“There are precious few fossil relics of the early Universe,” Brian Keating, Chancellor’s Distinguished Professor of Physics at UC San Diego, and author of Think Like a Nobel Prize Winner,” told The Daily Galaxy. “Just after the Big Bang,” Keating explained, “came the epoch of Big Bang nucleosynthesis, ending a few minutes later. The Cosmic Microwave Background, the universe’s oldest light, came about 380,000 years later. Then the great cosmic darkness began. That darkness had to end, or else we would not be here asking what caused the Universe to be so dim.
Epoch of Cosmic Dawn –First Atoms of the Universe
“The Epoch of Cosmic Dawn offers a new probe of the formerly dark ages,” Keating added, “By exploiting the extremely faint, but exquisitely well understood physics of hydrogen‘s spin flip/hyperfine structure, astronomers are on the precipice of detecting the faint radio fingerprints from universe’s first hydrogen atoms.
“These atoms are ideal tracers of the cosmic conditions prevailing at that primitive epoch. Unlike CMB photons,” Keating concludes, “these 21cm photons encode the exact moment they were formed, allowing the detected (redshifted) frequency to be used as a tomographic tracer of the 3D structure of the Universe.”
The 21-centimeter Emissions
MIT’s David Kaiser, the Germeshausen Professor of the History of Science, wrote in an email to The Daily Galaxy about the remnant glow from early moments after the big bang: “The CMB was emitted at nearly a single instant, about 380,000 years after the big bang, and it encodes a wealth of information about conditions that had prevailed very early in cosmic history. Only now are astronomers and cosmologists beginning to complement these careful observations of the CMB with new measurements of a different emission: the 21-centimeter emission line from neutral atomic hydrogen.
“Unlike the CMB,” Kaiser explained, “the 21-cm emissions occurred over a long stretch of cosmic history, and will enable cosmologists to measure subtle features of our evolving universe over time, including the era when the very first stars and galaxies began to form. Whereas the CMB gives us a rich snapshot of a single moment, observations of the 21-cm line should help us study processes that unfolded over a billion years, perhaps clarifying such long-standing mysteries as the roles black holes may play in galactic dynamics and the nature of dark matter. It is an incredibly exciting next step for the field, enabling us to map features of our universe as they changed over space and time.”
With new instruments coming on the line, such as the Hydrogen Epoch of Reionization Array (HERA) telescope –an array of 350 antennas situated next to MeerKAT in the Northern Cape province of South Africa–the first set of observations to the world revealed radio broadcasts from the first atoms of the Universe.
HERA radio astronomers have detected light from the oldest gas clouds that formed around 13 billion years ago when the universe emerged from a great cosmic dark age as the first stars and galaxies lit up.
“Even the most powerful optical and infra-red space telescopes like the Hubble Space Telescope or its upcoming successor, the James Webb Space Telescope, won’t be able to look that far back in time,” says radio astronomer Dr. Mario Santos, currently on the HERA board.
On September 17th, 2021, the HERA telescope released the first set of observations to the world that give a glimpse of what the universe looked like 13 billion years ago. A unique feature enables users to carry out observations and early science while construction of HERA continues. After the initial construction, scientists carried out Phase I observations using about 50 dishes.
“Observations of redshifted 21cm let us see primordial matter and how it is disturbed by stars, galaxies, or even stranger things,” Danny Jacobs, co-director of the Low Frequency Cosmology lab at Arizona State University where they observe high redshift cosmology at low radio frequencies, develop space-based astronomy such as HERA and PAPER (Precision Array for Probing the Epoch of Reionization) at Arizona State University, told The Daily Galaxy. “This kind of new window doesn’t come along very often,” he notes. “Telescopes like HERA are really pushing the envelope of what is possible and every new observation draws the net closer on what we know.”
“The upcoming observations with the enhanced HERA array allow us to observe the lighting up of the very first stars after the Big Bang,” says Dr. David DeBoer, HERA Project Manager, from the University of California, Berkeley.
In an email to The Daily Galaxy, DeBoer wrote: “HERA is built to measure the change of state of hydrogen gas in the early Universe over 13 billion years ago as it became more ionized due to radiation from compact sources. Since we can partially see through hydrogen over this cosmic epoch, we can measure this evolution over cosmic time. Recently HERA published new limits on the prevalence of hydrogen, which helps constrain some physical models of how the Universe evolved. Although only limits over a limited time and spatial scale, it advances our understanding of this period and is a positive harbinger of things to come as we continue to take data in South Africa.”
Previous Efforts to Detect this Epic Turning Point
All of these other sources or radiation are many orders of magnitude stronger than the signal we’re trying to detect, said said Jonathan Pober, an assistant professor of physics at Brown University and a team member of 2019 analysis of data collected by the Murchison Widefield Array (MWA) radio telescope. Even an FM radio signal that’s reflected off an airplane that happens to be passing above the telescope is enough to contaminate the data,” Their analysis of data collected by the MWA brings scientists closer than ever to detecting the ultra-faint signature of an epic turning point in cosmic history when the first stars and galaxies formed.
In a paper published in The Astrophysical Journal, the researchers presented the first analysis of data from a configuration of the MWA designed specifically to look for the signal of neutral hydrogen, the gas that dominated the universe during the cosmic dark age. The analysis sets a new limit — the lowest limit yet — for the strength of the neutral hydrogen signal.
“We can say with confidence that if the neutral hydrogen signal was any stronger than the limit we set in the paper, then the telescope would have detected it,” said Pober, corresponding author on the new paper who works in the field of “21 cm cosmology” — a program of research to observe neutral hydrogen from the early universe through its hyperfine 21 cm emission line. “These findings can help us to further constrain the timing of when the cosmic dark ages ended and the first stars emerged.
Epoch of Reionization (EoR)
Despite its importance in cosmic history, little is known about the period when the first stars formed, which is known as the Epoch of Reionization (EoR). The first atoms that formed after the Big Bang were positively charged hydrogen ions — atoms whose electrons were stripped away by the energy of the infant universe. As the universe cooled and expanded, hydrogen atoms reunited with their electrons to form neutral hydrogen. And that’s just about all there was in the universe until about 12 billion years ago, when atoms started clumping together to form stars and galaxies. Light from those objects re-ionized the neutral hydrogen, causing it to largely disappear from interstellar space.
The signal of neutral hydrogen from the dark ages
The goal of projects like the one happening at MWA is to locate the signal of neutral hydrogen from the dark ages and measure how it changed as the EoR unfolded. Doing so could reveal new and critical information about the first stars — the building blocks of the universe we see today. But catching any glimpse of that 12-billion-year-old signal is a difficult task that requires instruments with exquisite sensitivity.
When it began operating in 2013, the MWA was an array of 2,048 radio antennas arranged across the remote countryside of Western Australia. The antennas are bundled together into 128 “tiles,” whose signals are combined by a supercomputer called the Correlator. In 2016, the number of tiles was doubled to 256, and their configuration across the landscape was altered to improve their sensitivity to the neutral hydrogen signal. This new paper is the first analysis of data from the expanded array.
Neutral hydrogen emits radiation at a wavelength of 21 centimeters. As the universe has expanded over the past 12 billion years, the signal from the EoR is now stretched to about 2 meters, and that’s what MWA astronomers are looking for. The problem is there are myriad other sources that emit at the same wavelength — human-made sources like digital television as well as natural sources from within the Milky Way and from millions of other galaxies.
To zero in on the signal, the researchers use a myriad of processing techniques to weed out those contaminants. At the same time, they account for the unique frequency responses of the telescope itself.
“If we look at different radio frequencies or wavelengths, the telescope behaves a little differently,” Pober said. “Correcting for the telescope response is absolutely critical for then doing the separation of astrophysical contaminants and the signal of interest.”
Those data analysis techniques combined with the expanded capacity of the telescope itself resulted in a new upper bound of the EoR signal strength. It’s the second consecutive best-limit-to-date analysis to be released by MWA and raises hope that the experiment will one day detect the elusive EoR signal.