ASTRID example false color images of galaxies from all 4 redshift samples, across a range of stellar masses.

A Cosmic Look From Near Beginning to Now

TACC-supported ASTRID completes largest yet cosmological hydrodynamic simulations

By Jorge Salazar

Heidi Opdyke

In dark skies at night, distant star light twinkles and speaks to vast cosmic histories almost as old as time itself. New data from instruments such as NASA’s James Webb Space Telescope are helping astrophysicists probe deep cosmic mysteries such as the evolution of black holes and galaxies.

Using supercomputers to help make sense of the data, researchers from multiple institutions worked together to complete the largest cosmological hydrodynamic simulations, called ASTRID—a mind-boggling computational run that traces the evolution of the universe from its earliest time to the present.

Visualization shown of the ASTRID simulation at z = 0. The underlying background is the dark matter density, with the blue zoom showing the most massive dark matter halo in ASTRID
Researchers completed the largest yet cosmological hydrodynamic simulation, called ASTRID, that traces the evolution of the universe from its earliest time to the present, designated as z = 0 and referring to a zero redshift of star light. Visualization shown of the ASTRID simulation at z = 0. The underlying background is the dark matter density, with the blue zoom showing the most massive dark matter halo in ASTRID; inset showing a trichromatic gas map of temperature, neutral hydrogen gas fraction, and metallicity; stellar density inset; and mock observations for the Hubble Space Telescope Wide Field Camera. Credit: DOI: 10.3847/1538-4357/ae3c08

“The most important information about our recent study is that we’ve evolved the ASTRID simulation to z = 0, and that this data is available,” said Yihao Zhou, a PhD student in Carnegie Mellon University's (CMU) Department of Physics. Zhou and colleagues published their study in The Astrophysical Journal in February 2026.

The term "z" denotes the red shift of star light, in which light shifts toward the red end as its source moves away from an observer. High redshift corresponds to greater distances and earlier periods in the universe’s history, when the universe was expanding rapidly shortly after the Big Bang.

Illustration of ASTRID at z=0.5. The background shows the cosmic web of size 250 Mpc/h. The orange inset zooms into a massive cluster region, illustrating the gas density field colored by temperature. Red and yellow spikes mark massive black holes larger than 1e8 solar mass; yellow indicates BHs in high-accretion quasar mode, typically residing in star-forming galaxies, while red indicates BHs in low-accretion jet mode, usually found in quiescent galaxies.
Illustration of ASTRID at z=0.5. The background shows the cosmic web of size 250 Mpc/h. The orange inset zooms into a massive cluster region, illustrating the gas density field colored by temperature. Red and yellow spikes mark massive black holes larger than 1e8 solar mass; yellow indicates BHs in high-accretion quasar mode, typically residing in star-forming galaxies, while red indicates BHs in low-accretion jet mode, usually found in quiescent galaxies. The yellow and red insets show examples of the host galaxies. Credit: astrid.psc.edu

Zhou’s team started their cosmological simulations from z= 99, covering the ‘cosmic dawn’ of the universe. ASTRID traced 13.5 billion years of galactic structure and black hole evolution to z = 0, the present day.

“We have this very large simulation and piece of the universe, which captures the growth of black holes from the time at which they were formed all the way to today, as well as the galaxies that form, also from a very early time,” said study co-author and ASTRID principal investigator Tiziana Di Matteo, professor of physics and director of CMU’s McWilliams Center for Cosmology & Astrophysics.

The Frontera Supercomputer: A Virtual Universe At Unprecedented Scale

Di Matteo was awarded a 2024 Leadership Resource Allocation on the U.S. National Science Foundation (NSF)-funded Frontera supercomputer at the Texas Advanced Computing Center (TACC). The award gave the ASTRID team the computational power to model 166 billion particles across a volume of space 815 million light-years across, an enormous virtual laboratory for studying the universe.

Furthermore, the researchers solved complex physical interactions such as the gravity hydrodynamics for each time step, which necessitated building "gravity trees" to efficiently compute long- and short-range gravitational forces across billions of particles.

“This is a very demanding process, and only large compute clusters like Frontera can do this,” Zhou said.

The data input/output process for ASTRID was also computationally demanding with each simulation snapshot totaling 30 terabytes. “TACC’s Ranch archive system saved our ASTRID data,” Zhou added.

Yihao Zhou
Yihao Zhou
Tiziana Di Matteo
Tiziana Di Matteo

“The NSF has offered state-of-the-art resources in Frontera,” Di Matteo said. “That's why we were able to simulate the biggest volume and the highest resolution over a mass range of seven orders of magnitude and with the full physics.”

The ASTRID team plans to carry out future simulations at TACC’s U.S. NSF Leadership-Class Computing Facility using the upcoming Horizon supercomputer, which is expected to enter production later in 2026. Horizon will be deployed in two phases, with the first featuring graphical processing units (GPUs) and the second adding central processing units (CPUs).

“Our current code was designed to run on CPUs only,” Zhou said. “We are in the process of co-development to further refine our gravitational and black hole feedback models to make our simulation software more suitable for Horizon.”

“We are very excited about being ready to run on Horizon," Di Matteo added.

The NSF-funded Frontera supercomputer is a strategic national resource shared by thousands of scientists.
The NSF-funded Frontera supercomputer is a strategic national resource shared by thousands of scientists. Photo credit: Jorge Salazar, TACC

Black Hole Evolution

The timing for ASTRID couldn’t be better.

As new data pours in from science missions such as NASA’s James Webb Space Telescope (JWST), which is probing the earliest stages of galaxy and black hole formation, scientists are confronting a growing number of questions. JWST observations suggest that massive black holes were far more common in the early universe than previously thought, challenging existing theories about how these cosmic giants formed and grew so rapidly.

“We can look into the questions posed by JWST data using this very large volume of ASTRID simulations, with quite amazing resolution and physical realism, that allows us to make predictions and understand some of the current puzzles that are coming through,” Di Matteo said.

A slice of the Astrid simulation at redshift z = 5, and false color images of the Little Red Dot galaxies in Astrid. The three overlayed images are Little Red Dots present in this slice of Astrid and their position is indicated by the cross-hair.
A slice of the Astrid simulation at redshift z = 5, and false color images of the Little Red Dot galaxies in Astrid. The three overlayed images are Little Red Dots present in this slice of Astrid and their position is indicated by the cross-hair. Credit: DOI: 10.33232/001c.155493

The ASTRID simulations can evolve the universe across cosmic history, revealing the most frequent periods when black hole mergers were most common.

“We care about black hole mergers because even if we don't see the light or the accretion, we now have detectors that can see an event such as black hole mergers,” Di Matteo explained. “The Laser Interferometer Space Antenna (LISA), which will be launched in 2035, will find most of the massive black hole mergers in the early Universe.”

ASTRID includes dynamical friction models that improve the resolution of how a black hole gradually sinks into the galactic center when a galaxy merger happens.

“Our simulations have realistic black hole merger orbits, and a better estimation of the black hole merger timescales,” Di Matteo added.

Listening to the Universe’s Ripples

The ASTRID simulations also enable predictions of gravitational waves, first directly detected in 2015 by the Nobel-prize winning LIGO and Virgo collaborations. Because gravitational waves can pass through matter that blocks light, they offer astrophysicists a powering new way to probe the universe's most distant and hidden events, including the mergers of massive black holes. Di Matteo, Zhou, and colleagues use ASTRID to guide future planned gravitational detectors from LISA and the Pulsar Timing Array collaborations.

“With ASTRID, we make predictions for which environments would host gravitational wave sources that are most likely to be detected. In the future, people can use our predictions to discover gravitational sources,” Zhou explained.

What’s more, the ASTRID team has made predictions of galaxy evolution for large surveys currently running such as the Vera Rubin Observatory Legacy Survey of Space and Time (SST).

“For the first time, ASTRID allows us to capture this piece of the process where galaxies merge,” Di Matteo said. ASTRID simulates the central galactic black holes, which approach each other. “Eventually they merge, and they produce this gravitational wave signal that we can produce in the simulation, as if we are already, observing a gravitational wave signal."

ASTRID example false color images of galaxies from all 4 redshift samples, across a range of stellar masses.
ASTRID example false color images of galaxies from all 4 redshift samples, across a range of stellar masses. Credit: DOI: 10.33232/001c.129991

Keyhole Of The Universe

New data from JWST shows an abundance of black holes formed in the early universe, observed to co-evolve in almost every massive galaxy.

“Massive black holes are the key to the structure of the universe, and I would say the most interesting part of our universe,” Zhou stated.

Black holes are not merely cosmic consumers—they can transform the galaxies around them. As they grow by accreting gas, they unleash enormous amounts of energy through radiation and powerful jets, altering star formation and ultimately shaping the color, structure and evolution of nearly every massive galaxy in the universe.

“Our biggest question is about how, or what physical processes formed the ‘seeds’ of the early universe black holes,” Di Matteo said. “The simulations allow us to experiment with the seed models and look at the consequences that reveal the physical mechanisms that give way to predictions, something that the current generation and next generation telescopes or gravitational wave detectors can observe.”

Supercomputers transform the laws of physics into a living, evolving universe, one we can build, observe, and explore.

"We can’t replay the universe to study the deep past, and we’re limited in observing distant galaxies very directly because we don't yet have very good resolution," Zhou concludes. "With supercomputers, we can build the numerical universe that follows the gravity and evolution of stars and massive black holes over the entire cosmic time. Supercomputers help us to bridge our theory to real observations.”