Radio Astronomy on the Moon

So I’ve been able to sit in on a video-conference for a group that is putting out a plan for a low frequency radio telescope on the Moon. I’m just a wee student, so I observed quietly. There are a lot of technical challenges, and the science is fascinating and there is SO MUCH to think of and to do. And every once in a while, I couldn’t help but think, “DUDE, we’re talking about building things on the MOON.” How awesome is that? We are seriously considering a Lunar Base and astronaut involvement and lunar topography and lunar gravity and just WOW. The timescales in question are on the order of 20 years or so. Going to the Moon is fascinating enough, but going to do good science is even more impressive.

The reason for going to the Moon for this kind of experiment is all because of the Earth’s atmosphere. Specifically, the ionosphere, the layer of atmosphere where most of the atoms have been ionized. The plasma of free electrons becomes opaque to radiation at frequencies lower than about 10 MHz. This corresponds to very long wavelength radio radiation, or somewhere above and around the AM radio stations. The waves of light are actually on the order of meters in length! In fact, the ionosphere can be used like a mirror to send signals around the curve of the Earth. But why do we want to observe the universe at such long wavelengths?

My research involves the search for the signal of the epoch of reionization. Yeah, that’s a mouthful. In the past decade or so, we’ve been flooded with information about the Cosmic Microwave Background (CMB), or the microwave light from the period of time 300,000 years after the Big Bang. At this time, the universe was mostly hydrogen, with a bit of helium and a smattering of “other.” The universe was also mostly uniform, but not quite, since tiny fluctuations, which we see imprinted on the CMB, were the seeds for large matter structures that would, 13.6 billion years later, form things like galaxies where we live!

So we know that the universe was pretty uniform then, and we know about galaxies that exist today and a few billion years ago. But what happened in between? What were the first stars like? How did the first galaxies form? What we do know is that the neutral hydrogen universe of old has given way to a completely ionized universe. That is, the ultraviolet radiation from those first objects had to ionize hydrogen between the galaxies, splitting it into protons an electrons. We know roughly when that happened, but we don’t know all the details yet. What we can do is image the neutral hydrogen from early times and watch as it disappears, first in little bubbles around the first objects, later expanding into a web until the entire intergalactic medium is ionized!

Now, hydrogen has a spectral line at 1.4 GHz. However, due to the expanding universe, this signal is redshifted, or, it appears at longer wavelengths (lower frequencies) depending on how far back in time you look. The hydrogen should be going through it’s most rapid ionization at times that correspond to frequencies between ~100 and 200 MHz. This is where many ground based experiments are looking. However, to probe the early reionization epochs, as well as the intrinsic structure of the neutral hydrogen before the stars formed, we need to probe even lower frequencies, down to 10s of MHz or lower! Hence, we need to leave the Earth to avoid the ionosphere. Next stop, Moon! Not to mention, we get away from all that pesky man-made radio interference.

So anyway that’s a lot of information, and I’m always willing to chew someone’s ear off about it. For this meeting that I attended, the antenna design that is being drafted involves at least 400 of these things being spread out across an area of lunar surface. In talking about the need for a robot to deploy these, a simple robot with one function, I immediately thought of

WALL-E! Aww he’s so cute.

So, 2025? I’ll be about 40, and my eyesight will have only gotten worse. Think they’ll still send me? I’ll spend my time studying ground-based arrays and the ionosphere in the meantime.

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