Scientists develop virtual universe machine to study galaxy formation

James Marshall
August 13, 2019

How do galaxies like the Milky Way emerge? How do they expand and evolve over time? For several decades, the science underlying galaxy formation has been a mystery. In lieu of a time machine, scientists from the University of Arizona are using a supercomputer to simulate millions of universes and galaxies.

Peter Behroozi, an assistant professor at the UA Steward Observatory, and his colleagues addressed this challenge by creating millions of distinct universes on a supercomputer.

How are galaxies formed?

The findings of the team from the simulations challenge the fundamental ideas about the role of dark matter in galaxy formation and how the galaxy evolves over time and how birth is given to stars.

Behroozi is the lead author of the study.

Scientists wanting to study the evolution of galaxies over billions of years have to use computer simulations. Ocelote has been programmed to generate millions of small universes so as to compare their formation to the real universe. 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.

The results have helped scientists understand why galaxies cease to form new stars even when they have plenty of hydrogen gas to do so.

For instance, it is considered that a majority of the galaxies include supermassive black holes at their centers. Matter falling into these black holes radiates tremendous energies, acting as cosmic blowtorches that prevent gas from cooling down enough to collapse into stellar nurseries.

In the same way, stars that end in supernova explosions support this process.

"But we found the opposite: galaxies of a given size were more likely to form stars at a higher rate, contrary to the expectation", he added.

This study is the first to create self-contained universes that are such exact replicas of a real galaxy. One is the age of the galaxy; those further away had shifted into the red spectrum, known as 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.

To understand the universe better, we need a "universe sample" to test it and see what it's going to happen next. Galaxies might produce stars for significantly longer than earlier thought. In other words, we are forced to conclude that galaxies formed stars more efficiently in the early times than we thought.

"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".

It is possible to understand a lot about the universe through deduction and observation, but this causes discrepancies in data processing and gives irregular results.

Rather than attempt to portray every nuance of the entire universe (even a single totally modeled galaxy would require an excessive amount of computing power), the group devised a system that had just sufficient resolution to scale from supernovae to a "sizeable chunk" of observed space. Even so, this could still symbolize a boon for science - certainly for anyone anxious that it may take many years or centuries before computers might come near offering helpful recreations. However, they provide enough information to show us how galaxies progressively change under different applied laws of physics. Two-thousand processors crunched the data simultaneously over three weeks.

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