Last Journey Simulation Follows Matter Through Universe

David Morton Rintoul
5 min readFeb 2, 2021

The Last Journey Simulation used Mira, one of the world’s most powerful supercomputers, to model how mass flows through our Universe. Find out how it breaks ground for future cosmological studies.

One of the most perplexing things about our current understanding of the Universe is that Scientists hardly understand any of it. Oh, they grasp ordinary matter well enough.

The matter we’re used to living and working with, like my writing desk and chair, isn’t the issue. We know that matter is made of atoms, what atoms are made, and the elementary particles beyond that.

Unfortunately, in recent years, we’ve come to realize that ordinary matter only makes up about 5% of our Universe. The rest consists of dark energy (68%) and dark matter (27%).

Cosmologists Don’t Know What Dark Energy and Dark Matter Are

Cosmologists don’t honestly know what dark energy and dark matter are. They only see their effects. The expansion of the Universe is accelerating, so some sort of unseen energy must be fueling that acceleration.

We call it dark energy because we can’t perceive it directly. The same thing goes for dark matter.

When astronomers observe distant galaxies, the observed gravitational effects seem to outweigh the masses of all the stars they contain. There must be some sort of invisible matter out there that causes the discrepancy.

Scientists Can’t Perceive or Grasp 95% of the Known Universe

So, despite all of their ingenious efforts over centuries, scientists can’t perceive or grasp what seems to be 95% of the known Universe. They’re working to address this sorry state of affairs in many ways.

One of those efforts is the subject of a paper published this week in the journal Astrophysical Letters Supplementary Series. It tells the story of a massive computer simulation called the Last Journey.

A team of researchers at the U.S. Department of Energy’s Argonne National Laboratory ran the Last Journey on the laboratory’s powerful IBM Blue Gene/Q system supercomputer called Mira. Mira is retired now, but it was the third most powerful computer on Earth in its heyday.

One of the Five Largest Computer Simulations Ever Run

One of the five most extensive computer simulations ever run, the Last Journey, tells how mass became distributed throughout the Universe. To do this, developers had to consider how dark matter forms into clumps that become the “halos” within which the known galaxies formed and developed.

Katrin Heitmann is the deputy division director for Argonne’s High Energy Physics (HEP) division. She explained the simulation like this, “”We worked with a tremendous volume of the universe, and we were interested in large-scale structures, like regions of thousands or millions of galaxies, but we also considered dynamics at smaller scales.”

Even with all of the computing power that Mira delivers, the simulation took over six months. It also had to be run in stages.

The Simulation Took Over Six Months

The team used two different software packages for the simulation. One is called Hardware/Hybrid Accelerated Cosmology Code (HACC), and the other is called CosmoTools, which is HACC’s analysis framework.

The investigators ran the simulation in stages. First, HACC would calculate the position of all of the data points representing matter. It would pause so the CosmoTools could record the required data, including the locations of billions of simulated dark matter halos.

Heitman described the process, saying, “”Running the full machine is challenging because reading the massive amount of data produced by the simulation is computationally expensive, so you have to do a lot of analysis on the fly. That’s daunting because if you make a mistake with analysis settings, you don’t have time to redo it.”

Completed the Simulation in Half the Expected Time

This staggered and demanding approach may sound awkward, but it was the most efficient use of Mira’s resources. As a result, the team completed the simulation in half the expected time.

The resulting models will be invaluable resources for future cosmological studies. For example, researchers will use them to learn more about how dark energy and dark matter contributed to the development of the Universe as well as the physics behind galaxy formation.

A whole range of planned real-world experiments hinge on the model the Last Journey generated. These include the Stage-4 ground-based cosmic microwave background experiment (CMB-S4), the Legacy Survey of Space and Time (carried out by the Rubin Observatory in Chile), the Dark Energy Spectroscopic Instrument and two NASA missions, the Roman Space Telescope and SPHEREx.

“Primary Mission — To Help HIgh-Impact Science Get Done”

Katherine Riley, director of science at Argonne Leadership Computing Facility (ALCF), explained the benefits. “This huge data set they are building will feed into many different efforts. In the end, that’s our primary mission — to help high-impact science get done. When you’re able to not only do something cool but to feed an entire community, that’s a huge contribution that will have an impact for many years.”

I mentioned that Mira is now retired, but a replacement is in the works. Argonne’s new supercomputer will be called Aurora. It will have the capacity to run even more massive simulations.

Aurora will be part of a new class of exascale computers. They’ll be 50 times faster than any supercomputer operating today.

Every Culture Has a Story About How the Universe Formed

Every culture has a story about how the Universe formed and our place in it. Science has relegated those stories to the realms of myth and allegory.

Our emerging global culture is hungry for a new, science-based origin story they can accept as literally true. That story will never be complete until we understand the nature of dark energy and dark matter and their roles in the development and formation of our Universe.

Riley concluded by saying, “More and more, we find tightly coupled relationships in the physical world, and to simulate these interactions, scientists have to develop creative workflows for processing and analyzing. With these iterations, you’re able to arrive at your answers — and your breakthroughs — even faster.”

We always have more to learn if we dare to know.
Mira’s Last Journey: exploring the dark universe
The Last Journey. I. An Extreme-scale Simulation on the Mira Supercomputer
The 5 Big Questions We Need Cosmology to Answer
Measuring Dark Energy Just Got Way More Precise
Mass Discrepancy: Is Dark Matter Hiding in Plain Sight?

I’m a freelance writer and commercial blogger delivering content services to selective business to business marketing clients. I have extensive experience in content creation, technical writing and training, working as a consultant and later in management roles with many of Canada’s most successful organizations. Specialties: Content Marketing, Social Media, Technical Writing, Training and Development View all posts by David Morton Rintoul

Originally published at on February 2, 2021.



David Morton Rintoul

I write for those who find meaning in discoveries about space, living things, and humanity. I also write content marketing stories for select B2B clients.