Supercomputer provides new suite of Lyman-α forest simulations to illustrate the large-scale structure of the universe

Like a celestial beacon, distant quasars emit the brightest light in the universe. They emit more light than our entire galaxy, the Milky Way. Light comes from torn matter as it is swallowed by a supermassive black hole. Cosmological parameters are important numerical constraints that astronomers use to trace the evolution of the entire universe billions of years after the Big Bang.

Light from quasars reveals clues to the large-scale structure of the universe when it shines through enormous clouds of neutral hydrogen formed shortly after the Big Bang, on a time scale of 20 million years -light or more.

Using light data from quasars, the Frontera supercomputer at the Texas Advanced Computing Center (TACC) helped astronomers develop PRIYA, the largest suite of hydrodynamic simulations ever designed to simulate large-scale structures in the universe.

“We created a new simulation model to compare data that exists in the real universe,” said Simeon Bird, assistant professor of astronomy at the University of California, Riverside.

Bird and his 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 his colleagues published their work announcing PRIYA in October 2023 in the Journal of Cosmology and Astroparticle Physics (JCAP).

“We compare the eBOSS data to a variety of simulation models with different cosmological parameters and different initial conditions of the universe, such as different matter densities,” Bird explained. “You find which one works best and how far you can stray from it without breaking reasonable agreement between the data and the simulations. This knowledge tells us how much matter there is in the universe , or the amount of structure there is in the universe. universe.”

The PRIYA simulation suite is connected to large-scale cosmological simulations, also co-developed by Bird, called ASTRID, which are used to study the formation of galaxies, the coalescence of supermassive black holes and the reionization period at the beginning of the history of the Universe. universe. PRIYA goes even further. It uses information about the galaxy and black hole formation rules found in ASTRID and modifies the initial conditions.

“With these rules, we can take the model we developed that matches galaxies and black holes, then we modify the initial conditions and compare it to eBOSS’s Lyman-???? forest data of neutral hydrogen gas “Bird said. said.

The ‘Lyman- ???? “forest” comes from the “forest” of tight absorption lines on a graph of the quasar spectrum resulting from electronic transitions between the energy levels of neutral hydrogen atoms. The “forest” indicates the distribution, density and temperature of enormous intergalactic clouds of neutral hydrogen. Additionally, the lumpy appearance of the gas indicates the presence of dark matter, a hypothetical substance that cannot yet be seen, as evidenced by its observed tugging on galaxies.

PRIYA simulations were used to refine cosmological parameters in the work submitted to JCAP September 2023 and authored by Simeon Bird and colleagues at UC Riverside, MA Fernandez and Ming-Feng Ho.

Previous analysis of neutrino mass parameters did not agree with data from the cosmic microwave background (CMB), described as the afterglow of the Big Bang. Astronomers use CMB data from the Plank Space Observatory to place tight constraints on neutrino mass.

Neutrinos are the most abundant particle in the universe. It is therefore important to determine their mass value for cosmological models of the large-scale structure of the universe.

“We performed a new analysis with much larger and better designed simulations than before. The previous discrepancies with the Planck CMB data disappeared and were replaced by another tension, similar to that observed in other measurements of structures large scale at low redshift.” » said Bird. “The main result of the study is to confirm that the σ8 tension between CMB measurements and weak lensing exists until redshift 2, ten billion years ago.”

“A well-constrained parameter from the PRIYA study is σ8, which corresponds to the amount of neutral hydrogen gas structures on a scale of 8 megaparsecs, or 2.6 million light years. This indicates the number of clusters of dark matter floating out there,” Bird said. .

Another constrained parameter was ns, the scalar spectral index. This is related to how the awkwardness of dark matter varies depending on the size of the region analyzed. This indicates how quickly the universe expanded just moments after the Big Bang.

“The scalar spectral index defines the behavior of the universe from the beginning. The general idea of ​​PRIYA is to determine the initial conditions of the universe and the high-energy physics behavior of the universe,” said Bird.

Bird explained that supercomputers were needed for PRIYA simulations simply because they were so big.

“The memory requirements for PRIYA simulations are so large that you can’t put them on anything other than a supercomputer,” Bird said.

The PRIYA simulations on Frontera are among the largest cosmological simulations ever conducted, requiring more than 100,000 baseline hours to simulate a 3072 system³ (about 29 billion) particles in a “box” 120 megaparsecs on a side, or about 3.91 million light years in diameter. PRIYA simulations consumed over 600,000 node hours on Frontera.

“Frontera was very important for the research because the supercomputer had to be large enough that we could run one of these simulations quite easily, and we had to run a lot of them. Without something like Frontera, we wouldn’t be able to solve It’s not that it would take a long time – they just wouldn’t be able to run at all,” Bird said.

Additionally, TACC’s Ranch system provided long-term storage for PRIYA simulation data.

“The ranch is important because we can now reuse PRIYA for other projects. This could double or triple our scientific impact,” Bird said.

“Our appetite for more computing power is insatiable,” Bird concluded. “It’s crazy that we’re sitting here on this little planet observing most of the universe.”