Galactic Evolution Examined by ‘Universe Machine’

Researchers have turned to a massive supercomputer — dubbed the ‘UniverseMachine’ — to model the formation of stars and galaxies. In the process, they created a staggering 8 million ‘virtual universes’ with almost 10¹⁴ galaxies.

To say that the origins and evolution of galaxies and the stars they host have been an enigma that scientists have sought to explore for decades is the ultimate understatement.

In fact, desire to understand how the stars form and why they cluster the way they do, predates science, religion and possibly civilisation itself. As long as humans could think and reason — way before we knew what either a ‘star’ or a ‘galaxy’ was— we looked to the heavens with a desire to have knowledge of its nature.

We now know more than we ever have, but the heavens and their creation still hold mysteries for us. Observing real galaxies can only provide researchers with a ‘snapshot’ of how they appear at one moment. Time is simply too vast and we exist for far too brief a spell to observe galaxies as they evolve.

Now a team of researchers led by the University of Arizona have turned to supercomputer simulations to bring us closer to an answer for these most ancient of questions.

Astronomers have used such computer simulations for many years to develop and test models of galactic creation and evolution — but it only works for one galaxy at a time — thus failing to provide a more ‘universal’ picture.

To overcome this hurdle, Peter Behroozi, an assistant professor at the UA Steward Observatory, and his team generated millions of different universes on a supercomputer. Each universe was programmed to develop with a separate set of physical theories and parameters.

As such the team developed their own supercomputer — the UniverseMachine, as the researchers call it —to create a virtual ‘multiverse’ of over 8-million universes and at least 9.6 x 10¹³ galaxies.

The results could solve a longstanding quirk of galaxy-formation — why galaxies cease forming new stars when the raw material — hydrogen — is not yet exhausted.

The study seems to show that supermassive black holes, dark matter and supernovas are far less efficient at stemming star-formation than currently theorised.

The team’s findings — published in the journal Monthly Notices of the Royal Astronomical Society — challenges many of the current ideas science holds about galaxy formation. In particular, the results urge a rethink of how galaxies form, how they birth stars and the role of dark matter — the mysterious substance that makes up 80% of the universe’s matter content.

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

What makes the study notable is it is the first time each universe simulated has contained 12 million galaxies, spanning a time period of 400 million years after the ‘big bang’ to the present day. As such, the researchers have succeeded in the creation of self-consistent universes which closely resemble our own.

Putting the multiverse to the test — how the universe is supposed to work

To compare each universe to the actual universe, each was put through a series of test that evaluated the appearance of the simulated galaxies they host in comparison to those in the real universe.

Common theories of how galaxies form stars involve a complex interplay between cold gas collapsing under the effect of gravity into dense pockets giving rise to stars. As this occurs, other processes are acting to counteract star formation.

The Hubble Space Telescope took this image of Abell 370, a galaxy cluster 4 billion light-years from Earth. Several hundred galaxies are tied together by gravity. The arcs of blue light are distorted images of galaxies far behind the cluster, too faint for Hubble to see directly. (Image: NASA, ESA, and J. Lotz and the HFF Team/STScI)

For example, we believe that most galaxies harbour supermassive black holes in their centres. Matter forming accretion discs around these black holes and eventually being ‘fed’ into them, radiate tremendous energies. As such, these systems act almost as a ‘cosmic blowtorch’ heating gas and preventing it from cooling down enough to collapse into stellar nurseries.

Supernova explosions — the massive eruption of dying stars — also contributes to this process. In addition to this, dark matter provides most of the gravitational force acting on the visible matter in a galaxy — thus, pulling in cold gas from the galaxy’s surroundings and heating it up in the process.

Behroozi elaborates: “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.

“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.”

But what the team found was the opposite.

Behroozi says: “Galaxies of a given size were more likely to form stars at a higher rate, contrary to the expectation.”

Bending the rules with bizarro universes

In order to match observations of actual galaxies, the team had to create virtual universes in which the opposite was the case — universes in which galaxies continued to birth stars for much longer.

Had the researchers created universes based on current theories of galaxy formation — universes in which the galaxies stopped forming stars early on — those galaxies appeared much redder than the galaxies we see in the sky.

Ancient galaxies such as z8_GND_5296 appear red for two reasons; the lack of young blue stars and the stretching in the wavelength of emitted light due to cosmic redshift. (V. Tilvi, Texas A&M University/S.L. Finkelstein, University of Texas at Austin/C. Papovich, Texas A&M University/CANDELS Team and Hubble Space Telescope/NASA)

Galaxies appear red for two reasons.

If the galaxy formed earlier in the history of the universe cosmic expansion — the Hubble flow — means that it will be moving away from us more rapidly, causing significant elongation in the wavelength of the light it emits shifting it to the red end of the electromagnetic spectrum. A process referred to as redshift.

In addition to this, another reason an older galaxy may appear red is intrinsic to that galaxy and not an outside effect like redshift. If a galaxy has stopped forming stars, it will contain fewer blue stars, which typically die out sooner, and therefore be left with older — redder — stars.

Behroozi point out that isn’t what the team saw in their simulations, however. He says: “If galaxies behaved as we thought and stopped forming stars earlier, our actual universe would be coloured all wrong.

“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.”

Computing the multiverse is as difficult as it sounds

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.

Behroozi explains the computing challenge the team had to overcome: “Simulating a single galaxy requires 10 to the 48th computing operations. 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.”

In addition to utilizing computing resources at NASA Ames Research Center and the Leibniz-Rechenzentrum in Garching, Germany, the team used the Ocelote supercomputer at the UA High-Performance Computing cluster.

Two-thousand processors crunched the data simultaneously over three weeks. Over the course of the research project, Behroozi and his colleagues generated more than 8 million universes.

He explains: “We took the past 20 years of astronomical observations and compared them to the millions of mock universes we generated.

“We pieced together thousands of pieces of information to see which ones matched. Did the universe we created look right? If not, we’d go back and make modifications, and check again.”

Behroozi and his colleagues now plan to expand the Universe Machine to include the morphology of individual galaxies and how their shapes evolve over time.

As such they stand to deepen our understanding of how the galaxies, stars and eventually, life came to be.