Stampede2 supercomputer simulates star seeding, heating effects of native black holes
Just milliseconds after the universe’s Big Bang, chaos reigned. Atomic nuclei fused and broke apart in hot, frenzied motion. Incredibly strong pressure waves built up and squeezed matter so close together that they formed black holes, which astrophysicists call primordial black holes.
Did primordial black holes aid or hinder the formation of the universe’s first stars, which were eventually born about 100 million years later?
Supercomputer simulations helped investigate this cosmic question, thanks to simulations on the Stampede2 supercomputer at the Texas Advanced Computing Center (TACC), part of the University of Texas at Austin.
“We found that the standard view of the formation of the first star is not really changed by primordial black holes,” said Boyuan Liu, a postdoctoral researcher at the University of Cambridge. Liu is the lead author of computational astrophysics research, published in August 2022 in the Monthly Notices from the Royal Astronomical Society.
In the early universe, the Standard Model of astrophysics states that black holes seeded the formation of halo-like structures due to their gravitational pull, analogous to how clouds are formed by being seeded by dust particles. This is a plus for star formation, where these structures served as scaffolding that helped fuse into the first stars and galaxies.
However, a black hole also causes heating from gas or debris falling into it. This forms a hot accretion disk around the black hole, emitting energetic photons that ionize and heat the surrounding gas.
And that’s a minus for star formation, as gas needs to cool to condense to a density high enough to trigger a nuclear reaction, setting the star on fire.
“We found that these two effects — heating and seeding of black holes — almost cancel each other out, and the final impact is small for star formation,” Liu said.
Depending on which effect wins out over the other, star formation can be accelerated, slowed or prevented by primordial black holes. “This is why primordial black holes can be important,” he added.
Liu stressed that it is only with state-of-the-art cosmological simulations that one can understand the interplay between the two effects.
As for the importance of primordial black holes, the research also implied that they interact with the first stars and produce gravitational waves. “They could also potentially trigger the formation of supermassive black holes. These aspects will be explored in follow-up studies,” Liu added.
For the study, Liu and colleagues used cosmological hydrodynamic zoom-in simulations as their tool for state-of-the-art numerical schemes of gravitational hydrodynamics, chemistry and cooling in structure formation and early star formation.
“An important effect of primordial black holes is that they are seeds of structures,” Liu said. His team built the model that implemented this process, as well as the heating of primordial black holes.
They then added a subgrid model for black hole accretion and feedback. The model calculates at each time step how a black hole accumulates gas and also how it heats its environment.
“This is based on the environment around the black hole known in the on-the-fly simulations,” Liu said.
XSEDE awarded the science team assignments on TACC’s Stampede2 system.
“Supercomputing resources in computational astrophysics are absolutely essential,” said study co-author Volker Bromm, professor and chair of the Department of Astronomy, UT Austin.
Bromm explained that in theoretical astrophysics, the prevailing paradigm for understanding the formation and evolution of cosmic structure is the use of ab initio simulations, which follow the “gamebook” of the universe itself – the prevailing equations of physics.
The simulations use data from the initial conditions of the universe to high precision based on observations of the cosmic microwave background. Next, simulation boxes are set up that follow the cosmic evolution time step by time step.
But the challenges in computational simulation of structure formation lie in how large scales of the universe — millions to billions of light-years and billions of years — interact with the atomic scales where stellar chemistry takes place.
“The microcosm and the macrocosm interact,” Bromm said.
“The resources from TACC and XSEDE have been absolutely essential for us to push the boundaries of computational astrophysics. Everyone who works at UT Austin – faculty, postdocs, students – benefits from having such a top-notch supercomputing center. Bromm added.
“If we look at one typical structure that can form the first stars, we need about a million elements to completely dissolve this halo or structure,” Liu said. “That’s why we have to use supercomputers at TACC.”
Liu said that with Stampede2, a simulation on 100 cores can be completed in just a few hours instead of years on a laptop, not to mention the bottlenecks with memory and reading or writing data.
“The overall game plan with our work is that we want to understand how the universe transformed from the simple initial conditions of the Big Bang,” explains Bromm.
The structures that emerged from the Big Bang were driven by the dynamic importance of dark matter.
The nature of dark matter remains one of the greatest mysteries in science.
The clues of this hypothetical but undetectable substance are unmistakable, seen in the impossible rotational speeds of galaxies. The masses of all the stars and planets in galaxies like our Milky Way don’t have enough gravity to keep them from flying apart. The “x-factor” is called dark matter, but labs haven’t directly detected it yet.
However, gravitational waves have been detected, first by LIGO in 2015.
“It is possible that primordial black holes could explain these gravitational wave events that we have detected over the past seven years,” Liu said. “This just motivates us.”
Bromm said: “Supercomputers enable unprecedented new insights into how the universe works. The universe offers us extreme environments that are extremely challenging to understand. This also gives the motivation to build ever more powerful computational architectures and devise better algorithmic structures. and power for the benefit of all.”
The study, “Effects of stellar-mass primordial black holes on first star formation,” was published in August 2022 in the Monthly Notices from the Royal Astronomical Society. The authors of the study are Boyuan Liu, Saiyang Zhang and Volker Bromm of the University of Texas at Austin. Liu is now at Cambridge University.
Did black holes form immediately after the Big Bang?
Boyuan Liu et al, Effects of stellar primordial black holes on the first star formation, Monthly Notices from the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac1472
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