“The universe has thrown us a curveball,” said Catherine Heymans, a professor of astrophysics at the University of Edinburgh, in reaction to the most recent discoveries regarding dark energy. Physicists for decades assumed this mysterious force behind the accelerating expansion of the universe was a steady, unvarying aspect of the cosmos. But new evidence indicates that dark energy could be changing a discovery that could radically redefine our knowledge of the universe and its final fate.
The findings are from the Dark Energy Spectroscopic Instrument (DESI), a cutting-edge telescope attached to the Nicholas U. Mayall 4-meter Telescope in Arizona. In the last three years, DESI has painstakingly examined light from close to 15 million quasars and galaxies, building the biggest 3D map of the universe ever made. By observing the “red shift” of light how its wavelengths elongate as it moves through space DESI has mapped the expansion of the universe for the past 11 billion years. The data, when combined with other observations such as the cosmic microwave background and the observation of supernovae, indicate that dark energy’s effect is diminishing with time.
“This is the biggest hint we have about the nature of dark energy in the approximately 25 years since we discovered it,” said Adam Riess, a Nobel Prize-winning astrophysicist at Johns Hopkins University. Riess was one of the first to observe dark energy in the late 1990s, when measurements of distant supernovae showed the universe’s expansion was accelerating. At the time, dark energy was thought to be a cosmological constant a fixed force tied to the vacuum energy of space itself. But DESI’s findings challenge this long-held assumption, suggesting instead that dark energy may be dynamic, changing as the universe evolves.
If this evolving dark energy hypothesis holds, it could have profound implications for the future of the cosmos. The conventional Lambda-CDM theory of cosmology, which has dominated physicists for generations, expects the universe to continue expanding infinitely, with the galaxies moving ever further apart until nothing is left but a chilly emptiness. However, the prospect of dissipating dark energy brings other fates into the realm of possibility. One possibility is that expansion may slow down to a uniform rate, but never accelerate continuously. A second, more sensational consequence is the “big crunch,” in which the universe turns around, shrinks, and finally falls inward upon itself.
“This result about dark energy is something that we did not expect to happen in our lifetime,” said Mustapha Ishak-Boushaki, a member of the DESI team and cosmologist at the University of Texas at Dallas. The results are intriguing, but they’re not yet conclusive. The statistical significance of the data now stands at 4.2 sigma a good sign but not the 5 sigma needed to validate a discovery in physics. DESI researchers are hopeful, though, as the instrument keeps gathering data in its fourth year of observation. Before the survey is completed, DESI will have weighed the positions of about 50 million galaxies and quasars, perhaps making the case for changing dark energy.
Compounding the mystery, DESI’s results confirm those from other large-scale experiments, including the Dark Energy Survey (DES). Combined, those datasets suggest increasing discord with Lambda-CDM.
The implications of this study go beyond cosmology. If dark energy is actually evolving, it may indicate the existence of new, unknown elements of the universe. Some scientists theorize it could be connected to a new form of particle or a subtle imperfection in Einstein’s theory of general relativity. Others propose that it may open the door to a new theory of fundamental physics, one that more accurately explains the mysterious forces governing our universe. “It sounds like it will be a paradigm shift, something that will change our understanding and the way we are putting all the pieces together,” Ishak-Boushaki said.
DESI’s precision in tracking dark energy is unmatched, thanks to its ability to measure baryon acoustic oscillations subtle patterns in the distribution of matter left over from the early universe. These patterns act as a “standard ruler,” allowing researchers to gauge the strength of dark energy at different points in cosmic history. “For a couple of decades, we’ve had this standard model of cosmology that is really impressive,” explained postdoctoral researcher Willem Elbers from Durham University. “As our data are getting more and more precise, we’re finding potential cracks in the model and realizing we may need something new to explain all the results together.”
In the future, astronomers are looking to a collection of future experiments to further investigate the nature of dark energy. NASA’s Nancy Grace Roman Space Telescope and the Vera Rubin Observatory in Chile will deliver complementary datasets, while DESI itself is still honing its analysis. “Whatever the nature of dark energy is, it will shape the future of our universe. It’s pretty remarkable that we can look up at the sky with our telescopes and try to answer one of the biggest questions that humanity has ever asked.”
As scientists continue to expand our knowledge, this much is certain: the universe is much more complicated and much more wonderful than we ever thought.
