New DESI results strengthen indications that dark energy changes over time
PRESS RELEASE
The fate of the Universe depends on the balance between matter and dark energy, the fundamental ingredient that drives its accelerating expansion. New results from the Dark Energy Spectroscopic Instrument (DESI) use the largest 3D map of the universe ever built to characterize the influence of dark energy on the expansion of the universe over the past 11 billion years. This study shows hints that dark energy, previously thought to be a cosmological constant, may evolve in unexpected ways.
DESI is an international experiment involving over 900 participants from more than 70 institutions around the world, and it is led by the US Department of Energy’s Lawrence Berkeley National Laboratory (United States). The collaboration published its findings in multiple papers submitted to the ArXiv digital repository and in a presentation at the American Physical Society Global Physics Summit in Anaheim, California.
The fate of the Universe depends on the balance between matter and dark energy, the fundamental ingredient that drives its accelerating expansion. New results from the Dark Energy Spectroscopic Instrument (DESI) use the largest 3D map of the universe ever built to characterize the influence of dark energy on the expansion of the universe over the past 11 billion years. This study shows hints that dark energy, previously thought to be a cosmological constant, may evolve in unexpected ways.
DESI is an international experiment involving over 900 participants from more than 70 institutions around the world, and it is led by the US Department of Energy’s Lawrence Berkeley National Laboratory (United States). The collaboration published its findings in multiple papers submitted to the ArXiv digital repository and in a presentation at the American Physical Society Global Physics Summit in Anaheim, California.
“The results we have obtained are very interesting”, says Andreu Font-Ribera, IFAE scientist and member of the DESI team that developed the study. “It seems that we are on the verge of a change of paradigm for our models of the Universe, and this is very exciting”, he notes.
Alone, he DESI data are consistent with the standard model of the universe, ΛCDM (where CDM stands for cold dark matter and Λ represents the simplest explanation of dark energy, where it behaves like a cosmological constant). However, when combined with other measurements, there are growing indications that the impact of dark energy may be weakening over time. This suggests that models other than the commonly accepted one may provide a better explanation of cosmic evolution. Such other measurements include the light left over from the dawn of the universe (the cosmic microwave background or CMB), exploding stars (supernovae) and distortions in the propagation of light caused by gravity (weak gravitational lensing).
“In my opinion, it is still too early to claim categorically that we have discovered an evolving dark energy”, says Eusebio Sánchez, a science researcher at CIEMAT, who has participated in the analysis of the data. “However, the fact that different independent projects are observing similar results makes the situation particularly interesting”.
So far, the preference for a changing dark energy has not reached “5 sigma”, the statistical convention used in physics that sets the threshold needed to consider whether a measurement is considered a discovery. Analysis of different combinations of the DESI data with the CMB and several sets of supernovae finds a range between 2.8 and 4.2 sigma (a 3 sigma event has a 0.3% chance of being a statistical fluctuation, but many 3 sigma events in physics have disappeared as more data have been collected). The analysis has used a double-blind technique that hides the results until the end, mitigating any unconscious bias in the data.
The new analysis uses the data from the first three years of observation: nearly 15 million of the best-measured galaxies and quasars. This is a major breakthrough. The precision of the experiment has improved compared to the first DESI analysis, which also observed tracks of changing dark energy, because the data set has increased by a factor of more than two.
“If confirmed, this would be one of the most important results in cosmology of the last decades because it opens the door to new ideas beyond the standard ΛCDM model”, says Juan García-Bellido, researcher at the IFT-UAM/CSIC, who has collaborated in this measurement. “If the results get higher significance with future measurements, we could explore ideas such as new theories of gravity or quintessence, which predict a variable acceleration of the expansion of the Universe”.
DESI tracks the influence of dark energy by studying the distribution of matter in the Universe. Events in the early universe left very subtle patterns in the distribution of matter, known as baryon acoustic oscillations (BAOs). These oscillations are used to calibrate distances or standard rulers, and their apparent size at different times depends directly on the expansion of the universe. From the measurement of the standard ruler at different distances we obtain the strength of dark energy throughout cosmic history. The distance measurements provided by DESI in this new study are the most accurate to date.
“We are at a very exciting moment because for a long time we have believed that the Universe behaved in a certain way, but now, with increasingly precise data, we realize that there are aspects that we still don’t fully understand”, says Laura Casas, a PhD student at IFAE in Barcelona, who has led the validation of the analysis in the Lyman-alpha forests. “Although much remains to be studied, the hints about evolving dark energy are a fascinating finding”.
The collaboration will soon begin to work on additional analyses to extract even more information from the current dataset, and DESI will continue to accumulate new data. Further experiments starting in the coming years will provide complementary datasets for future analyses.
“The observational results we are obtaining about the evolution of the Universe open up a wide range of possible theories that can explain these observations”, says Francisco Javier Castander, researcher at ICE-CSIC and IEEC, who contributed to the experiment. “Regardless of the nature of dark energy, its properties will determine the future of the Universe. It is very rewarding to see how the instrument we have built allows us to observe the sky and study the Universe in detail, to answer one of the most important questions that humanity has ever asked”.
Some videos presenting the new analysis are available on DESI’s YouTube channel. In addition to making the latest results public at the American Physical Society (APS) meeting, the DESI collaboration has also announced that its first public data release (DR1) is now available for exploration. This dataset contains millions of celestial objects and will enable a wide range of astrophysical investigations that add to DESI’s cosmological goals.
The DESI collaboration
DESI receives funding from the following institutions: the Department of Energy’ Office of Science and the National Energy Research Scientific Computing Center (NERSC), in the United States; the National Science Foundation (NSF), in the United States; the Division of Astronomical Sciences (AST), through a contract with the NSF’s National Optical Astronomy Observatory; the Science and Technology Facilities Council (STFC), in the United Kingdom; the Gordon and Betty Moore and Heising-Simons Foundations in the United States; the Commissariat à l’Energie Atomique et des Energies Alternatives (CEA) in France; the National Council for Science and Technology (CONACYT) in Mexico; the Spanish Ministry of Science, Innovation and Universities; and DESI member institutions.
The DESI collaboration is grateful to be allowed to conduct astronomical research on Du’ag at Kitt Peak, Arizona, a mountain of special significance to the Tohono O’odham Nation Indian Reservation.
At the national level, the participants are the Research Centre for Energy, Environment and Technology (CIEMAT), the Institute of Space Sciences (ICE, CSIC), the UB’s Institute of Cosmos (ICCUB), the Institute of High-Energy Physics (IFAE), the Institute of Theoretical Physics (IFT-UAM/CSIC), the Institute of Astrophysics of Andalusia (IAA) and the Institute of Astrophysics of the Canary Islands (IAC).
A full list of participating institutions and further related information can be found on the DESI website.
Multimedia gallery
DESI observes the sky from the Mayall Telescope, shown here beneath the Milky Way. Credit: KPNO/NOIRLab/NSF/AURA/R.T. Sparks
The Dark Energy Spectroscopic Instrument maps the universe by collecting spectra from millions of galaxies and quasars. Credit: Marilyn Sargent/Berkeley Lab
Star trails over the Mayall Telescope that houses DESI. Credit: Luke Tyas/Berkeley Lab and KPNO/NOIRLab/NSF/AURA
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