Scientists, including MIT's Jacqueline Hewitt and Nicholas Kern, shared the long-awaited result: We're getting closer and closer to understanding the universe's first star. Historically, humans have been creating and sharing stories of thinking about the creation of stars — what they were and how the first stars were created.
Now, with new results from the Hydrogenation Epoch Array (HERA), a radio telescope located in the Karoo Astronomical Reserve in South Africa, MIT scientists are a small step closer to understanding this history, but significant.
HERA researchers are looking for the earliest signs of star formation and galaxy structure. Specifically, scientists including Jacqueline Hewitt, a professor of physics at MIT's Julius A. Stratton, are trying to understand what happened during a period known as the cosmic dawn that occurred about 400 million years after the Big Bang. In early fall 2021, Hewitt, Nicolas Kern, a Pappalador physics researcher at the MIT's Cabry Institute of Astrophysics and Space Research, and other researchers from a collaborative collaboration finalized long-awaited results that were collected and analyzed over a four-year period during the first phase of the HERA telescope's construction.
Their study, published Feb. 7, 2022 in the Astrophysical Journal, proposes a new upper limit on radio signals from hydrogen in the universe, suggesting early star formation and giving scientists a clearer idea of when the first stars and galaxies formed. These findings narrow down theoretical models that make assumptions about the origin of the cosmic dawn.
Hera's findings are so important in part because they were collected in the early stages of HERA's development. The telescope operates as an array of radio antennas and currently has only a fraction of its final size — data collected from 39 of HERA's 52 deployed antennas. In its complete form, there will be a total of 350 antennas. Once fully built, HERA will be sensitive enough to collect larger data sets and information from farther away, further détente back in the past.
To look back at the dawn of the universe, HERA uses low-frequency radio waves to identify signals that are not easily observed. This is different from other telescopes, such as the Hubble Space Telescope, which observes structures like galaxies, which make up only 5% of the observable material in space. The other 95% of the material is intergalactic, including low-density hydrogen. With HERA, scientists can observe what's going on between galaxies and use that information to infer what galaxies are doing that we can't observe, and how galaxy formation affects the space around it.
Members of the HERA Collaborative Project at a scientific conference. The HERA team consists of agencies from the United States, Canada, Europe and South Africa.
To understand this period in the history of the universe, scientists are looking for "spin flip signals," also known as the 21-centimeter line, which is the wavelength of neutral hydrogen. This radio signal comes from interstellar matter between galaxies and is produced by the emission and/or absorption of hydrogen atoms through this transition.
Determining the "era of revival," or the time when the signal was observed, is what matters. Astronomers want to know if [the signal] is absorbed, which means it's before X-rays, or in emission, i.e. after X-rays, and wants to see if it disappears due to heavy ionization.
The signal has two characteristics, or processes, that can be captured. When a star heats hydrogen, the signal is first altered. The second part, what HERA has been looking for so far, is the disappearance of the 21-centimeter signal, which occurs when hydrogen is ionized by the energy generated by additional star formation. This signal indicates that the star has been created.
The 21-centimeter line from cosmic dawn has not yet been explicitly detected. However, the new results from HERA provide data on the nature of spin-flipped signals in the universe at 500 million years — 10 times more sensitive than previous results.
With these results, the HERA team has been able to provide evidence that excludes several possible theories about galaxy formation. Most notably, these data show that there must be some mechanism to heat hydrogen in space, which means that galaxies must have black holes.
With funding from the Gordon and Betty-Moore Foundation and the National Science Foundation, HERA will operate under 350 antennas and employ a new antenna design that will enable telescopes to capture lower frequency radio waves and higher redshift observation points, effectively seeing farther away in time.
Hewitt, a project leader to expand HERA's signal capacity, has been studying the question of when the earliest stars formed since 2004. She has led the prototyping of new low-frequency components and is developing more techniques to analyze current and future datasets. The new antenna design from the University of Cambridge, which should be installed in early 2022, will greatly increase the range of information they are able to obtain.