Scientists have long sought to understand the universe’s accelerating growth, but physical explanations for dark energy, the material responsible for this effect, have remained elusive.
Now, a new model linking star formation to cosmic acceleration gives the right growth over cosmic time and the right balance of energies today.
In new research published in Physical Review Letters, a team of researchers — including Arizona State University Regents Professor Rogier Windhorst and Assistant Research Scientist Kevin Croker, both of the School of Earth and Space Exploration — has reached these conclusions through the analysis of cutting-edge data that suggest dark energy’s influence isn’t constant but changes over time.
From Arizona’s desert to the edges of the cosmos
The Dark Energy Spectroscopic Instrument (DESI) instrument, housed at the Kitt Peak National Observatory on Iolkam Du’ag in southern Arizona, uses 5,000 robotic “eyes” to gather light from millions of galaxies. With help from DESI’s ultra-precise maps of the cosmos, researchers examined how dark energy may be linked to star formation and the life cycle of matter.
As stars collapse into black holes, the cosmologically coupled black hole (CCBH) model proposes that their mass is converted into dark energy. CCBH is one of several hypotheses aimed at explaining the DESI results that showed dark energy strength changes with time. The cosmological expansion history is sensitive to the neutrino content of the universe, but the standard model for dark energy didn’t leave enough room for their presence. This connection not only helps explain how dark energy might grow over cosmic time but also reconciles apparent gaps in the universe’s matter budget — especially related to neutrinos.
The CCBH hypothesis was introduced about five years ago by study co-authors Croker and University of Hawaiʻi Associate Professor Duncan Farrah. Mathematical descriptions of black holes as tiny droplets of dark energy, instead of “spaghettifying” monsters wrapped in one-way layers, have been explored by researchers for over half a century. But their utility in explaining the universe’s accelerating growth has only been recently considered.
Measuring ghost particles
Neutrinos are the second-most abundant particles in the universe after photons, yet they interact so weakly with matter that trillions pass through us unnoticed every second. While it’s known they have mass, determining exactly how much has been notoriously difficult.
The cosmological growth rate is sensitive to the amount of “ghost particles” within the universe, but the standard model for dark energy didn’t leave enough room for their presence.
“The data would suggest that the neutrino mass is negative and that, of course, is likely unphysical,” said Windhorst, a co-author of the new study.
If black holes have been converting stellar matter into dark energy for billions of years, a key consequence of the CCBH model, then the total amount of matter left in the universe today is lower than previously assumed. That adjustment makes room for the missing neutrino mass.
Theoretical innovation, observational precision
For Croker, who joined the DESI collaboration as an external researcher, the results are a significant milestone.
“Working with DESI on the three-year data, it’s been a game changer,” Croker said of working as a DESIDESI is an international project led by the Lawrence Berkeley National Laboratory, with support from the U.S. Department of Energy, the National Science Foundation and over 70 institutions globally. In addition to its primary support from the DOE Office of Science, DESI is also supported by the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. Additional support for DESI is provided by the NSF; the Science and Technology Facilities Council of the United Kingdom; the Gordon and Betty Moore Foundation; the Heising-Simons Foundation; the French Alternative Energies 2 and Atomic Energy Commission; the National Council of Humanities, Sciences, and Technologies of Mexico; the Ministry of Science and Innovation of Spain; and by the DESI member institutions. external collaborator on this project. “You’ve got some of the sharpest and most creative researchers in the field lending their hands and hearts. It’s an absolute privilege.”
The CCBH model also has helped ease other tensions in cosmology, such as the differing values of the Hubble constant — the current expansion rate of the universe. By allowing dark energy to evolve as stars form and die, the model offers a dynamic explanation for observations that other static models struggle to fit.
This press release was written by Matt Davenport University of Michigan, with contributions from Kim Baptista at Arizona State University.