In the Cosmic Calendar, which maps the chronology of the Universe across a single Earth year, modern humans don't appear until the very last minute of December 31.
Everything we understand about the evolution of the Universe, we've had to piece together. We simply haven't been around for pretty much any of the 13.7 billion-year history of the cosmos to observe it in action.
That detective work has been pretty epic. And one of the tools in our kit is simulations of the formation and evolution of the vast structures that span observable space.
Now, using supercomputers, an international team of scientists led by the University of Helsinki in Finland has produced the largest and most accurate simulation yet of the evolution of the local Universe. This can help us understand the dynamics at play as the Universe continues to evolve, including the mysterious dark matter and dark energy.
"The simulations simply reveal the consequences of the laws of physics acting on the dark matter and cosmic gas throughout the 13.7 billion years that our Universe has been around," says cosmologist Carlos Frenk of Durham University in the UK.
"The fact that we have been able to reproduce these familiar structures provides impressive support for the standard Cold Dark Matter model and tells us that we are on the right track to understand the evolution of the entire Universe."
The simulation is called SIBELIUS-DARK, and it covers a volume of space extending 600 million light-years from the Solar System. This includes several clusters of galaxies, including Virgo, Coma, and Perseus; the Milky Way and Andromeda galaxies; the Local Void; and the Great Attractor.
The dark matter distribution of the SIBERIUS-DARK volume. (McAlpine et al., MNRAS, 2022)
Within this volume of space, the simulation needed to account for around 130 billion particles. Computing these particles over the entire lifespan of the Universe – and at a higher resolution than ever before – took several weeks on the DiRAC COSmology MAchine (COSMA) supercomputer at Durham University, producing a petabyte of data. Then, the researchers had to compare the results with observational surveys of the real Universe.
This allows them to explore something called the Cold Dark Matter model of cosmology, the current standard for mapping the evolution of the Universe. It relies on a vast cosmic web of dark matter, the mysterious invisible mass responsible for adding gravity to the Universe beyond what can be accounted for by normal matter.
According to this model, dark matter accumulates in clumps called haloes. Hydrogen and other gases feed into these haloes, eventually forming stars and then galaxies. This model explains quite a few properties of the observable Universe. However, most simulations incorporating it simulate a random patch of the Universe.
Our patch of the Universe is a little outside the randomized norm. The Milky Way is floating in a void, or a relative underdensity of galaxies compared to the average distribution in the wider Universe. So the researchers decided to recreate our own corner of the Universe, to see if the Cold Dark Matter model could reproduce what we see in our immediate vicinity.
It could.
"This project is truly groundbreaking," says cosmologist Matthieu Schaller of Leiden University in the Netherlands. "These simulations demonstrate that the standard Cold Dark Matter Model can produce all the galaxies we see in our neighborhood. This is a very important test for the model to pass."
But SIBELIUS-DARK also showed that the Local Void might be unusual, in that it appears to have evolved from a local large-scale underdensity of dark matter from the outset. What produced this underdensity in the early cosmic web will need to be the subject of future explorations.
Meanwhile, the team will be conducting further analyses of the simulation to test the Cold Dark Matter model of cosmology.
"By simulating our Universe, as we see it, we are one step closer to understanding the nature of our cosmos," says physicist Stuart McAlpine of the University of Helsinki.
"This project provides an important bridge between decades of theory and astronomical observations."
The research has been published in the Monthly Notices of the Royal Astronomical Society.