First stars and 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 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 computational simulation of structure formation lie in the way large scales of the universe -- millions to billions of light years and billions of years -- mesh with the atomic scales where stellar chemistry happens.

"The microcosm and the macrocosm interact," Bromm said.

"TACC and XSEDE resources have been absolutely vital for us to push the frontier of computation astrophysics. Everyone who is at UT Austin -- faculty members, postdocs, students -- benefits from the fact that we have such a premier supercomputing center. I'm extremely grateful," Bromm added.

"If we look into one typical structure that can form the first stars, we need around one million elements to fully resolve this halo or structure," Liu said. "This is why we need to use supercomputers at TACC."

Liu said that using Stampede2, a simulation running on 100 cores can complete in just a few hours versus 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 was transformed from the simple initial conditions of the Big Bang," explained Bromm.

The structures that emerged from the Big Bang were driven by the dynamical importance of dark matter.

The nature of dark matter remains one of the biggest mysteries in science.

The clues of this hypothetical yet unobservable substance are undeniable, seen in the impossible rotational speeds of galaxies. The mass of all the stars and planets in galaxies like our Milky Way do not have enough gravity to keep them from flying apart. The 'x-factor' is called dark matter, yet laboratories have not yet directly detected it.

However, gravitational waves have been detected, first by LIGO in 2015.

"It is possible that primordial black holes can explain these gravitational wave events that we have been detecting over the past seven years," Liu said. "This just motivates us."

Said Bromm: "Supercomputers are enabling unprecedented new insights into how the universe works. The universe provides us with extreme environments that are extremely challenging to understand. This also gives motivation to build ever-more-powerful computation architectures and devise better algorithmic structures. There's great beauty and power to the benefit of everyone."

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Astronomers develop novel way to 'see' the first stars through the fog of the early Universe

A team of astronomers has developed a method that will allow them to 'see' through the fog of the early Universe and detect light from the first stars and galaxies.

The researchers, led by the University of Cambridge, have developed a methodology that will allow them to observe and study the first stars through the clouds of hydrogen that filled the Universe about 378,000 years after the Big Bang.

Observing the birth of the first stars and galaxies has been a goal of astronomers for decades, as it will help explain how the Universe evolved from the emptiness after the Big Bang to the complex realm of celestial objects we observe today, 13.8 billion years later.

The Square Kilometre Array (SKA) -- a next-generation telescope due to be completed by the end of the decade -- will likely be able to make images of the earliest light in the Universe, but for current telescopes the challenge is to detect the cosmological signal of the stars through the thick hydrogen clouds.

The signal that astronomers aim to detect is expected to be approximately one hundred thousand times weaker than other radio signals coming also from the sky -- for example, radio signals originating in our own galaxy.

Using a radio telescope itself introduces distortions to the signal received, which can completely obscure the cosmological signal of interest. This is considered an extreme observational challenge in modern radio cosmology. Such instrument-related distortions are commonly blamed as the major bottleneck in this type of observation.

Now the Cambridge-led team has developed a methodology to see through the primordial clouds and other sky noise signals, avoiding the detrimental effect of the distortions introduced by the radio telescope. Their methodology, part of the REACH (Radio Experiment for the Analysis of Cosmic Hydrogen) experiment, will allow astronomers to observe the earliest stars through their interaction with the hydrogen clouds, in the same way we would infer a landscape by looking at shadows in the fog.

Their method will improve the quality and reliability of observations from radio telescopes looking at this unexplored key time in the development of the Universe. The first observations from REACH are expected later this year.

The results are reported today in the journal Nature Astronomy.

"At the time when the first stars formed, the Universe was mostly empty and composed mostly of hydrogen and helium," said Dr Eloy de Lera Acedo from Cambridge's Cavendish Laboratory, the paper's lead author.

He added: "Because of gravity, the elements eventually came together and the conditions were right for nuclear fusion, which is what formed the first stars. But they were surrounded by clouds of so-called neutral hydrogen, which absorb light really well, so it's hard to detect or observe the light behind the clouds directly."

In 2018, another research group (running the 'Experiment to Detect the Global Epoch of Reioniozation Signature' -- or EDGES) published a result that hinted at a possible detection of this earliest light, but astronomers have been unable to repeat the result -- leading them to believe that the original result may have been due to interference from the telescope being used.

"The original result would require new physics to explain it, due to the temperature of the hydrogen gas, which should be much cooler than our current understanding of the Universe would allow. Alternatively, an unexplained higher temperature of the background radiation -- typically assumed to be the well-known Cosmic Microwave Background -- could be the cause" said de Lera Acedo.

He added: "If we can confirm that the signal found in that earlier experiment really was from the first stars, the implications would be huge."

In order to study this period in the Universe's development, often referred to as the Cosmic Dawn, astronomers study the 21-centimetre line -- an electromagnetic radiation signature from hydrogen in the early Universe. They look for a radio signal that measures the contrast between the radiation from the hydrogen and the radiation behind the hydrogen fog.

The methodology developed by de Lera Acedo and his colleagues uses Bayesian statistics to detect a cosmological signal in the presence of interference from the telescope and general noise from the sky, so that the signals can be separated.

To do this, state-of-the-art techniques and technologies from different fields have been required.

The researchers used simulations to mimic a real observation using multiple antennas, which improves the reliability of the data -- earlier observations have relied on a single antenna.

"Our method jointly analyses data from multiple antennas and across a wider frequency band than equivalent current instruments. This approach will give us the necessary information for our Bayesian data analysis," said de Lera Acedo.

He added: "In essence, we forgot about traditional design strategies and instead focused on designing a telescope suited to the way we plan to analyse the data -- something like an inverse design. This could help us measure things from the Cosmic Dawn and into the epoch of reionisation, when hydrogen in the Universe was reionised."

The telescope's construction is currently being finalised at the Karoo radio reserve in South Africa, a location chosen for its excellent conditions for radio observations of the sky. It is far away from human-made radio frequency interference, for example television and FM radio signals.

The REACH team of over 30 researchers is multidisciplinary and distributed worldwide, with experts in fields such as theoretical and observational cosmology, antenna design, radio frequency instrumentation, numerical modelling, digital processing, big data and Bayesian statistics. REACH is co-led by the University of Stellenbosch in South Africa.

Professor de Villiers, co-lead of the project at the University of Stellenbosch in South Africa said: "Although the antenna technology used for this instrument is rather simple, the harsh and remote deployment environment, and the strict tolerances required in the manufacturing, make this a very challenging project to work on."

He added: "We are extremely excited to see how well the system will perform, and have full confidence we'll make that elusive detection."

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ALMA scientists detect signs of water in a galaxy far, far away

Water has been detected in the most massive galaxy in the early Universe, according to new observations from the Atacama Large Millimeter/submillimeter Array (ALMA). Scientists studying SPT0311-58 found H20, along with carbon monoxide in the galaxy, which is located nearly 12.88 billion light years from Earth. Detection of these two molecules in abundance suggests that the molecular Universe was going strong shortly after the elements were forged in early stars. The new research comprises the most detailed study of molecular gas content of a galaxy in the early Universe to date and the most distant detection of H20 in a regular star-forming galaxy. The research is published in The Astrophysical Journal.

SPT0311-58 is actually made up of two galaxies, and was first seen by ALMA scientists in 2017 at its location, or time, in the Epoch of Reionization. This epoch occurred at a time when the Universe was just 780 million years old -- roughly 5-percent of its current age -- and the first stars and galaxies were being born. Scientists believe that the two galaxies may be merging, and that their rapid star formation is not only using up their gas, or star-forming fuel, but that it may eventually evolve the pair into massive elliptical galaxies like those seen in the Local Universe.

"Using high-resolution ALMA observations of molecular gas in the pair of galaxies known collectively as SPT0311-58 we detected both water and carbon monoxide molecules in the larger of the two galaxies. Oxygen and carbon, in particular, are first-generation elements, and in the molecular forms of carbon monoxide and water, they are critical to life as we know it," said Sreevani Jarugula, an astronomer at the University of Illinois and the principal investigator on the new research. "This galaxy is the most massive galaxy currently known at high redshift, or the time when the Universe was still very young. It has more gas and dust compared to other galaxies in the early Universe, which gives us plenty of potential opportunities to observe abundant molecules and to better understand how these life-creating elements impacted the development of the early Universe."

Water, in particular, is the third most abundant molecule in the Universe after molecular hydrogen and carbon monoxide. Previous studies of galaxies in the local and early Universe have correlated water emission and the far-infrared emission from dust. "The dust absorbs the ultraviolet radiation from the stars in the galaxy and re-emits it as far-infrared photons," said Jarugula. "This further excites the water molecules, giving rise to the water emission that scientists are able to observe. In this case, it helped us to detect water emission in this massive galaxy. This correlation could be used to develop water as a tracer of star formation, which could then be applied to galaxies on a cosmological scale."

Studying the first galaxies to form in the Universe helps scientists to better understand the birth, growth, and evolution of the Universe, and everything in it, including the Solar System and Earth. "Early galaxies are forming stars at a rate thousands of times that of the Milky Way, said Jarugula. "Studying the gas and dust content of these early galaxies informs us of their properties, such as how many stars are being formed, the rate at which gas is converted into stars, how galaxies interact with each other and with the interstellar medium, and more."

According to Jarugula, there's plenty left to learn about SPT0311-58 and the galaxies of the early Universe. "This study not only provides answers about where, and how far away, water can exist in the Universe, but also has given rise to a big question: How has so much gas and dust assembled to form stars and galaxies so early in the Universe? The answer requires further study of these and similar star-forming galaxies to get a better understanding of the structural formation and evolution of the early Universe."

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