On April 15, the Dark Energy Spectroscopic Instrument (DESI) scientific project successfully completed the main objective of its five-year mission by creating the most detailed three-dimensional map of the Universe. The five-year survey was completed ahead of schedule, accurately mapping more than 47 million galaxies and quasars—far exceeding the initial target of 34 million—as well as 20 million stars in our neighborhood.
The precise coordinates and spectroscopic data for all of these celestial bodies make up the largest, high-resolution 3D map of the Universe created to date. Scientists are analyzing the relevant data to investigate dark energy, an unknown form of energy with gravitationally repulsive properties, which causes the universe’s accelerating expansion.
So far, the instrument's performance has exceeded all expectations. For this reason, and given the highly intriguing finding that an analysis of data from the first three years of its operation indicates that dark energy may be changing, DESI will continue collecting data through 2028.
According to the Standard Model of Cosmology (L-CDM), which describes the evolution of the Universe from an initially superheated and superdense state, the energy and matter it contains consist of 68% dark energy, 27% cold dark matter (Cold Dark Matter, CDM), and just 5% of ordinary, so-called baryonic matter. In this context, the Standard Model of Cosmology holds that dark energy is an intrinsic property of empty space and, as such, remains completely constant over time. This type of dark energy is called the cosmological constant and is denoted by the letter Λ.
Essentially, this is the same constant that Einstein himself somewhat arbitrarily introduced into the equations of the General Theory of Relativity (GTR), in his effort to arrive at a static model of the Universe. For the record, it is worth recalling here that when Einstein formulated the General Theory of Relativity in 1915–16, most scientists believed that the universe was static and unchanging.
Even Einstein himself found it impossible to remain unaffected by this deeply ingrained perception, even though solving the equations of general relativity leads quite naturally to the theoretical prediction of an expanding universe. So, against this general backdrop of rejection of a cosmological evolution, Einstein decided to modify the equations of the General Theory of Relativity, introducing an additional term—known as the cosmological constant—in order to counteract the gravitational pull that tended to cause it to contract. Unable to recognize in time the inherent instability of his static cosmological model, Einstein thus missed the opportunity to be the first to predict the expansion of the Universe.
Ultimately, following the observations of the American astronomer Edwin Hubble, who proved in 1929 that the universe is indeed expanding, the “necessity” of the cosmological constant was temporarily “lost,” and Einstein himself was forced to remove it from the equations of general relativity, admitting that its introduction was “the biggest blunder of his life.” However, with the discovery in 1998 that the expansion of the universe is accelerating, the cosmological constant returned to the forefront, this time as the simplest explanation for dark energy.
Let’s return, however, to our topic, bringing this historical digression to a close. By analyzing the enormous amount of data collected by DESI, researchers can detect the effect of dark energy over the last 11 billion years of cosmic expansion. It is interesting to note that an analysis of the data from its first three years of operation suggests that dark energy may vary over time, with a standard deviation of 2.8–4.2σ (roughly speaking, the larger the sigma, the smaller the probability that the result is due to random variation.) The 5σ) are considered a confirmed scientific discovery.
With the complete five-year dataset, researchers will have significantly more information to determine whether this finding is further supported or not. If confirmed, this would mark a significant shift in how we understand the Universe and its possible fate, and would require a reevaluation of the Standard Cosmological Model.
The analysis and processing of the complete dataset has already begun, and the first results on dark energy are expected in 2027. As we mentioned, DESI will continue its observations at least until 2028, expanding the boundaries of its three-dimensional map of the Universe by about 20% and covering regions of the sky that are more difficult to observe.
DESI is installed on the Nicholas U. telescope. Mayall at the U.S. National Research Foundation, located at the Kitt Peak National Observatory in Arizona. More than 900 researchers (including 300 doctoral students) and more than 70 scientific institutions and universities from around the world are participating in this effort.
Photo: Star trails above the Mayall telescope (Credit: Luke Tyas/Berkeley Lab and KPNO/NOIRLab/NSF/AURA)
Source: DESI Completes Planned 3D Map of the Universe – Berkeley Lab News Center