Unraveling the mysteries of dark energy at UF
As the universe expands faster and farther, so too does our understanding of the forces governing its growth. One of modern astronomy’s most profound mysteries is dark energy, an enigmatic force theorized to be responsible for the universe’s accelerating expansion. Recently, groundbreaking data from the Dark Energy Spectroscopic Instrument, or DESI, has brought new urgency to the study of this cosmic force, with findings that could challenge long-held theories of space and time.
Zachary Slepian, Ph.D., an associate professor in the Department of Astronomy and a member of the Astraeus Space Institute at the University of Florida, is at the forefront of this cosmic investigation. DESI is a US Department of Energy-led international collaboration, and Slepian is helping it peer farther and deeper into our Universe than ever before. DESI involves more than 900 researchers from over 70 institutions worldwide, and UF is a full member through Slepian’s group. Thus, UF is at the heart of the largest, both in terms of person-power and cosmic volume, map of the cosmos humankind has ever made.
“I have been interested in dark energy since I first heard about it as a student in college — not only has it shaped our past 7 billion years, but it will determine the ultimate fate of our universe,” Slepian said. “One of the discoverers, Saul Perlmutter, gave a lecture at Princeton when I was an undergraduate there, and he won the Nobel for it the year I graduated.”
This early exposure sparked Slepian’s lifelong dedication to understanding the forces shaping the cosmos.
DESI, equipped with 5,000 robotically-positioned optical fibers, is scanning galaxies at a speed and scale previously thought impossible. DESI can map as many as 150,000 stars and galaxies every night, requiring about 15-30 minutes for every 5,000 points of interest. By studying the light emitted from these galaxies, DESI aims to measure the effect of dark energy on the universe’s expansion. Early DESI data suggests that dark energy may not be constant in time, as scientists once believed it to be, but could instead be growing — an observation that challenges our understanding of dark energy's true nature.
Slepian’s background in astronomy and his expertise in advanced algorithms have been an important UF contribution to this groundbreaking project. He explains that algorithms to measure clustering of pairs, triplets, and quadruplets of galaxies are central to extracting information from the massive volumes of data DESI generates. His experience with computational techniques and theoretical modeling enables him to tackle some of the project’s most complex challenges.
Powerful Collaboration
UF's Research Computing group — particularly Ying Zhang — has been exceptionally effective in supporting this work, leveraging UF’s powerful setup of over 600 NVIDIA A100 cores on HiPerGator. Slepian’s work has also greatly benefited from UF's unique and ongoing collaboration with NVIDIA, spearheaded by Kaleb Smith, Ph.D. Slepian’s affiliations with the Astraeus Space Institute and with the AI Institute have also provided not only critical resources — including funding, seminars, and advanced computing infrastructure—but also valuable opportunities for collaboration with fellow scholars across disciplines.
Slepian’s specific contributions to the project focus on harnessing clusters of triplets and quadruplets of galaxies. These methods help disentangle the intricate relationships between galaxies, dark energy, and the larger cosmic web. Slepian has also led several working groups in DESI focused on these topics. Through his efforts, Slepian is helping refine the measurements of dark energy’s effects on the universe.
Collaboration is key to the success of the DESI project, and Slepian emphasizes the importance of constant communication with colleagues around the world. UF’s contributions, particularly in terms of computing power, funding, and student and postdoctoral scholar involvement, have made it a critical player in advancing the global effort to understand dark energy.
“Ever since I was a child, I’ve dreamt of understanding how the universe works,” said Slepian group graduate student William Ortolá Leonard. “Being part of the DESI project has brought that dream to life — teaching me not only how we uncover the cosmos’ secrets, but also the many different paths scientists take to get there.”
“Working with DESI is central to my Ph.D. research and has helped me earn two prestigious fellowships: the National Science Foundation Graduate Research Fellowship and the Theory-Experiment Cross Training Fellowship from the John Templeton Foundation. I’m incredibly grateful to be part of a collaboration that is advancing both science and scientists.”
“DESI allows me to connect theoretical curiosity with cutting-edge observational data, making me feel genuinely part of a profound effort to understand the fundamental structure and symmetries of the universe," said Slepian group postdoctoral scholar Alessandro Greco, Ph.D.
DESI’s early results have already begun to challenge existing theories of dark energy, and Slepian is excited about the potential implications. “Depending on dark energy’s nature, the universe will either coast in the future, crunch, or rip,” he said: the universe is unpredictable and could continue to grow, collapse back down, or expand so fast it tears.
Slepian’s careful analysis of early findings — and his leadership in interpreting their implications — is shaping our understanding of dark energy and the fabric of the universe itself. While much remains to be discovered, his work not only highlights UF’s contributions to one of the most ambitious scientific efforts of our time, but also inspires the next generation of researchers poised to explore the unknown.