Tiny satellite will use the dark side of the moon to eavesdrop on whispers from the early universe

A tiny UK-developed satellite, roughly the size of a small carry-on suitcase, could help answer one of the biggest questions in cosmology: What happened during the roughly 150 million years of the cosmic dark ages, before the universe’s first stars appeared?

An international team of scientists led by the University of Cambridge will use the far side of the moon as a “shield” so the satellite—called CosmoCube—can block out all the noise from Earth and listen for a faint whisper from the very early universe.

This whisper, known as the 21-centimeter line, is a signal emitted by hydrogen atoms in the period between the afterglow of the Big Bang and cosmic dawn, when nuclear fusion lit up the first stars. No one has directly observed this era before.

Detecting this signal from more than 13.5 billion years ago is extremely difficult with Earth-based telescopes because Earth’s ionosphere blocks the right frequencies, and interference from FM radio, satellites and telecommunications drowns it out.

However, the moon provides a natural shield. As CosmoCube orbits the far side of the moon, it will be shielded from all the noise from Earth for roughly 40 minutes of each 2-hour orbit. Over an expected 2-year mission, it will build up 1,000 hours of data on one of the last unexplored periods of the universe, helping us understand how the universe transitioned from dark and nearly empty to the complexity we see today.

Dark matter and a lunar radio shield

The mission has received funding from the UK Space Agency, and the researchers hope CosmoCube can be launched within the next five years. Details are published in the journal Nature Astronomy.

In addition to exploring the universe in the period before the first stars, CosmoCube will also explore the role of dark matter—the mysterious force that holds galaxies together.

“This emission from hydrogen after the Big Bang, but before the first stars, will hopefully allow us to understand the role of dark matter in the early universe, how it worked to pull together hydrogen into the first stars and galaxies,” said lead author Professor Eloy de Lera Acedo of Cambridge’s Cavendish Laboratory.

To study this period, CosmoCube will operate at extremely low frequencies—between 10 and 50 MHz—far outside the range of ground-based telescopes, which is why the moon will be used as CosmoCube’s “fortress of solitude.”

“There’s no other place where you can get the sort of shielding you need to detect such a faint signal, while at the same time looking at all of space,” said de Lera Acedo, who is also affiliated with the Kavli Institute for Cosmology. “The far side of the moon is really the only option: It solves multiple problems at once, opening a clear window to the very early universe.”

How the satellite will listen

Once in orbit around the moon, CosmoCube will unfold a long, lightweight radio antenna sensitive enough to detect the 21-centimeter signal from hydrogen atoms in the early universe when the satellite is on the moon’s far side.

While in lunar orbit, CosmoCube will constantly check and correct its own electronics using a “Dicke switched” calibrator, which will flip between the sky and several built-in reference sources. This will help cancel out tiny drifts and noise inside the satellite that could otherwise masquerade as cosmic signals.

Once CosmoCube’s data is back on Earth, the team will use advanced Bayesian statistical methods to remove foreground noise—mainly radio emissions from our own galaxy. They will also reconstruct how the antenna responds to different parts of the sky using computer simulations and in-flight measurements, allowing them to subtract any remaining distortions.

“Apart from the science, what makes our mission unique is its size: We’re probing the earliest, deepest parts of the dark ages that others don’t reach, but with a compact, relatively low-cost platform,” said de Lera Acedo.

The race to keep the moon quiet

However, the far side of the moon may not stay quiet for long: Other missions are being planned by the U.S., India and other countries to take advantage of the moon’s silence.

CosmoCube features a state-of-the-art, fully integrated miniature radiometer using the latest analog and digital technology, the so-called RF-Systems-on-Chip (RFSoCs). The CosmoCube space platform (“SSTL-21”) is being developed in the UK by Surrey Space Technology Limited (SSTL), which specializes in manufacturing small satellites.

Instrument development is well underway, with functioning lab prototypes, environmental testing and key collaboration with industry partners. In the UK, academic partners include Portsmouth University and STFC RAL Space, with participation from EU countries such as Malta. The CosmoCube team recently participated in the ESA mini-Fast missions Call for Ideas, targeting a mission cost under 50 million euros.

“CosmoCube is aiming to do some ambitious science from a very small satellite in a challenging environment, and to do that requires some clever design techniques,” said co-author Dr. Will Grainger of STFC RAL Space. “We’ve worked with the project partners to develop representative models of the satellite and its payload. These have been tested in our facilities to ensure the thermal performance allows the payload to operate and perform the required sensitive measurements under the different temperature conditions it will experience while in orbit around the moon. In the future, we hope to further develop the full payload in preparation for a full mission.”

“This could be a real UK success story: The hardware, the software, the implementation and the technology are all being developed here, and it could help us answer one of the most profound questions in the universe,” said de Lera Acedo.

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Gaby Clark

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