe announced, surprisingly and contrary to the dominant thinking at that time that the cosmos was static, that space must be expanding or collapsing. But this fact was so extraordinary that Einstein himself decided to modify his formulas, introducing a constant into them that would counteract the tendency towards a dynamic universe and ensure its immutability.
A few years later, in the early 1930s, when Edwin Hubble experimentally discovered the expansion of the universe (a discovery supported by the theoretical development that other scientists had made from general relativity), Einstein recognized his mistake and resigned. to the use of its constant (called cosmological).
Finally, in the last decade of the last century, it was demonstrated that the cosmos is not only expanding, but that it is doing so increasingly rapidly, and the term dark energy was coined to refer to the enigmatic force that fuels this acceleration. In the current cosmological model, this energy would behave similarly to Einstein's original cosmological constant, but this time driving the expansion and not slowing it down.
The point is that, in the equations that account for the development of the universe, a term appears again that represents the cosmological constant and that, in numerous interpretations, is considered as an energy associated with empty space. This being so, the density of this energy would be expected to be constant: as the universe expands, more space is created which, in turn, contributes more dark energy, so the result is that its density remains unchanged. and does not dilute.
However, there is increasing evidence to suggest that the density of this dark energy could vary with time (and/or space).
This being the case, it is not surprising that the DESI collaboration has raised so much expectation. It is an instrument installed on the Nicholas U. Mayall telescope at the Kitt Peak Observatory in Arizona (USA), and has 5,000 small robotic devices capable of directing the weak radiation received towards specialized receivers. The project plans to observe, for five years, millions of galaxies in different distance ranges (which is equivalent to saying in different periods in the history of the cosmos) to study how the universe has evolved and be able to validate the standard cosmological model or detect small divergences in it.
The observation of our universe, on a large scale, reveals the presence of large structures, formed by clusters of galaxies and enormous empty spaces. But the accumulations of matter do not appear randomly distributed, and astronomers have been able to identify a certain pattern that is interpreted as a vestige of the early cosmos.
If we study the probability of finding a galaxy around any other, we obtain a curve that decreases as we move further away. However, and significantly, a small increase in this probability is detected towards 490 million light years away. That is to say, there is a statistically higher presence of matter forming a type of sphere with a radius of 490 million light years.
The reason for the existence of this pattern must be sought in the waves that were formed in the moments after the Big Bang due to minute fluctuations in the density of the plasma (the hot soup of particles that made up the cosmos at that time). These undulations, which are called baryon acoustic oscillations (in reference to the fact that they were formed due to the movement of protons and neutrons, representatives of a group of particles collectively called baryons, and which moved at the speed of sound in that medium). ), were imprinted in space, grew as the universe expanded and became the seeds that concentrated matter around them.
Therefore, the study of the evolution of the large structures of the cosmos from the acoustic oscillations of baryons allows us to determine how the expansion has developed over billions of years.
The DESI collaboration also focuses on analyzing the light from very old galaxies, which has therefore traveled towards us for much of the history of the universe. As Andreu Font-Ribera, a researcher at the Institut de Physics d'Altes Energies in Barcelona and who has co-led this part of the study, comments, the analysis allows us to detect the concentrations of gas that this light must have passed through on its path.
Specifically, Font-Ribera explains that the clouds of neutral hydrogen in the universe absorb part of the light that passes through them, and they always do so at a certain wavelength: precisely what is needed to excite the electron of the hydrogen atom of the first to the second energy level (a transition called Lyman-alpha).
Thus, when the light of a galaxy decays, the absence of radiation right at the Lyman-alpha wavelength reveals the presence of concentrations of neutral hydrogen between the object and us. But even more interesting: the expansion of space shifts the wavelength value at which we perceive the Lyman-alpha transition, which means that, as the light from the distant galaxy passes through different accumulations of hydrogen on its journey towards us instruments, the multiple absorptions that its radiation has undergone appear slightly moved in the spectra, creating a characteristic pattern known as the Lyman-alpha forest.
Therefore, the analysis of the Lyman-alpha forest allows, in the words of Font-Ribera, to reconstruct what the distribution of gas was like in different stages of the universe and to study the effect that the expansion has had on it.
In order to capture the light of extraordinarily distant objects, we have resorted to the observation of quasars, galaxies with supermassive black holes inside that devour matter. This material, as it falls towards the black hole, is heated, by friction, to millions of degrees and emits powerful radiation that is about 1,000 times brighter than that generated by a normal galaxy and that, therefore, our instruments can receive. despite the distance.
Using DESI data, scientists have been able to carry out simulations considering different values for the fundamental parameters of the universe. The result is that the study confirms the current cosmological model: a universe dominated by 69% dark energy of constant density, and with 31% matter (in which the so-called cold dark matter is the majority, an ingredient that does not It is made up of atoms and whose nature we still do not know).
The alarm has arisen, however, when researchers have combined DESI with information from other projects.
Licia Verde, ICREA researcher at the Institute of Cosmos Sciences of the University of Barcelona and participant in DESI, explains that when the data are analyzed together with those from the cosmic microwave background (the light that was released shortly after the Big Bang ) and with some of those derived from the study of supernovae (star explosions), the models seem to have a certain preference towards a non-constant acceleration of the expansion, that is, towards a dark energy that could be variable in time.
Despite this, researchers are cautious and warn that it is not yet a complete detection. At the moment, the evidence found is within a confidence range of 99.87%, still insufficient to rule out the possibility that what was found is due to a purely statistical effect. But Verde warns that the indication is significant enough that "we cannot sweep it under the rug" and that further investigation must be carried out.
Furthermore, Aurelio Carnero, researcher at the Institute of Astrophysics of the Canary Islands and also a member of DESI, recalls that there are already other symptoms that seem to indicate that "there is something that does not fit" in our knowledge about the universe, referring to the difference that is obtained from the value of the current expansion rate of the cosmos when calculated using different methods (a problem called the "Hubble tension" and which has recently been confirmed).
Given the high precision required in this type of project, and in an attempt to avoid unintentional bias, studies carried out with DESI observations have used a mechanism consisting of modifying the data during the project phase in which scientists have should develop their analysis models.
Thus, the research team did not have access to the real data until the models were completed and trained with the simulated information. As Andreu Font-Ribera highlights, this approach, in any case necessary, has considerably increased the difficulty of scientists' work.
Confirmation of variation in dark energy density would have profound consequences for cosmology. It would force us to rethink our model of the universe and provide fundamental clues to understand what this mysterious energy really is or if gravity, on a large scale, does not work as we think.
Although it would be an exciting possibility, its validation could still be a long way off. In this sense, Aurelio Carnero points out that harmony will be required between different studies that use different techniques. Among these projects, Carnero highlights those based on the EUCLID space telescope (belonging to the European space agency and which is already in orbit 1.5 million kilometers from Earth) and the Vera Rubin (an observatory under construction that will have with an instrument with more than 8 meters of mirror and promoted by the National Science Foundation of the United States).
