In the coming decade, multiple space agencies and commercial space providers are determined to return astronauts to the Moon and build the necessary infrastructure for long-duration stays there. This includes the Lunar Gateway and the Artemis Base Camp, a collaborative effort led by NASA with support from the ESA, CSA, and JAXA, and the Russo-Chinese International Lunar Research Station (ILRS). In addition, several agencies are exploring the possibility of building a radio observatory on the far side of the Moon, where it could operate entirely free of radio interference.
For years, researchers have advocated for such an observatory because of the research that such an observatory would enable. This includes the ability to study the Universe during the early “Cosmic Dark Ages,” even before the first stars and galaxies formed (about 50 million years after the Big Bang). While there have been many predictions about what kind of science a lunar-based radio observatory could perform, a new research study from Tel Aviv University has predicted (for the first time) what groundbreaking results this observatory could actually obtain.
This study was led by Prof. Rennan Barkana and Dr. Rajesh Mondal, an astrophysics professor and a postdoctoral researcher (respectively) with the School of Physics and Astronomy at Tel Aviv University. The paper that describes their conclusions, “Prospects for precision cosmology with the 21 cm signal from the dark ages,” has been published in Nature Astronomy. As they argue, the study’s findings show that the measured radio signals can be used to test the Standard Model of Cosmology and determine the composition of the Universe.
The Cosmic Dark Ages, which occurred roughly 130,000 to 1 billion years after the Big Bang, has traditionally remained elusive to astronomers (hence the name). Essentially, light from this cosmological period is redshifted to the point where it is only visible in the radio spectrum. What’s more, the only sources of photons from this period are the remnant radiation from the Big Bang – which is visible today as the Cosmic Microwave Background (CMB) – or are visible as the 21 cm line (or hydrogen line) caused by the reionization of neutral hydrogen.
These radio waves can only be studied from space, where they are free of atmospheric interference and other radio sources. On the far side of the Moon, a radio observatory would also be safe from radio interference caused by our Sun. Establishing an observatory there would still be a major challenge. As Prof. Barkana explained in a recent Tel Aviv University statement:
“NASA’s new James Webb space telescope discovered recently distant galaxies whose light we receive from the cosmic dawn, around 300 million years after the Big Bang. Our new research studies an even earlier and more mysterious era: the cosmic dark ages, only 50 million years after the Big Bang. Conditions in the early Universe were quite different from today.
“The new study combines current knowledge of cosmic history with various options for radio observations, in order to reveal what can be discovered. Specifically, we computed the intensity of radio waves as determined by the density and temperature of the hydrogen gas at various times, and then showed how the signals can be analyzed in order to extract from them the desired results.”
For astronomers hoping to push the boundaries of cosmology, the Cosmic Dark Ages offer an opportunity to study the first stars and galaxies in the Universe. For their study, Barkana and Mondal argue that a lunar radio observatory could measure radio signals to determine the composition of the early Universe, the expansion rate of the cosmos (thereby testing the theory of Dark Energy), and perhaps gain insight into the mystery of Dark Matter. These are all integral to the Standard Model of Cosmology, known as the Lambda-Cold Dark Matter (LCDM) model.
They also found that with an array consisting of multiple radio antennas, scientists could accurately measure the amount of hydrogen and helium shortly after the Big Bang. A precise determination of both would reveal valuable information on how ordinary matter formed from hydrogen, which fueled the creation of the first stars, gradually giving rise to heavier elements, planets, and eventually life. Last, they found that with an even larger array of lunar antennas, it will also be possible to measure the weight of cosmic neutrinos – a critical parameter in developing physics beyond the Standard Model of Particle Physics. As Prof. Barkana concluded:
“When scientists open a new observational window, surprising discoveries usually result. With lunar observations, it may be possible to discover various properties of dark matter, the mysterious substance that we know constitutes most of the matter in the Universe, yet we do not know much about its nature and properties. Clearly, the cosmic dark ages are destined to shed new light on the Universe.”