By Teodor Teofilov
A University of Arizona-led team of scientists is one step closer to answering the questions about how galaxies such as our Milky Way come into existence and how they grow and change over time thanks to the use of supercomputer simulations.
The scientists used a supercomputer to simulate the creation and evolution of 8 million universes, each containing 12 million galaxies and spanning the time from 400 million years after the Big Bang to the present day. In the process they discovered something new about our universe.
After the Big Bang, space was made up almost entirely of hydrogen and helium. Some astronomers think that gravity pulled dust and gas together to form individual stars, and those stars drew closer together into collections that ultimately became galaxies. Others think that the mass of what would become galaxies drew together before the stars within them were created.
However there are some galaxies that stop creating new stars even though they have plenty of gas and dust. Scientists have attributed this to a combination of factors.
For example, it is thought that supermassive black holes at the centers of galaxies might be radiating tremendous energies, acting as a sort of cosmic Bunsen burner that prevents gas from cooling down enough to star-forming temperatures, with stars going supernova adding to this process.
It could be because of dark matter as well, because it accounts for most of the gravitational force that acts on visible matter in a galaxy. Dark matter would pull in cold gas from the galaxy’s surroundings and heat it up in the process.
“As we go back earlier and earlier in the universe, we would expect the dark matter to be denser, and therefore the gas to be getting hotter and hotter,” said in a press release Peter Behroozi, an assistant professor at the University of Arizona, whose research focuses on how the evolution of dark matter and dark energy drive the formation of galaxies. “This is bad for star formation, so we had thought that many galaxies in the early universe should have stopped forming stars a long time ago.”
To match the observations of actual galaxies the team had to create virtual universes in which the opposite was the case and the galaxies kept churning out stars for much longer. This means that if the researchers created universes based on our current theories of galaxy formation, where galaxies stopped forming stars early on, then those galaxies will be much redder than the ones we see in the sky.
There are two reasons for which galaxies appear red. The first has to do with a galaxy’s age — if it formed earlier in the history of the universe, it will be moving away faster and will shift the light into the red spectrum (an effect called redshift). The other is that if a galaxy has stopped forming stars, it will have fewer blue stars, which typically die out sooner, and will be left with the older, redder stars.
“But we don’t see that,” Behroozi said. “If galaxies behaved as we thought and stopped forming stars earlier, our actual universe would be colored all wrong.”
According to Behroozi, creating mock universes of unprecedented complexity required an entirely new approach that was not limited by computing power and memory, and provided enough resolution to span the scales from the “small” – individual objects such as supernovae – to a sizeable chunk of the observable universe.
“Simulating a single galaxy requires 10 to the 48th computing operations,” Behroozi explained. “All computers on Earth combined could not do this in a hundred years. So to just simulate a single galaxy, let alone 12 million, we had to do this differently.”
Once they had tweaked their simulations to allow early galaxies to continue to efficiently form stars for longer did those galaxies evolve into forming virtual universes that look like our one.
Behroozi and his team, through the use of a supercomputer to simulate universes, concluded that galaxies formed stars more efficiently in the early times than thought.
“In other words, we are forced to conclude that galaxies formed stars more efficiently in the early times than we thought,” Behroozi said. “And what this tells us is that the energy created by supermassive black holes and exploding stars is less efficient at stifling star formation than our theories predicted.”
The study is the first to create self-consistent universes that are such exact replicas of the real one, but even this is just a fraction of the estimated between 100 billion and 200 billion galaxies that astronomers believe populate our universe.
With supercomputers getting more powerful all the time and scientists using creative solutions to overcome computing power limits, it won’t be long before they can simulate the creation of our entire universe and solve more of its mysteries in the process.
The team detail their findings in a study called the “UNIVERSEMACHINE: The Correlation between Galaxy Growth and Dark Matter Halo Assembly from z = 0-10,” which was published in the journal Monthly Notices of the Royal Astronomical Society and is co-authored by Risa Wechsler at Stanford University, Andrew Hearin at Argonne National Laboratory and Charlie Conroy at Harvard University. The study was funded by NASA, the National Science Foundation and the Munich Institute for Astro- and Particle Physics.
You might be interested in reading about the possibility of our world being a simulation, so check out Scary Science: We Might Be Living In a Video Game.