These are exciting times to explore the largest unanswered questions in physics thanks to high-tech experiments and very precise data. That's particularly true of dark energy, the name given to the mysterious driver of the universe's accelerating expansion.
In a report published in the Physical Review Letters, a collaboration of researchers has released new data strengthening the case that dark energy's influence on the universe—long believed to be constant—is actually changing over cosmic time. The team and external collaborators show how the data can be understood as a signal of matter being converted into dark energy.
The new findings stem from an isolated mountain in southern Arizona called Iolkam Du'ag. Here, the Tohono O'odham Nation stewards Kitt Peak National Observatory, where the Dark Energy Spectroscopic Instrument, or DESI, peers deep into the universe's past using 5,000 robotic eyes—each focused on a different galaxy every 15 minutes.
Working every hour of nearly every night, DESI has already mapped millions of galaxies and other types of ancient, luminous objects, many from when the universe was less than half its current size.
In the current study, the researchers focused on an interpretation of black holes as tiny bubbles of dark energy. Because black holes are made when massive stars exhaust their nuclear fuel and collapse, this cosmologically coupled black hole, or CCBH, hypothesis requires the conversion of stellar matter into dark energy.
This conveniently links the rate of dark energy production, and matter consumption, to something that has been measured for decades by the Hubble Space Telescope and now the James Webb Space Telescope: the rate of star formation.
"This paper is fitting the data to a particular physical model for the first time and it works well," said DESI collaboration member Gregory Tarlé, professor emeritus of physics at the University of Michigan and corresponding author of the new report.
A major focus of the study is the mass of ghost-like particles called neutrinos, the second most abundant particle in the universe. Scientists know these particles have masses that are greater than zero, and so contribute to the matter budget in the universe, but their exact values have yet to be measured.
Interpreting the new DESI data with the CCBH model gives a measurement greater than zero, in agreement with what scientists already know about these ghost particles and an improvement over other interpretations that prefer zero, or even negative, masses.
"It's intriguing at the very least," Tarlé said. "I'd say compelling would be a more accurate word, but we really try to reserve that in our field."
DESI is an international experiment that brings together more than 900 researchers from over 70 institutions. The project is led by Lawrence Berkeley National Laboratory, and the instrument was constructed and is operated with funding from the U.S. Department of Energy Office of Science. DESI is mounted on the U.S. National Science Foundation's Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory—a program of NSF NOIRLab—in Arizona.
Exorcising the ghost particles
The CCBH hypothesis was introduced about five years ago by study co-authors Kevin Croker, assistant research scientist at Arizona State University, and University of Hawaii 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.
Yet, the idea that the dark energy within such black holes could be influencing the universe at large was unorthodox. It made enough sense mathematically, however, to attract a small nucleus of curious researchers who started examining how well the hypothesis accounted for observations and cosmological data.
"Historically, this is the way physics is done. You come up with as many ideas as you can and you shoot them down as fast as you can," said DESI researcher Steve Ahlen, emeritus professor of physics at Boston University and an early collaborator on the CCBH development.
"You don't shy away from ideas that are new and different, which is clearly what we need to come up with these days when there are so many mysteries."
The first data to bolster the CCBH hypothesis came from the unexpected growth of supermassive black holes at the centers of dormant elliptical galaxies, relative to the growth of those galaxies' stellar populations. But it was data from the first year of DESI, which showed the dark energy density tracking the rate of star formation, that convinced Croker and Farrah to join forces with the DESI Collaboration.
"Working with DESI on the three-year data, it's been a game-changer," Croker said of working as a DESI 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."
Other than packets of light called photons, neutrinos are the most abundant particles in the universe. In the time it takes you to read this sentence, hundreds of trillions of neutrinos will pass through your body. But neutrinos rarely interact with their surroundings, meaning they're constantly zipping through other matter, completely undetected, which is why they're sometimes referred to as ghost particles.
Scientists know neutrinos have mass, but precisely how much is challenging to measure on account of their ethereal nature. While enormous experiments currently running on Earth work to pin down these numbers, the night sky offers a powerful and complementary avenue for answers.
DESI's galactic maps contain information on how fast the universe has grown over the past 10 billion years, in turn providing a cosmic inventory of matter and dark energy. But matter comes in three types: cold dark matter, baryons and neutrinos. Early universe measurements from the afterglow of the Big Bang measure the amount of dark matter and baryons long ago. But according to DESI, it seems like there is less matter today when compared to the ancient past. This leaves little room for the neutrinos.
"The data would suggest that the neutrino mass is negative and that, of course, is likely unphysical," said Rogier Windhorst, Regents' Professor at ASU's School of Earth and Space Exploration and co-author of the new study.
Interpreted with the CCBH hypothesis, however, that unphysical issue disappears. Because stars are made of baryons, and black holes convert dead star matter into dark energy, the amount of baryons today has decreased relative to the Big Bang measurements. This allows neutrinos to contribute to the matter budget in the way expected from other measurements.
"You find that the neutrino mass probability distribution points to not only a positive number, but a number that's entirely in line with ground-based experiments," Windhorst said. "I find this very exciting."
CCBH: More bang for the buck
While this result gets top billing, the work also highlights other helpful features of the CCBH model.
"The CCBH hypothesis quantifiably links phenomena you would not initially expect to be related," Farrah said. "It is the mixing of scales, large and small, that runs so counter to our trained linear intuition."
Matter slows down the growth of the universe, whereas dark energy speeds it up. Because matter is converted to dark energy in the CCBH hypothesis, accelerated expansion happens earlier and so the expansion rate today, the Hubble rate, is a bit larger. This extra boost brings the cosmological measurement of the Hubble rate closer to other measurements, like those from distant exploding stars called supernovae.
The CCBH hypothesis also explains the observed amount of dark energy: It's not just some magical number set when the universe was born. Dark energy comes from dead stars, so there isn't any until you have stars, and stars do not form until the universe has grown sufficiently large and cool. Once stars are produced, the amount of dark energy made is directly related to how many stars are made.
"Working on this project has been both challenging and incredibly fun," said study co-author Gustavo Niz, a researcher at the University of Guanajuato, Mexico. "This is just another milestone in establishing CCBH as a viable theory. It will take more data, rigorous analysis and broader scrutiny to determine whether it can become a new paradigm for explaining our universe. Of course, it could also be ruled out as new data emerges."
Croker said the hypothesis performs well when looking at the universe in the rough, "but data from other experiments that study individual black holes isn't as compelling. That's why the hypothesis is interesting. Many different observers can actually test it, hammer it out in real time."
According to Ahlen, that's the way science goes. But for scientists who have been working on DESI from the beginning, it's exciting to see that data coming in is enabling researchers to test new and different hypotheses.
"This is so cool, to be at this point after working on an experiment for so long, to be coming up with exciting results," said Tarlé, who led the team that built DESI's robotic eye system. "It's just wonderful."
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.
The DESI collaboration is honored to be permitted to conduct scientific research on Iolkam Du'ag (Kitt Peak), a mountain with particular significance to the Tohono O'odham Nation.
