First stars and black holes
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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 tightly together that black holes formed, which astrophysicists call primordial black holes. Did primordial black holes help or hinder formation of the universe’s first stars, eventually born about 100 million years later?
Supercomputer simulations helped investigate this cosmic question, thanks to simulations on the Stampede2 supercomputer of the Texas Advanced Computing Center (TACC), part of The University of Texas at Austin.
“We found that the standard picture of first-star formation is not really changed by primordial black holes,” said Boyuan Liu, a post-doctoral researcher at the University of Cambridge. Liu is the lead author of computational astrophysics research published August 2022 in the Monthly Notices of the Royal Astronomical Society.
In the early universe, the standard model of astrophysics holds that black holes seeded the formation of halo-like structures by virtue of their gravitational pull, analogous to how clouds form by being seeded by dust particles. This is a plus for star formation, where these structures served as scaffolding that helped matter coalesce into the first stars and galaxies.
However, a black hole also causes heating by gas or debris falling into it. This forms a hot accretion disk around the black hole, which emits energetic photons that ionize and heat the surrounding gas.
And that’s a minus for star formation, as gas needs to cool down to be able to condense to high enough density that a nuclear reaction is triggered, setting the star ablaze.
“We found that these two effects — black hole heating and seeding — almost cancel each other out and the final impact is small for star formation,” Liu said.
Depending on which effect wins over the other, star formation can be accelerated, delayed or prevented by primordial black holes. “This is why primordial black holes can be important,” he added.
Liu emphasized that it is only with state-of-the-art cosmological simulations that one can understand the interplay between the two effects.
Regarding the importance of primordial black holes, the research also implied that they interact with the first stars and produce gravitational waves. “They may also be able to trigger the formation of supermassive black holes. These aspects will be investigated 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 the gravity hydrodynamics, chemistry and cooling in structure formation and early star formation.
“A key 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 incorporating the heating from primordial black holes.
They then added a sub-grid model for black hole accretion and feedback. The model calculates at each timestep how a black hole accretes gas and also how it heats its surroundings.
“This is based on the environment around the black hole known in the simulations on the fly,” Liu said.
XSEDE awarded the science team allocations on the Stampede2 system of TACC.
“Supercomputing resources in computational astrophysics are absolutely vital,” said study co-author Volker Bromm, professor and chair, Department of Astronomy, UT Austin.
Bromm explained that in theoretical astrophysics, the ruling paradigm for understanding the formation and evolution of cosmic structure is to use ab initio simulations, which follow the ‘playbook’ of the universe itself — the governing equations of physics.
The simulations use data from the universe’s initial conditions to high precision based on observations of the cosmic microwave background. Simulation boxes are then set up that follow the cosmic evolution timestep by timestep.
But the challenges in computat…
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