A supercomputer assisted astronomers in developing PRIYA, the universe’s greatest collection of hydrodynamic models of large-scale structure. Distant quasars, like celestial beacons, emit the brightest light in the universe. They produce more light than our entire galaxy, the Milky Way. The light comes from matter that has been blasted apart by a gigantic black hole. Astronomers utilize cosmological parameters, which are fundamental numerical restrictions, to chart the evolution of the entire cosmos billions of years after the Big Bang.
Quasar light, which shines through massive clouds of neutral hydrogen gas generated shortly after the Big Bang and spans 20 million light years or more, gives information about the universe’s large-scale structure.
The National Science Foundation (NSF)-funded Frontera supercomputer at the Texas Advanced Computing Center (TACC) assisted astronomers in developing PRIYA, the largest suite of hydrodynamic simulations ever performed for simulating large-scale structure in the universe, using quasar light data.
“We’ve created a new simulation model to compare data that exists in the real universe,” said Simeon Bird, an assistant professor of astronomy at the University of California, Riverside.
We’ve created a new simulation model to compare data that exists in the real universe. We compare eBOSS data to a variety of simulation models with different cosmological parameters and different initial conditions to the universe, such as different matter densities.
Simeon Bird
Bird and colleagues developed PRIYA, which takes optical light data from the Extended Baryon Oscillation Spectroscopic Survey (eBOSS) of the Sloan Digital Sky Survey (SDSS). He and colleagues published their work announcing PRIYA October 2023 in the Journal of Cosmology and Astroparticle Physics (JCAP).
“We compare eBOSS data to a variety of simulation models with different cosmological parameters and different initial conditions to the universe, such as different matter densities,” Bird explained. “You find the one that works best and how far away from that one you can go without breaking the reasonable agreement between the data and simulations. This knowledge tells us how much matter there is in the universe, or how much structure there is in the universe.”
The PRIYA simulation suite is linked to ASTRID, a large-scale cosmological simulation suite co-developed by Bird that is used to research galaxy formation, the coalescence of supermassive black holes, and the re-ionization phase early in the universe’s history. PRIYA takes things a step further. It alters the beginning conditions based on the galaxy information and black hole generation criteria found in ASTRID.
“With these rules, we can we take the model that we developed that matches galaxies and black holes, and then we change the initial conditions and compare it to the Lyman-𝛼 forest data from eBOSS of the neutral hydrogen gas,” Bird went on to say.
The ‘Lyman-𝛼 forest’ gets its name from the ‘forest’ of closely packed absorption lines on a graph of the quasar spectrum resulting from electron transitions between energy levels in atoms of neutral hydrogen. The ‘forest’ indicates the distribution, density, and temperature of enormous intergalactic neutral hydrogen clouds. What’s more, the lumpiness of the gas indicates the presence of dark matter, a hypothetical substance that cannot be seen yet is evident by its observed tug on galaxies.
Simeon Bird and his UC Riverside colleagues, M.A. Fernandez and Ming-Feng Ho, used PRIYA simulations to revise cosmological parameters in a paper submitted to JCAP in September 2023.
Previous analyses of neutrino mass parameters did not accord with data from the Cosmic Microwave Background radiation (CMB), often known as the Big Bang’s afterglow. Astronomers use Plank space observatory CMB data to establish stringent constraints on neutrino mass. Because neutrinos are the most prevalent particles in the cosmos, determining their mass value is critical for cosmological models of the universe’s large-scale structure.
“We made a new analysis with simulations that were a lot larger and better designed than anything before. The earlier discrepancies with the Planck CMB data disappeared, and were replaced with another tension, similar to what is seen in other low redshift large-scale structure measurements,” Bird said. “The main result of the study is to confirm the σ8 tension between CMB measurements and weak lensing exists out to redshift 2, ten billion years ago.”
One well-constrained parameter from the PRIYA study is on σ8, which is the amount of neutral hydrogen gas structures on a scale of 8 megaparsecs, or 2.6 million light years. This indicates the number of clumps of dark matter that are floating around there,” Bird said.
Another parameter constrained was ns, the scalar spectral index. It is connected to how the clumsiness of dark matter varies with the size of the region analyzed. It indicates how fast the universe was expanding just moments after the Big Bang.
“The scalar spectral index sets up how the universe behaves right at the beginning. The whole idea of PRIYA is to work out the initial conditions of the universe, and how the high energy physics of the universe behaves,” Bird said.
Supercomputers were needed for the PRIYA simulations, Bird explained, simply because they were so big. “The memory requirements for PRIYA simulations are so big you cannot put them on anything other than a supercomputer,” Bird said.
TACC awarded Bird a Leadership Resource Allocation on the Frontera supercomputer. Additionally, analysis computations were performed using the resources of the UC Riverside High Performance Computer Cluster.
The PRIYA simulations on Frontera are some of the largest cosmological simulations yet made, needing over 100,000 core-hours to simulate a system of 3072^3 (about 29 billion) particles in a ‘box’ 120 megaparsecs on edge, or about 3.91 million light-years across. PRIYA simulations consumed over 600,000 node hours on Frontera.
“Frontera was critical to the research because the supercomputer had to be large enough to run one of these simulations fairly easily, and we needed to run a lot of them.” We couldn’t fix them without something like Frontera. “It wouldn’t take long; they just wouldn’t be able to run at all,” Bird explained.
TACC’s Ranch system also provided long-term storage for PRIYA simulation data.
“Ranch is significant because we can now reuse PRIYA for other projects.” “This has the potential to double or triple our scientific impact,” Bird added. “Our appetite for more compute power is insatiable,” Bird said. “It’s crazy that we’re sitting here on this little planet observing most of the universe.”
