Posted on Feb 24, 2022 in Science
A stunning new simulation reproduces iconic structures within our Local Universe, such as the Virgo, Coma and Perseus clusters of galaxies, the ‘Great Wall’, and the ‘Local Void’. At the center of the simulation is a pair of galaxies, the virtual counterparts of our own Milky Way and our massive neighbor, the Andromeda galaxy (see image above). An international team of researchers, led by the University of Helsinki, and including members from Durham University in the UK, used the supercomputer simulations, named SIBELIUS-DARK, to recreate the entire evolution of the cosmos, from the Big Bang to the present.
Laws of Physics Acting on Dark Matter
“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,” said Professor Carlos Frenk, Ogden Professor of Fundamental Physics at the Institute for Computational Cosmology, at Durham University.
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The simulation, is part of the “Simulations Beyond the Local Universe” (SIBELIUS) project, and is the largest and most comprehensive ‘constrained realization’ simulation to date. The team meticulously compared the virtual Universe to a series of observational surveys to find the correct locations and properties for the virtual analogies of the familiar structures.
Under-Density of Our Local Group of Galaxies
It was found that our local patch of the Universe may be somewhat unusual as the simulation predicted a lower number of galaxies on average due to a local large-scale ‘underdensity’ of matter. While the level of this underdensity is not considered to be a challenge to the standard model of cosmology, it could have consequences for how we interpret information from observed galaxy surveys. Our Local Group of galaxies simply happens to reside in a small underdensity of the cosmic web, and so galaxy surveys must probe sufficient distances to get a more representative sample of the cosmos.
“The simulation is designed to reproduce the density field of observed galaxies as a starting point,” Stuart McAlpine told The Daily Galaxy. “That means we aim to reproduce the right abundance of galaxies at the correct locations of the sky,” he explained. “Once this is established, we then test many other aspects of the simulations against the observations. For example, when looking at the abundance of galaxies in the K-band as a function of their distance from us, the SIBELIUS-DARK prediction and the observations match well.”
“Particles” Represent Mass of Dark Matter, Gas, or Stars
The simulation covers a volume up to a distance of 600 million light years from Earth and is represented by over 130 billion simulated ‘particles’, requiring many thousands of computers working in tandem over several weeks and producing large amounts of data. Despite its vast size, the hydrodynamic simulation can still not resolve individual objects like stars, so each ‘particle’ represents a certain mass of dark matter, gas, or stars that feels the force of gravity, pressure, and radiation from neighboring ‘particles’. The simulation was performed on the DiRAC COSmology MAchine (COSMA) operated by the Institute for Computational Cosmology at Durham University.
These ‘cosmological simulations developed by the team used relevant physics equations to describe how dark matter and cosmic gas evolve throughout the Universe’s lifetime.
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Dark Matter Haloes
First, the dark matter coalesces into small clumps, called haloes, and the surrounding gas is gravitationally attracted towards these clumps, eventually fragmenting into stars to form galaxies. Overtime, haloes grow large enough to host galaxies like our own Milky Way.
Cold Dark Matter Model
Over the past 20 years, cosmologists have developed a ‘standard model’ of cosmology – the ‘Cold Dark Matter’ model – which can explain a plethora of observed astronomical data, from the properties of the heat left over from the Big Bang, to the number and spatial distribution of galaxies we observe around us today. The term ‘cold’ refers to the preferred hypothesis that dark matter is composed of massive, slowly moving particles, in contrast to Hot Dark Matter in the form of neutrinos moving near the speed of light.
When simulating a virtual cold dark matter universe, most cosmologists follow a ‘typical’, or random, patch, one that is similar to our own observed Universe, yet only in a statistical sense.
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Simulations Represent a Specific Patch of Universe
The simulations carried out in this study are different. By using advanced generative algorithms (models how the data was generated in order to categorize a signal), the simulations are conditioned to reproduce our specific patch of Universe, thus containing the present day structures in the vicinity of our own galaxy that astronomers have observed over decades.
“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,” said Stuart McAlpine, Postdoctoral researcher at the University of Helsinki, said:
“By simulating our Universe, as we see it, we are one step closer to understanding the nature of our cosmos. This project provides an important bridge between decades of theory and astronomical observations.”
“The true power of the simulation however is to investigate the phenomenon that are more difficult to observe, McAlpine wrote in his email to The Daily Galaxy. “For example, How unusual is our patch of Universe? We can investigate the by looking at the overall density of matter around us, how crowded is the Local Volume, something that is very hard to measure observationally. Turns out, while a little bit under-dense, the region is perfectly compatible with the Lambda-Cold Dark Matter prediction.”
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The Last Word
“Our attempt to reconstruct the local large-scale structure from cold dark matter initial conditions could have failed,” Carlos Frenk wrote in an email to The Daily Galaxy. “That it didn’t shows that the standard model of cosmology is consistent with what we see in the Universe around us. This is another test that the cold dark matter model passes, another triumph for this elegant theory!”
The international research team will further analyze the simulation created with the hope of providing further stringent tests of the standard model of cosmology.
Image credit: Stuart McAlpine
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Stuart McAlpine, Carlos Frenk and Monthly Notices of the Royal Astronomical Society
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