A research team from the University of California, Santa Cruz, have used the Oak Ridge Leadership Computing Facility’s Summit supercomputer to run one of the most complete cosmological models yet to probe the properties of dark matter — the hypothetical cosmic web of the universe that largely remains a mystery some 90 years after its existence was definitively theorized.
According to the Lambda-cold dark matter model of Big Bang cosmology — which is the working model of the universe that many astrophysicists agree provides the most reasonable explanations for why it is the way it is — 85% of the total matter in the universe is dark matter. But what exactly is dark matter?
“We know that there's a lot of dark matter in the universe, but we have no idea what makes up that dark matter, what kind of particle it is. We just know it's there because of its gravitational influence,” said Bruno Villasenor, a former doctoral student at UCSC and lead author of the team’s paper, which was recently published in Physics Review D. “But if we can constrain the properties of the dark matter that we see, then we can discard some possible candidates.”
By producing over 1,000 high-resolution hydrodynamical simulations on the Summit supercomputer located at the Department of Energy’s Oak Ridge National Laboratory, the team modeled the Lyman-Alpha Forest, which is a series of absorption features formed as the light from distant bright objects called quasars encounters material along its journey to Earth. These patches of diffuse cosmic gas are all moving at different speeds and have different masses and extents, forming a “forest” of absorption lines.
The researchers then simulated universes with different dark matter properties that affect the structure of the cosmic web, changing the fluctuations of the Lyman-Alpha Forest. The team compared the results from the simulations with fluctuations in the actual Lyman-Alpha Forest observed by telescopes at the W. M. Keck Observatory and the European Southern Observatory’s Very Large Telescope and then eliminated dark matter contenders until they found their closest match.
Consequently, the team’s results were contrary to the Lambda-CDM model’s primary contention that the universe’s dark matter is cold dark matter — hence the model’s abbreviation, which references dark matter’s slow thermal velocities rather than its temperature. Instead, the study’s top prospect indicated the opposite supposition: We may indeed be living in a universe of warm dark matter, with faster thermal velocities.
“Lambda-CDM provides a successful view on a huge range of observations within astronomy and cosmology. But there are slight cracks in that foundation. And what we're really trying to do is push at those cracks and see whether there are issues with that fundamental foundation. Are we on solid ground?” said Brant Robertson, project leader and a professor at UCSC’s Astronomy and Astrophysics Department.
Beyond possibly unsettling a few long-held assumptions about dark matter, and the universe itself, the UCSC project also stands out for its computational feat. The team accomplished an unprecedentedly comprehensive set of simulations produced with state-of-the-art simulation software that accounts for the physics that shape the structure of the cosmic web and leverages the computational power of the largest supercomputers in the world.