Peering into the Dark Ages, from the Far Side of the Moon

We observe the Universe through its electromagnetic radiation (light) generated or reflected by astrophysical objects, sent towards the Earth. In recent times, we've also looked at and learned about the Universe using gravitational waves, neutrinos and cosmic rays. For the lower half of the electromagnetic (EM) spectrum - from radio to optical - we use our terrestrial telescopes, be that the familiar metallic tubes with mirrors, or the classical dish-shaped antennas. To probe other parts of the electromagnetic spectrum, such as UV, X-rays and Gamma-rays, we've sent observatories into space. This is because Earth's atmosphere creates havoc on incoming light at these wavelengths - or blocks it out altogether. This is not necessarily a bad thing, as high-energy EM radiation can harm life and evolution. 

The radio regime presents an interesting case, as radio signals are bounded by the atmosphere. Molecules like oxygen, water and carbon dioxide absorb radiation as it passes through the atmosphere on the upper end (around 1 terahertz or ~0.3-millimetre wavelength); while the ionosphere reflects radio waves back into space, on the lower end (around 30 MHz or ~10-metre wavelength).  

These reflected low-frequency radio waves hold a wealth of information about the Universe, and in particular, an early epoch during its history - but we can't access them from Earth. The solution seems to be launching radio telescopes into orbit beyond Earth's ionosphere to collect data at these frequencies. But this is both inefficient and practically impossible; these telescopes are better when they're bigger (so putting them in space is costly/risky); plus our entire Solar System is flooded with human-made radio frequency interference (RFI) that emanates from Earth. Not to mention, some natural RFI generated by the Sun (radio bursts from flares, coronal mass ejection shocks), and planets like Jupiter (decametric emissions) and Saturn’s emissions too. 

One practical location shielded from RFI (constantly from Earth’s radio pollution, monthly from the Sun, and periodically from the planets) is the far side of the Moon. This has inspired new pathfinder missions being developed to send demonstrable technology to this side of the Moon so they can hopefully take measurements of these low-frequency radio waves - which no one on Earth has yet observed. 

The first, LuSEE-lite, expected to leave Earth late in 2024 will undertake observations of surface plasma physics and waves, DC electrostatic potentials, and observe the impacts of dust on the lander in the harsh environment. The target land site for this first payload is the Schrödinger Basin, on the far side of the Moon’s south pole. 

For the second payload, LuSEE-Night is expected to land on the Lunar farside at mid-latitudes in 2025, as part of a mission working in collaboration with the US Department of Energy, Brookhaven National Laboratory, University of California, Space Science Laboratory and NASA. 

It is this mission that will look at these low-frequency radio waves from deep space to probe what is known as the cosmic dark ages, as well as test how instrumentation survives the long lunar day and night. Both missions were selected in June 2019 by NASA as part of the Lunar Surface Instrument and Technology Payloads for the Commercial Lunar Payloads Services program. 

After the Big Bang, nucleosynthesis created the lightest elements - hydrogen, helium and tiny amounts of beryllium and lithium nuclei. These elements were ionised (stripped of their electrons) and floating in a hot plasma that continually absorbed and scattered light. As the Universe expanded, and at approximately 380,000 years after the Big Bang, the overall temperature cooled enough for electrons to latch onto their protons, forming neutral hydrogen. This left an isotropic imprint on the Universe around us - we know it today as the Cosmic Microwave Background (CMB). 

Neutral hydrogen can exist in one of two energy states - the first is a higher energy state in which a 21cm wavelength photon is absorbed by the atom, causing the spin axis of both sub-atomic particles to be rotating in the same direction. The second, once that atom emits the 21cm photon, it returns to its lower energy state, in which the electron spin axis is opposing that of the proton spin axis. 

This absorbing and emitting of photons by neutral hydrogen continued until 100 million years after The Bang, starting to dwindle down with the birth of the first stars. That's because as these new stars started to burn brightly, they produced lots of ultraviolet radiation, knocking electrons away from their proton counterparts, ionising them again, and commencing the Epoch of Reionisation. It is this period, between the last scattering of the CMB, and the first light from the first stars shining in the Epoch of Reionisation, that is referred to as the cosmic dark ages.

When we measure the frequency of the 21cm photon being absorbed and emitted with today’s neutral hydrogen, we register it at 1.420 GHz. If we were around in the cosmic dark ages, we’d also measure it at this same frequency. However, the Universe has expanded since then, and so the wavelength (and thus frequency) of the 21cm signal has also been stretched (a term known as redshifted). This redshifting has caused the original 1.420 GHz signal to fall below 100 MHz when we observe it today. 

By observing the global sky-averaged spectrum of this redshifted 21cm neutral hydrogen signal, astronomers expect to learn more about the temperature and ionisation state of neutral hydrogen during the cosmic dark ages, learning about the thermal history of the Universe to a time prior to when the first star’s light shone. This could radically change our perception of the evolution of the Universe, as well as potentially introduce new physics beyond the standard model. 

Cosmological models predict that the 21cm absorption feature in the highly-redshifted signal CMB spectrum should have two dips - the Epoch of Reionisation at 70 - 80 MHz, and recombination after the CMB (i.e., the start of the cosmic dark ages) at 10 - 20 MHz. This latter value falls below the threshold at which Earth’s ionosphere starts to become opaque (roughly 20 MHz). 

So if we want to observe these redshifted signals from the cosmic dark ages, we need to set up our telescope somewhere outside of Earth’s ionosphere, in a shadow where none of Earth’s RFI can flood it out, and at a location where the Sun and other Solar System bodies can be blocked out at periodic intervals. Given our technology, it has to be relatively nearby as well. 

Enter stage right, the far side of the Moon. 

The LuSEE-Night lander is a low-frequency radio astronomy pathfinder demonstrator program, consisting of a 4-channel, 50 MHz bandwidth Nyquist base-band receiver system and 2 orthogonal dipole antennas that measure 6-metres tip-to-tip. These antennas sit atop a 1m x 1m deck, approximately 70cm high, and are mounted atop the lander - so they will not be in direct contact with he Lunar surface. The antennas also reside on a motor-driven carousel and allow the orientation of the dipole to be changed in the plane of the Lunar surface - though, this rotation will only operate during the Lunar day when the solar panels are pulling direct energy from the Sun. 

LuSEE-Night is designed to full stokes, spectral density measurements at low radio frequencies (below 50 MHz), and is developed to operate continually across the full Lunar night when the Sun is below the horizon of the far side, which lasts approximately 14 days. A self-heating nighttime operations power pack will keep all the electronic equipment warm in the freezing conditions. A spectrometer with four single-ended antennas will also produce auto- and cross-correlated spectra.

To calibrate the system, LuSEE-night will use a far-field calibrator during its commissioning, where it will transmit a known waveform to the lander, allowing the instruments to correlate against this signal as part of the testing and switching on activities, prior to making any scientific observations. 

Once the lander has completed its job and is commissioned, it will effectively finish its operations and kill off, avoiding any issues of accidental RFI leakage that could flood the sensitive signals that LuSEE-Night is trying to observe. 

As well as conducting its radio astronomy objectives, scientists are planning on learning how the spacecraft (and equipment) will survive over the long lunar day and night - where temperatures can fluctuate between -130°C and 120°C. As LuSEE-Night is a demonstrator mission, they hope that these critical learnings will allow them to better plan for future, long-term science missions on the Lunar surface. 

Whilst LuSEE-Night might need to get away from Earth and head to the far side of the Moon to probe the epoch of recombination and cosmic dark ages, the end of this era - the Epoch of Reionisation - is also being assessed with an instrument much closer to home. In fact, it’s in Australia’s backyard - out at the Inyarrimanha Ilgari Bundara (CSIRO’s Murchison Radio-astronomy Observatory in outback Western Australia). 

The Experiment to Detect the Global EoR Signature (EDGES) consists of two instruments, each with a dipole antenna pointing directly upwards. They contain a radio receiver, that feeds data to a digital spectrometer. The two instruments are located about 150 meters apart and operate in the 50 - 100 MHz band, where the Epoch of Reionisation neutral hydrogen absorption signal is expected. 

In 2018, astronomers around the world were excited when results from the EDGES instruments indicated that an absorption feature, centred at 78 MHz was observed - thought to be generated by the ionisation of neutral hydrogen by the first stars. However, the results from this experiment are considered somewhat controversial - as another experiment, called the Shaped Antenna Measurement of the Background Radio Spectrum (SARAS) attempted to replicate these results independently in 2022, but did not replicate them to a high degree of confidence.

The pristine and quiet environment on the far side of the Moon has attracted the attention of radio astronomers, wanting to build telescopes away from Earth’s noisy RFI environment. So far, however, only small demonstrator programs have been implemented, with a number of different proposals put forward. 

The Lunar far side, whilst ideal for radio astronomy, does also come with a large number of challenges. The first is, getting there and landing safely. Due to the tidal lock between the Earth and the Moon, the far side always faces away from our planet, and so any missions carried out in that region are done blindly. So far, several missions that have been there have used small orbiting CubeSats to take observations, collect data and relay this back to Earth. This of course introduces RFI into this environment, so is not the most ideal. 

Secondly, the Lunar environment is harsh to humans and machines. Not only does the temperature fluctuate between extremes, but there is also ongoing bombardment from micrometeorites, and the Lunar regolith is statically charged. The Apollo astronauts reported it sticking to their suits, and being abrasive, like tiny shards of glass. Any radio telescope situated there must not only be able to withstand this environment, but also have solutions to ongoing maintenance as a response to this environment. After all, we can’t just go and re-coat our antennas as we do on Earth. 

Next, we have to consider the issue of data processing and data transfer. The infrastructure (cables, transmission towers, internet, etc.) we have on Earth is not on the Moon, and radio telescopes have the ability to collect an enormous amount of data during their observations (in the GB and TB range). So how are we to deal with this data? Process it on the Moon? If so, we need computers and power sources there (which can introduce RFI). Transfer it to the near side via cables, then transmit to Earth? If so, who is going to lay and maintain the cables? Use transmitters to send it to Earth? Who is going to build these transmitters? Clearly, there are more questions to be answered about this.

Lastly, we must also consider the political nature of the Moon - which is owned by no country, and subject only to loose international treaties and concepts that are run through the United Nations. We must consider landing and mining locations for where the telescope will live, and any impacts this might have on existing areas of importance in space archaeology. 

LuSEE-Night is a demonstrator program, as was the Chinese Change-4 mission, which also contained a radio astronomy payload experiment. These are small landers that don’t claim too much space, require too much power, or rely on large-scale infrastructure to conduct and report on their science. 

There are proposals, however, that consider building dish-antenna-like radio telescopes into existing craters - such as the Lunar Crater Radio Telescope proposal by NASA’s Jet Propulsion Laboratory. This concept considers a 3-kilometre crater being turned into a huge dish using wire mesh and robots. Whilst only at concept phase, many of the above challenges would first be required to be resolved, before a large-scale project like this could be undertaken. 

Until then, hopefully, little landers like LuSEE-Night might pave the way to humans observing, for the first time, the low-frequency radio bands of the electromagnetic spectrum - and maybe, just maybe, shining the first light on the age of cosmic darkness.