Key Takeaways:

  • Studying real galaxies only offers glimpses of the past. Researchers use computer simulations to explore galaxy evolution over billions of years.

  • This study challenges existing ideas about dark matter’s role, galaxy evolution, and star birth within them.

  • Millions of simulated universes were created, each obeying different galaxy formation theories. Comparing them helps identify the rules that govern our real universe.

  • Despite ample hydrogen gas (star fuel), galaxies stop forming stars. This study sheds light on why.

  • Contrary to expectations, simulations show early galaxies with higher star formation rates, challenging our understanding of dark matter’s influence.

How do galaxies such as our Milky Way come into existence? How do they develop and change with time? Supercomputer simulations have brought a team of scientists led by the University of Arizona one step closer to discovering the answers to the long-standing mystery of galaxy formation.

Researchers who wish to examine how galaxies change over billions of years must turn to computer simulations because real-world galaxy observations in space can only provide glimpses into the past. This method has historically been used by astronomers to develop and test individual new theories about how galaxies form. To overcome this obstacle, assistant professor Peter Behroozi of the UA Steward Observatory and his colleagues created millions of distinct universes on a supercomputer, each of which obeyed different physical theories for how galaxies should form.

Virtual 'universe machine' sheds light on galaxy evolution
A UA-led team of scientists generated millions of different universes on a supercomputer, each of which obeyed different physical theories for how galaxies should form. Credit: NASA, ESA, and J. Lotz and the HFF Team/STScI

The findings, published in the Monthly Notices of the Royal Astronomical Society, challenge fundamental ideas about the role dark matter plays in galaxy formation, how galaxies evolve over time and how they give birth to stars.

The study’s lead author, Behroozi, said, “We can create many different universes on the computer and compare them to the actual one, and that lets us infer which rules lead to the one we see.”

With computer simulations that each represent a sizable portion of the real universe and span the time from 400 million years after the Big Bang to the present, the study is the first to create self-consistent universes that are such exact replicas of the real one. The study’s 12 million galaxies span 400 million years.

Each “Ex-Machina” universe was put through a series of tests to evaluate how similar galaxies appeared in the generated universe compared to the true universe. The universes most similar to our own all had similar underlying physical rules, demonstrating a powerful new approach for studying galaxy formation.

The results from the “UniverseMachine,” as the authors call their approach, have helped resolve the long-standing paradox of why galaxies cease to form new stars even when they retain plenty of hydrogen gas, the raw material from which stars are forged.

According to widely accepted theories, star formation in galaxies is caused by a complex interaction between cold gas collapsing into dense pockets due to gravity and other processes that inhibit star formation.

For instance, it is believed that supermassive black holes are located in the centers of most galaxies. Massive energy released by matter colliding with these black holes serves as a cosmic blowtorch, keeping gas from cooling down sufficiently to collapse into star nurseries. Similarly, stars ending their lives in supernova explosions contribute to this process. In addition to being the primary source of gravitational force acting on visible matter within a galaxy, dark matter also plays a significant role by drawing in cold gas from the surrounding galaxy and heating it in the process.

“We would expect the dark matter to be denser and the gas to be getting hotter and hotter as we travel further back in time in the universe. We had assumed that many galaxies in the early universe should have stopped forming stars a long time ago because this is bad for star formation,” Behroozi said. “But we found the opposite: galaxies of a given size were more likely to form stars at a higher rate, contrary to the expectation.”

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