Only milliseconds After the Big Bang of the universe, chaos reigned. Atomic nuclei fuse and break apart in frantic, hot motion. The intense pressure waves build up and squeeze matter so tightly together that black holes are formed, which astrophysicists call primordial black holes.
Supercomputer simulations probed primordial black holes and their influence on the formation of the first stars in the universe. Black holes can help stars form by seeding structures that form around them thanks to their massive gravity. They also hinder star formation by heating the gas that falls on them. Stampede2 simulations show that these effects essentially cancel each other out. Shown here is an artist’s concept illustrating a hierarchical scheme for merging black holes. Credit: LIGO / Caltech / MIT / R. Injury (IPAC).
Do primordial black hole helped or hindered the formation of the universe’s first stars, which were finally born about 100 million years later?
Supercomputer simulations have helped investigate this cosmic question, thanks to simulations on the Stampede2 supercomputer by the Texas Advanced Computing Center (TACC), part of the University of Texas at Austin.
“We found that the standard picture of early star formation is not actually altered by primordial black holes,” says Boyuan Liu, a postdoctoral researcher at the University of Cambridge. Liu is the lead author of the computational astrophysics study 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 seed the formation of halo-like structures by their gravity, similar to how clouds formed by dust particles. This is a plus for star formation, where these structures act as scaffolding for matter to coalesce into the first stars and galaxies.
However, black holes also cause warming due to 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, because the gas needs to cool in order for it to condense to a density high enough to trigger a nuclear reaction, causing the star to ignite.
“We found that these two effects – black hole heating and granulation – almost cancel each other out, and that the final effect is small on star formation,” Liu said.
Depending on which effect wins out over the other, star formation can be accelerated, delayed, or prevented by primordial black holes. He added: “This is why primordial black holes can be important.
The matter fields at the time of cloud collapse (i.e. the beginning of star formation) as expected distributions of dark matter (top) and gas (bottom) in the four simulations were targeted at same region but with different primordial black holes, as measured by the parameter f_PBH. Primordial black holes are drawn with black dots and circles showing the size of the structure containing the collapsing cloud. The data section has a physical range of 2000 light years and a thickness of 1000 light years. The age of the universe at the time of the first collapse decreases with f_PBH giving f_PBH < 0.001 when the "seeding" effect prevails. It then increases from f_PBH = 0.001 to f_PBH = 0.01 and higher as the "heating" effect becomes more important. Credit: Liu et al.
Liu emphasizes that only with modern cosmological simulations can the interplay between the two effects be understood.
Regarding the importance of primordial black holes, the study also implies that they interacted with the first stars and generated gravitational waves. “They can also trigger formation supermassive black hole. These aspects will be investigated in further studies,” added Liu.
For the study, Liu and colleagues used cosmological hydrodynamic zoom simulations as their tool for modern numerical schemes of gravity. hydrodynamicschemistry and cooling during early star formation and structure.
“A major effect of primordial black holes is that they are seeds of structures,” says Liu. His team built a model of how this process works, as well as incorporating heating from primordial black holes.
Then they add an extra mesh model for the black hole accretion and feedback. The model calculates at each time step how a black hole accumulates gas and also how it heats up its surroundings.
“This is based on the environment around the black hole known in flight simulations,” says Liu.
XSEDE awarded the science team for Stampede2 TACC system.
“Supercomputing resources in computational astrophysics are absolutely critical,” said study co-author. Volker Brommprofessor and chair, Department of Astronomy, UT Austin.
Bromm explains that in theoretical astrophysics, the dominant model for understanding the formation and evolution of cosmic structure is to use ab Initio simulation, following the ‘books’ of the universe itself – the governing physical equations.
The simulations use data from the initial conditions of the universe with high precision based on observations of the cosmic microwave background. Then, simulation boxes are set up in step-by-step cosmic evolution in time.
But the computational challenges of simulating structure formation lie in how the massive scales of the universe – millions to billions of light years and billions of years – lie within the grid of atomic scales where the process takes place. star chemistry.
“The microcosm and the macro model interact,” says Bromm.
“The resources of TACC and XSEDE have been crucial for us to push the frontiers of computational astrophysics. Everyone at UT Austin – faculty, postdocs, students – benefits from the fact that we have such a premier supercomputing center. I am extremely grateful,” Bromm added.
“If we consider a typical structure that could have formed the first stars, we need about a million elements to completely resolve this halo or structure,” says Liu. “This is why we need to use supercomputers at TACC.”
Liu said that using Stampede2, a simulation running on 100 cores could complete in just a few hours compared to a year on a laptop, not to mention bottlenecks with memory and reading or writing data.
“The overall game plan with our work was that we wanted to understand how the universe was transformed from the simple initial conditions of the Big Bang,” explains Bromm.
The structures emerging from the Big Bang are driven by the kinetic importance of dark matter.
The nature of dark matter remains one of the greatest mysteries in science.
There’s no denying the clues of this hypothetical but unobservable substance, seen in the unimaginable rotational speeds of galaxies. The masses of all the stars and planets in galaxies like our own Milky Way don’t have enough gravity to stop them flying apart. The ‘x-factor’ is known as dark matter, but laboratories have yet to detect it directly.
However, gravitational waves were detected, for the first time by LIGO in 2015.
“It is possible that primordial black holes could explain the gravitational wave events we have detected over the past seven years,” said Liu. “This just motivates us.”
“Supercomputers are enabling unprecedented new insights into how the universe works,” said Bromm. The universe provides us with extremely incomprehensible harsh environments. This also gives the impetus to build more and more powerful compute architectures and come up with better algorithmic structures. There is great beauty and strength for the benefit of all. “
The study, “Effect of stellar-mass primordial black holes on early star formation,” was published in August 2022 in the Monthly Notices of the Royal Astronomical Society. The study authors are Boyuan Liu, Saiyang Zhang and Volker Bromm of the University of Texas at Austin. Liu is currently studying at Cambridge University.
Written by Jorge Salazar
Source: TACC
