The most extensive and accurate 3D map of the universe created so far has confirmed that 69% of the cosmos is composed of dark energy of an unknown nature that is accelerating cosmic expansion. But the results of the research, still preliminary, indicate that this dark energy may not have been constant throughout the history of the universe, but rather evolved. If these indications are confirmed, they would have profound consequences, as they would force a rethinking of the models on which the current understanding of the universe is based.
The new map of the universe in 3D has been produced with data from the first year of observations of the Dark Energy Spectroscopic Instrument (DESI, for its initials in English). It is an instrument installed in one of the telescopes of the Kitt Peak Observatory, in Arizona (USA). So far it has observed 5.7 million objects in different distance ranges, which is equivalent to saying in different periods in the history of the cosmos. Among the stars analyzed, there are galaxies that emitted their light more than 11,000 million years ago.
About 900 scientists from more than 70 institutions, including several Spanish research centers, are participating in the project. It aims to observe up to 40 million galaxies over five years. With this, it is hoped that it will be possible to better understand how the universe has evolved throughout its history.
The first results largely validate the standard cosmological model, as they confirm that the universe is 69% dominated by dark energy of constant density, and contains 31% matter. Within this matter, the majority is the so-called cold dark matter, an ingredient that is not made up of atoms and whose nature is still unknown.
The surprise has emerged after combining DESI observations with data from other projects, reports Licia Verde, Icrea researcher at the Institute of Cosmos Sciences of the University of Barcelona and participant in DESI. When the data are analyzed together with data from the cosmic microwave background (the light released shortly after the big bang) and some from the study of supernovae (exploding stars), models suggest that the acceleration of the expansion of the universe is not constant. Therefore, dark energy could be variable in time. This conclusion, if confirmed, would force a revision of the current cosmological model.
When the accelerated expansion of the universe was discovered at the end of the last century, the term dark energy was coined to refer to the enigmatic force that accelerates the expansion. This energy was considered to behave like a constant and, for historical reasons, was called the cosmological constant, a term Einstein used in the equations of general relativity that describe the cosmos. However, DESI's early results indicate that it may not be constant.
These results have raised great expectations in the cosmological community. But the researchers warn that this is not yet a full-blown discovery. With the current data, the probability that the acceleration of the expansion of the universe is not constant is 99.87%. This percentage is still insufficient to rule out that what has been found is due to a statistical mirage. But Licia Verde warns that the sign is significant enough not to be ignored.
Confirming that the expansion of the universe does not behave as the current cosmological model indicates may still take some time. Aurelio Carnero, researcher at the Institut d'Astrofísica de les Canarias and also a member of DESI, points out that there will be a need for coordination between different studies that use different techniques. Among these projects, Carnero highlights those based on the Euclid space telescope of the European Space Agency (ESA), launched in 2023, and the Vera Rubin telescope, which will enter service in Chile in 2025.
