Faint ‘whispers’ from the Moon may unveil the the first billion years of the Universe’s evolution, which has yet to be observed in detail. Very little is known about the first stars and galaxies that came into existence in this early period. One avenue to explore this epoch is to study the faint radio waves from neutral hydrogen atoms.
“Before there were stars and galaxies, the Universe was pretty much just hydrogen, floating around in space,” said Benjamin McKinley at Australia’s Curtin University and the International Centre for Radio Astronomy Research. “Since there are no sources of the optical light visible to our eyes, this early stage of the Universe is known as the ‘cosmic dark ages’.
The Radio Signal is the Only Observable Thing in that Early Period of the Universe
In 2018, reports International Center for Radio Astronomy Research (ICRAR), the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) detected a very unusual signal, which could be our first glimpse at the period when the first stars and galaxies began to heat the gas in the early Universe, a period known as Cosmic Dawn.
When asked how the radio signal will help confirm or challenge theories about the evolution of the Universe, McKinley replied in an email to The Daily Galaxy; “The radio signal is really important, because it is really the only observable thing in that very early period of the Universe. We are peering back to before there are stars and galaxies and black holes, or at least to the first of these types of more complex objects. In this period the Universe is mostly just neutral Hydrogen atoms, and they emit this 21-cm wavelength radio light. As the Universe is expanding, this light (radio waves) gets stretched too (redshifted) and so by look at how the signal changes as a function of wavelength, we are seeing how it changes over time as the Universe expands.
“This information alone can tell us about both cosmology –i.e. how the Universe is expanding, what are the effects of gravity, dark matter and dark energy etc– and also about the first sources of light such as stars and galaxies,” McKinley explained. “We have plenty of theories about how the Universe should look in its infancy, but very little evidence to determine which models are correct!”
“Neutral hydrogen atoms can randomly undergo an energy transition where their electron’s spin orientation ‘flips’, resulting in the emission of a photon with 21-cm wavelength,” explains McKinley. “The early Universe was abundant with neutral hydrogen and, due to the expansion of the Universe, these early 21-cm photons have now been stretched (redshifted) to wavelengths between 1 – 6 m. We should therefore be able to observe a redshifted 21-cm signal from the early Universe using radio telescopes, however,” he notes, “there are complications. There are extremely bright foregrounds in the way that obscure the signal, including radio emission from our own Galaxy and from other extra-galactic sources, such as accreting supermassive black holes. Also, the instruments used to observe the radio emission can introduce small, but significant structure into an observed signal, due to imperfections in calibration. These two effects couple together, making the whole process of detection extremely difficult.”
The Moon – Key to How the First Stars and Galaxies Shaped the Universe
The Moon may be the key to unlocking how the first stars and galaxies shaped the early Universe. A team of astronomers led by McKinley and the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) observed the Moon with a radio telescope to help search for the faint signal from hydrogen atoms in the infant Universe.
In research published in the Oxford University Press Monthly Notices of the Royal Astronomical Society, the astronomers describe how they have used the Murchison Widefield Array (MWA) radio telescope to help search for radio signals given off by the hydrogen atoms.
“If we can detect this radio signal it will tell us whether our theories about the evolution of the Universe are correct.”
Like trying to hear the flap of a hummingbird’s wing”
McKinley said that in your car radio, you can tune into various channels and the radio waves are converted into sounds. “The radio telescope, the Murchison Widefield Array (MWA) which is located in the Western Australian desert far away from earth-based FM radio stations, takes the radio signals from space and which we can then convert into images of the sky.”
This radio signal from the early Universe is very weak compared to the extremely bright objects in the foreground, which include accreting black holes in other galaxies and electrons in our own Milky Way. The key to solving this problem is being able to precisely measure the average brightness of the sky. However, built-in effects from the instruments and radio frequency interference make it difficult to get accurate observations of this very faint radio signal.
In this work, the astronomers used the Moon as a reference point of known brightness and shape. This allowed the team to measure the brightness of the Milky Way at the position of the occulting Moon. The astronomers also took into account ‘earthshine’–radio waves from Earth that reflect off the Moon and back onto the telescope. Earthshine corrupts the signal from the Moon and the team had to remove this contamination from their analysis.
“So far I have experimented with using the Moon as a known thermal reference source,” explains McKinley. “It should be possible to determine the mean temperature of the sky occulted by the Moon using an interferometer (such as the MWA) if the spectrum and shape of the Moon is known. This project has encountered challenges such as dealing with reflected ‘earthshine’ from terrestrial radio transmitters, and also emission from the Galaxy bouncing off the Moon. Efforts are now concentrating on better understanding these sources of interference.”
Eureka–The earliest evidence of hydrogen observed
In a subsequent 2018 study published in the journal Nature, astronomers from MIT and Arizona State University reported that a table-sized radio antenna in a remote region of western Australia has picked up faint signals of hydrogen gas from the primordial universe. The scientists have traced the signals to just 180 million years after the Big Bang, making the detection the earliest evidence of hydrogen yet observed.
They also determined that the gas was in a state that would have been possible only in the presence of the very first stars, reported MIT. These stars, blinking on for the first time in a universe that was previously devoid of light, emitted ultraviolet radiation that interacted with the surrounding hydrogen gas. As a result, hydrogen atoms across the universe began to absorb background radiation — a pivotal change that the scientists were able to detect in the form of radio waves.
The findings provide evidence that the first stars may have started turning on around 180 million years after the Big Bang.
“This is the first real signal that stars are starting to form, and starting to affect the medium around them,” says study co-author Alan Rogers, a scientist at MIT’s Haystack Observatory. “What’s happening in this period is that some of the radiation from the very first stars is starting to allow hydrogen to be seen. It’s causing hydrogen to start absorbing the background radiation, so you start seeing it in silhouette, at particular radio frequencies.”
“There is a great technical challenge to making this detection. Sources of noise can be a thousand times brighter than the signal they are looking for. It is like being in the middle of a hurricane and trying to hear the flap of a hummingbird’s wing,” said Peter Kurczynski, currently Chief Scientist of NASA’s Cosmic Origins Program, who studies the formation and evolution of galaxies as well as new technologies to explore the universe.
The Search Continues
“We have a new PhD student who has joined us at the International Centre for Radio Astronomy, who will be continuing this work using the Lunar Occultation technique to try and detect the sky-averaged, redshifted 21-cm signal from the early Universe,” Ben McKinley wrote in his email to The Daily Galaxy..
“The last big news in this field.” he continued, “was from Bowman et al in 2018, who claimed to detect the signal using the EDGES instrument. But there has been a lot of controversy around whether or not they had really made a detection, and hence a lot of new experiments to try and detect the signal. This type of experiment is usually done with a single-element antenna, but our approach is to try and use a multi-element system; an interferometer. So, we use the MWA and observe the Moon, which we use as a reference against the background sky. The main problem we faced in 2018 was that the Moon reflects a lot of radio signals from Earth, so the first job of our new student is to better understand these reflections, using an upgraded MWA which has much better angular resolution.
“Another thing we are doing is to use a different interferometer, the Engineering Development Array-2, which is a prototype for the upcoming Square Kilometer Array telescope. In this case we take advantage of the closest antennas in the array to emulate a single element.”
Image at the top of the page shows radio waves from our galaxy, the Milky Way, reflecting off the surface of the Moon and observed by the Murchison Widefield Array radio telescope located in outback Western Australia. Dr. Ben McKinley, Curtin University/ICRAR/ASTRO 3D. Moon image courtesy of NASA/GSFC/Arizona State University.