Supercomputer helps astronomers pinpoint dark matter

A team of researchers from the University of California, Santa Cruz, utilized the powerful Summit supercomputer at the Oak Ridge Leadership Computing Facility to conduct an extensive study on dark matter. Dark matter, which comprises approximately 85% of the total matter in the universe according to the widely accepted Lambda-cold dark matter model, remains a mysterious cosmic web whose composition is unknown.

Bruno Villasenor, the lead author of the team’s recently published paper in Physical Review D and a former doctoral student at UCSC, explained that while the existence of dark matter is evident due to its gravitational effects, its particle nature remains unidentified. However, by gaining insights into the properties of observed dark matter, scientists can rule out certain candidate particles.

To investigate further, the team employed the Summit supercomputer to generate over 1,000 high-resolution hydrodynamical simulations. These simulations focused on the Lyman-Alpha Forest, a collection of absorption features formed when light from distant quasars encounters cosmic material during its journey to Earth. These diffuse gas patches exhibit diverse movements, masses, and sizes, collectively resembling a “forest” of absorption lines.

The researchers proceeded to simulate different universes with varying properties of dark matter that influence the structure of the cosmic web, resulting in fluctuations in the Lyman-Alpha Forest. By comparing these simulation outcomes with observed fluctuations in the Lyman-Alpha Forest data collected by telescopes at the W. M. Keck Observatory and the European Southern Observatory’s Very Large Telescope, the team gradually eliminated potential dark matter candidates until they identified the closest match.

This comprehensive study sheds light on the elusive nature of dark matter and offers valuable insights into its properties through the analysis of the Lyman-Alpha Forest.

The gas distribution tracing the cosmic web and a set of skewers that cross the simulation box along which the Lyman-Alpha Forest absorption features are computed from the simulation and used to compare to observations. Credit: Bruno Villasenor/UCSC

In a surprising turn, the findings of the UCSC research team contradicted the dominant Lambda-CDM model’s assertion that dark matter in the universe is cold dark matter, characterized by slow thermal velocities rather than temperature. Instead, the study suggested an alternative possibility: our universe may consist of warm dark matter, with faster thermal velocities.

Brant Robertson, the project leader and a professor at UCSC’s Astronomy and Astrophysics Department, acknowledged the success of the Lambda-CDM model in explaining various astronomical and cosmological observations. However, the team aimed to challenge the foundational assumptions of this model and explore potential flaws. The goal was to determine if the current understanding of dark matter is on solid ground.

In addition to its implications for our understanding of dark matter and the universe, the UCSC project was notable for its computational achievement. The team conducted an exceptionally comprehensive set of simulations using cutting-edge simulation software, which accounted for the physical processes shaping the structure of the cosmic web. These simulations harnessed the computational power of the world’s largest supercomputers.

The UCSC team employed Cholla, a GPU-optimized hydrodynamics code, as the basis for their simulations on the Summit supercomputer. Originally developed by Evan Schneider, an assistant professor at the University of Pittsburgh, Cholla aimed to facilitate a better understanding of how gases in the universe evolve over time by acting as a fluid dynamics solver. However, the UCSC team needed additional physics solvers to tackle their dark matter research. Over a span of three years during his doctoral dissertation at UCSC, Villasenor integrated these physics solvers into Cholla.

Villasenor explained that he expanded Cholla by incorporating the physics of gravity, dark matter, the expanding universe, and the chemical properties of gases, hydrogen, and helium. This integration of various physics was crucial for conducting cosmological hydrodynamical simulations and examining how radiation in the universe influences the gas distribution and its heating.

This figure illustrates how the temperature of the cosmic web changes as you change the intensity of the ionizing energy from quasars from left to right and as you change the timing of when these quasars form from bottom to top. This image was generated from another large grid simulation that we also ran on Summit. Credit: Bruno Villasenor/UCSC

Villasenor has achieved a remarkable feat by developing one of the most comprehensive simulation codes for modeling the universe. Traditionally, astrophysicists had to make choices regarding which parameters to include in their simulations. However, with the combination of Villasenor’s advancements and the computational power of the Summit supercomputer, researchers now have a significantly larger set of physical parameters at their disposal.

According to Robertson, this accomplishment fulfills a long-standing desire of researchers, made possible by the supercomputer systems at the Oak Ridge Leadership Computing Facility (OLCF). The ability to extensively vary the physics of the universe in multiple ways is a significant advancement, allowing for direct comparisons with observations.

The computational challenges posed by this endeavor far surpass anything previously attempted. Robertson emphasizes the magnitude of this achievement, noting that it is orders of magnitude beyond previous computational capabilities.

Schneider, who provided guidance to Villasenor during the expansion of Cholla, believes that his additions will be invaluable. As she prepares to conduct her own simulations on the new exascale-class Frontier supercomputer, Schneider anticipates leveraging the solvers integrated by Villasenor. She describes astrophysics software as an ever-evolving tool, with no ultimate version. By expanding the capabilities of Cholla, researchers can address a wider range of problems and tackle increasingly complex simulations.

The analogy Schneider uses to describe the development of Cholla is that of a multitool. The original code served as a pocketknife, and Villasenor’s additions can be likened to adding a screwdriver. Each addition expands the tool’s versatility and the range of problems it can solve. As further enhancements are made, researchers will be able to tackle even more intricate challenges in the field of astrophysics.

Source: Oak Ridge National Laboratory

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