This project (along with this followup) is very near and dear to my heart because it was my first foray into research and into cosmology. It was also one of the most fun things I got the opportunity to do in undergrad both because of the fantastic people and because of the really neat science goal, which was to try to make technology that will someday allow us to observationally see the formation of the first galaxies in the universe.
Trying to look at the formation of the first galaxies is no easy task. To illustrate why, let me walk you through some of the history of the universe around that time. So in the very beginning, the universe was incredibly dense and hot-- so hot that neutral atoms couldn’t form. If an electron and a proton combined to form a hydrogen atom, there was so much thermal energy available in the immediate surroundings that the atom would be pretty much instantly ionized. Thus, the universe was filled with a plasma which was opaque to light, since ionized particles are very good at scattering light. But the universe didn’t stay this way. Over time, it expanded and things started to cool down. Eventually, it became cool enough that there was a phase transition and neutral atoms could form. These neutral atoms were mostly hydrogen and helium gas, which is transparent to light. So once the universe underwent this phase transition, suddenly all the light that had been constantly scattered by the plasma was able to stream through the neutral gas. This light is known as the cosmic microwave background (CMB) and we’ve studied it extensively. But then what happened? Well we had all this gas lying around and no stars or galaxies had formed yet. So it seems like it was a pretty boring time-- except it wasn’t! Gravity was still doing its thing behind the scenes, collapsing matter right up to the point where it was dense enough to ignite and become a star. But we can’t see this going on because before the stars hadn’t formed yet and so there is nothing for us to point our telescopes to. It’s kind of like the CMB is a baby picture of the universe and early galaxies give us photos of the adult universe, but we have no pictures from the period in between.
That’s where 21 cm cosmology comes in! It turns out that due to quantum mechanics, neutral hydrogen gas does emit some radiation in the form of radio waves which we can potentially observe using radio telescopes. When the light gets emitted at a wavelength of 21 cm, it gets redshifted because the universe is expanding while the light is travelling to earth. So by looking at the redshifting, we can actually tell how long the light has been travelling, which means we know how far away the source was. So we can use the 21 cm line to actually make a 3D map of the hydrogen. Eventually, as stars and galaxies form, they emit ionizing radiation which makes the neutral hydrogen ionized again and it stops emitting 21 cm radiation… This would effectively end up looking like swiss cheese in our map, and we could use this to actually see the first stars and galaxies “turn on” as they begin emitting this high energy ionizing radiation. There’s also a great wealth of information we can learn from this technique about what the first galaxies were like and how they formed.
Nobody has ever detected this signal, because it’s coming from billions of light years away so it’s very faint. But different experiments are getting closer and closer to a detection, and we were a part of that. We worked on novel new techniques for doing radio astronomy using our prototype experiment which we deployed in northern Maine. Our site, near "The Forks," was chosen because it is very “radio quiet” i.e. there isn’t a lot of human-made radio interference. I have to say, I feel that it was quite an honor for me to participate in this endeavor, and needless to say it will be very exciting when one day this signal is discovered and we can look at our universe’s childhood.
With Max Tegmark and the OmniGang aka the MITEoR collaboration