What does the Moon have to do with our quest to understand the beginnings of our Universe?
Not much, you might think, given that the Moon is about 4.5 billion years old, while the Universe is said to be 13.8 billion years old.
But the Moon has an important role to play in a branch of astrophysics that hopes to unlock the secrets of exactly how the first stars formed and how those stars evolved into the majestic galaxies we see today.
The Moon is not only Earth’s natural satellite. By blocking out galactic background radio signals, it could be the key that unlocks the secrets of the early universe.
Ben McKinley is a research scientist at Curtin University in Perth, Western Australia, specializing in understanding the complex structure and behavior of radio galaxies.
We spoke to Ben to learn more about how our own Moon can help us in our quest to better understand the evolution of the Universe as we see it today.
What does radio astronomy tell us about the early Universe?
When the early universe expanded and cooled enough, the protons and electrons formed neutral hydrogen.
The first stars and galaxies then “reionized” the hydrogen, that is to say that their photons interacted with the atoms, causing the separation of the electrons. This is the era of reionization.
Neutral hydrogen emits photons at a wavelength of 21 cm.
But since the Epoch of Reionization, the wavelength of these will have been redshifted (stretched by the expansion of the Universe) between one and three meters, in the radio part of the electromagnetic spectrum.
The appearance of this red-shifted 21 cm signal tells us about those early stars and galaxies that reionized hydrogen.
What are the challenges of detecting this signal from the reionization epoch?
This goal is fraught with pitfalls, as the signal is weak and obscured by much brighter radio emissions in the foreground, from objects such as radio galaxies and electrons moving near the speed of light in our Milky Way. .
This is where the Moon comes in; it obscures the sky, providing variation to the otherwise featureless average radio signal we seek.
If we know the angular size of the Moon and its brightness as a function of frequency or wavelength, we can use observations from the Murchison Widefield Array – a low-frequency radio telescope in Western Australia – to infer the background signal medium at different frequencies.
Why do you use the Moon?
The Moon acts as a known reference against which we can measure the average sky background.
The overall average is otherwise invisible to the Murchison Widefield Array, which is sensitive only to angular variations in the signal.
In theory, any blackout shape could be used; the Moon happens to be an ideal size in the sky and we think we know its brightness as a function of frequency.
How does the Murchison Widefield Array help?
The Murchison Widefield Array could observe the Moon between 72 MHz and 230 MHz.
The telescope pointed to the Moon one night and the following night at the same local sidereal time, the same area was observed.
In the second observation, the Moon would not be in the field of view because it would have moved away, due to its orbit around the Earth.
This allowed us to subtract one set of images from the other, removing the rest of the sky and leaving an imprint of the Moon.
The different images were then analyzed to extract the global sky background that the Moon occulted.
Did you have any problems?
The main problem was that we couldn’t assume the Moon was a blackbody. [an object that absorbs radiation perfectly] a constant temperature because the Moon reflects radio signals from Earth back to the telescope.
So what happens is you get a bright spot in your images at the center of the Moon because it acts like a mirror.
This is mainly a problem between the frequencies of 87MHz and 110MHz.
However, it turns out that sunlight reflections and surface heating by the Sun is a much weaker effect, and so for our purposes the phase of the Moon doesn’t matter.
How many observations did you make?
We made observations over several nights for a total of about 250 hours and are still analyzing and publishing the data.
So far, we only have one night each of an on-off moon pair.
Since we can’t cover the entire frequency range at once, we had to split the observations, so we ended up with about 70 minutes of “on the Moon” time per frequency channel.
This was sufficient to demonstrate that our process was working and could measure the global signal from the Milky Way’s foreground sky – it was about 10,000 times brighter than the expected global signal of 21cm.
This interview originally appeared in the November 2022 issue of BBC Sky at Night Magazine.
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