A lunar telescope that could explore the cosmic dark ages

Multiple space agencies will send missions to the moon this decade and the next, with plans to establish infrastructure that will allow for many returns. This includes NASA’s Lunar Gateway and Artemis Base Camp, the Chinese-Roscosmos International Lunar Research Station (ILRS), and the ESA’s Moon Village. With so many space agencies and commercial space companies focused on lunar exploration, there are also multiple plans for establishing research facilities and scientific experiments.

In particular, NASA, China, and the ESA have proposed creating radio astronomy experiments that would operate on the far side of the moon. In a recent paper, an international team of European astronomers proposed an ultra-long wavelength radio interferometer that could examine the cosmological periods known as the Cosmic Dark Ages and Cosmic Dawn. Known as the Dark Ages Explorer (DEX), this telescope could provide fresh insights into one of the least understood periods in the history of the universe.

The study was led by Christiaan Brinkerink, a scientific engineer with the Radboud Radio Lab (RRL) at Radboud University Nijmegen. He was joined by researchers from the Netherlands Institute for Radio Astronomy (ASTRON), the Eindhoven University of Technology, the Delft University of Technology (TU Delft), the Laboratory for Instrumentation and Research in Astrophysics (LIRA), the Kapteyn Astronomical Institute, the Leiden Observatory, the Cambridge Institute of Astronomy, the Kavli Institute for Cosmology, the European Research Infrastructure Consortium (ERIC), and the ESA’s European Space Research and Technology Center (ESTEC).

The paper describing their concept is available on the arXiv preprint server and is currently being reviewed for publication in Experimental Astronomy.

Viewing the Cosmic Dawn

According to current cosmological models, the early universe (ca. 380,000 to 1 billion years after the Big Bang) was permeated by neutral hydrogen. During this era, known as the “Cosmic Dark Ages,” the only sources of light in the universe were photons released from the first electrons and protons coming together during the Recombination Epoch (ca. 380,000 years after the Big Bang) and those released by hydrogen during the Era of Reionization (ca. 250–500 million years after the Big Bang.)

Whereas the former are visible today as the Cosmic Microwave Background (CMB), the latter are only visible today as the “hydrogen line” (aka the 21 cm line). This refers to photons released by neutral hydrogen as it was reionized by the ultraviolet radiation released by the first stars and galaxies in the universe. This period is known to astronomers as “Cosmic Dawn” (ca. 50 million to one billion years after the Big Bang), and is considered the final frontier of astronomy and cosmology.

Due to the distances involved, light from this period is redshifted to the point that it is only visible in the Ultra-High Frequency (UHF) band of the microwave window. So far, astronomers have gotten a glimpse of what’s behind the veil of the Cosmic Dark Ages thanks to instruments like the James Webb Space Telescope (JWST). However, as Brinkerink told Universe Today via email, there are still many unanswered questions about this cosmological period:

“We have a fiducial model for how structure formation progressed from the time of the CMB up until the formation of the first stars (with the clumping of dark matter precipitated by cooling through the radiation from baryonic matter), but significant uncertainties remain about the rate at which this structure formation progressed.

“Observing the signal from neutral hydrogen (the redshifted 21-cm line) is basically the only way we have to directly study this time period. With the results from JWST showing a surprisingly large number of fully formed galaxies at early epochs, it is likely that we do not fully understand the phase of structure formation that came before the existence of the first stars.”

In particular, Webb’s early observations revealed a surprising number of galaxies, which were also brighter than expected. Astronomers also noted that the “seeds” of the first supermassive black holes (SMBHs) were larger than expected. These results were in “tension” with previous cosmological models, reinforcing previous research like the Experiment to Detect the Global EoR Signature (EDGES). This experiment showed an absorption feature in the global spectrum around 70 MHz (about twice the depth expected), suggesting that we do not fully understand the active processes during Cosmic Dawn.

The “tension” produced by these results has inspired new theories about early galaxy and SMBH formation. It has also provided an added incentive to create next-generation facilities to study the early universe more deeply. The DEX aims to address these mysteries by making measurements of the spectrum of the neutral hydrogen signal across a range of redshifts. Initially, this will cover the Cosmic Dawn period (redshifts of z = 28 to 14) and eventually the Dark Ages (z = 50 to 28). As Brinkerink explained:

“The redshift range is split up because these ranges place very different requirements on array size and antenna size and placement. Through measurements of the spatial power spectrum as a function of redshift, we can ‘make a movie’ of how the early universe evolved and see the role of Dark Matter in precipitating and accelerating this process.

“DEX will continuously generate sky snapshots (images) at many frequencies across its observing bandwidth, which are integrated over time (~minutes per integration) to manage the output data rate. Back on Earth, these sky snapshots can be put in a processing pipeline that extracts the spatial and spectral variations and performs foreground avoidance and/or subtraction (both can be performed with this data product).”

The far side of the moon has long been considered an ideal site for observatories, including radio, optical, and other types of telescopes. These facilities would be shielded from radio frequency interference (RFI) from Earth, and atmospheric distortion would not be a factor. However, the engineering challenges of building and maintaining such an observatory would be considerable.

Enter DEX

Their study builds on previous work conducted by the ESA. In 2020, the ESA created the Astrophysical Lunar Observatory Topical Team (ALO TT) to realize a lunar-based cosmological radio array on the far side of the moon. This team comprises approximately 60 researchers, engineers, and commercial partners from universities, institutes, and companies from Europe and beyond. This was followed by an ESA Concurrent Design Facility (CDF) study titled “Assessment of an Astrophysical Lunar Observatory on the far side of the moon,” which assessed the feasibility of a lunar observatory using present-day technologies with a high Technological Readiness Level (TRL).

The results, said Brinkerink, confirmed that such an array would be possible with technology available in the not-too-distant future:

“The CDF study performed by ESA together with us in 2021 showed that the scale at which a lunar array can be realized with existing technology does not yet connect to what we need to yield new science: a 4×4 array was considered doable, but for the science we need, at least a 32×32 array. Location-wise, DEX needs a spot on the lunar far side that is sufficiently flat to allow for the deployment of an array with a size of ~200x200m, pointing us in the direction of a crater floor with a relatively young surface (fewer craters and boulders present).

The study also focused on the moon’s southern polar region since this is where NASA plans to conduct regular missions through the Artemis Program. Data relay services will also be possible in this region since it will be in view of the Lunar Gateway much of the time. Temperature variations are lower during lunar cycles in the polar region, ranging from highs of about 54°C (130°F) to lows of -203°C (-334°F), versus highs of 121°C (250°F) and lows of -133°C (-208° F) near the equator. This creates greater engineering challenges, though Brinkerink and his colleagues note that mid-latitudes offer better sky and ultraviolet coverage.

Their design study considered many different power scenarios. The classic method, he said, where electric power is generated from a central facility and distributed using conductive cabling, would account for up to 50% of the total system’s mass, making it very expensive to transport and deploy. Such a system would also affect the placement of antennas since it would require power distribution centers close to the antennas. Therefore, the team considered alternatives, including optical fiber and free-space radio frequency data transmission.

They also considered distributed systems deployed close to the antennas, but ruled this out because it would create a source of locally generated RFI. In addition, the team identified several technological developments necessary to make DEX possible. In particular, they found that the mass budget for sending the necessary elements to the moon could be addressed with a foil-based structure for the radiating elements. As Brinkerink described:

“These allow for unfolding, unrolling, or inflating to be used as deployment methods. The low-noise amplifiers need to be temperature-tolerant because of their exposed placement directly in the lunar environment. Even with protective measures in place, the temperature range that they will be exposed to is still larger than the survival range for more standard solutions.

“Furthermore, we need a reliable and efficient array deployment system that makes sure that all antennas are placed in a predictable pattern, as any deviations from the nominal antenna positions will result in a degradation of the quality of the scientific output (in the form of spatial mode mixing in the power spectra).”

Ultimately, the study determined that the number of antennas needed to achieve the observatory’s primary scientific goals is not yet feasible. Nevertheless, Brinkerink and his colleagues emphasize how it also establishes a pathway for technological development that could lead to a realistic and valuable experiment in the next decade or two. In the meantime, the development of these technologies will have spin-off applications here on Earth. According to Brinkerink:

“[C]ommunications systems on small satellites will benefit from the foil-based antenna technology, and radio receivers that need to operate in harsh thermal environments for extended periods of time can make use of the implementations developed for DEX. Science-wise, the measurement of the spatial power spectra from the Dark Ages and Cosmic Dawn link to the actual imaging of the structure of clumping matter in these epochs (Dark Ages/Cosmic Dawn tomography), which can help us understand the evolution of supermassive black holes and the role of early galactic feedback in galaxy growth.”