Well, I'm back to regular life. All of the summer educational programs are over here in Green Bank. While that means I can now come home before bunkhouse bedtime, it also means that I no longer get to count 'chaperoning the pool recreation time' as work.





Transition.





This post is going to be about a few things:

1. The Epoch of Reionization

2. Neutral Hydrogen

3. Cosmological redshift

4. Interferometers

5. The PAPER (Precision Array to Probe the Epoch of Reionization) project





Walk with me.





EoR: Pronounce it 'Eeyore'; we'll start a trend

The epoch of reionization, an overview of which can be found here , is the long-ago time when the plasma--one of the universe's main ingredients--went from being largely neutral to being largely ionized, due to interactions with photons. The EoR is important because once the photons had ionized the plasma, they could travel through it (to our telescopes) instead of being eaten for their energy. Without reionization, our telescopes wouldn't be able to see anything.





The EoR is also one of the earliest times we can ever hope to see, since photons from 300,000 years ABB < t < 1 billion years ABB (ABB=after Big Bang) never made it out of the universoup. And looking as far back toward the Big Bang as possible is a pretty important and interesting thing to do, if you ask me, which I guess you didn't.





Neutral Hydrogen: It's Everywhere All the Time

Hydrogen: the Smith of the universe. The iPod of elements. The Starbucks of atoms. Wherever you go, there it is.





Hydrogen emits light at a frequency of 1420.41 MHz because of its hyperfine transition (see this post ). By observing this transition with radio telescopes, we can learn about the distribution of matter in the universe.





Cosmological Redshift: Zipping Away

But we don't just see Hydrogen at 1420.41 MHz, because most of the hydrogen in the universe is moving in some direction relative to us. Relative motion causes the observed frequency of a wave to change: if it's coming toward you, the waves get squished together, and the frequency appears higher (blueshift); if it's going away from you, the waves get stretched out, and the frequency appears lower (redshift). Outside of the Milky Way, it's all moving away from us because the universe itself is expanding. You can tell how far away a galaxy is by determining how redshifted its hydrogen is. More often, though, astronomers know a distance and use a given redshift to see what they see (for more on cosmological redshift, see here ).





The farther out you look, the faster objects are receding from us.





The farther out you look, the further back in time you're seeing, because that light had to travel here, and it can only travel at the speed of light.









Neutral Hydrogen: Coming to a Clear Channel station near you.



So if you look far enough back/out, thought some scientists, you might be able to see signatures of hydrogen atoms in the early universe, during the EoR. By knowing more about the distribution of neutral hydrogen when the first stars and galaxies were in the process of forming, we can learn about the ways that the first stars and galaxies formed and, thus, about how the universe came to be the way it is today.





So how far is this hydrogen redshifted? An unbelievable amount: down to ~100 MHz. Like the RADIO, you guys. Like Magic 107.7 Lite Rock Less Talk. That's how fast it's moving, that's how far away it is, that's how old it is.













Interferometers: More is Better

By pointing a bunch of smaller telescopes at the same object at the same time, you can synthesize, effectively, a single telescope that has the diameter of the largest separation between two small telescopes.





So if you have 15 telescopes spread out over your backyard, your neighbors would freak out, and the longest line you could draw between two telescopes would be the diameter of your synthesized telescope.





In order for this to work, you have to 'correlate' the signals, which happens in a correlator. It seems a lot like magic when you give a one-word name to something so complicated.





Young's Double-Slit Experiment: When you shine a light through two slits at the same time, you don't get two dots of light at the other end: you get an interference, or fringe, pattern. Why?









Results of Young's Double-Slit Experiment, which is not a euphemism.

Source.



Light is not just a particle that passes straight through the slit--it is also a wave that waves through the slit. So the waves from both slits constructively and destructively interfere as they come out the other side.





The same thing, basically, happens in an interferometer. You compare the waves arriving at one telescope (element) of your interferometer to the waves arriving at another element of your interferometer, and you get an interference pattern.





Rinse and repeat for combinations of elements; these combinations are known as 'baselines' (for your 15-element backyard interferometer, you would have 210 baselines to work with, and if your interferometer is smartly distributed, you will get a fairly complete image of wherever you were pointing).









PAPER Project: Not just PVC Pipe





One of many PAPER antennas. Source.











There are 32 of these antennas in Green Bank, WV, right now, and as of more recently, there are 32 in Karoo, South Africa. The idea is that, eventually, there will be a 128-element interferometer that can image the whole sky at these wavelengths once per day.





Preliminary results from the Karoo array have produced a map of the 110-180 MHz sky. Efforts are being made to identify foreground sources (objects that are not neutral hydrogen from long ago and far away but just happen to emit radio waves at these frequencies) by comparing PAPER images to catalogues of known objects and removing their flux contributions, leaving the image with only EoR hydrogen.





To read more about this project, visit Nicole Gugluicci's blog: One Astronomer's Noise . She was in South Africa doing the commissioning of the 32-element array, including stringing the coaxial cable from antennae-to-correlator herself.





Here is a paper about PAPER's instrumentation specifics.





Here is a paper about the early science results.













The Astronomical Journal, 139 (4), 1468-1480 DOI: Parsons, A., Backer, D., Foster, G., Wright, M., Bradley, R., Gugliucci, N., Parashare, C., Benoit, E., Aguirre, J., Jacobs, D., Carilli, C., Herne, D., Lynch, M., Manley, J., & Werthimer, D. (2010). THE PRECISION ARRAY FOR PROBING THE EPOCH OF RE-IONIZATION: EIGHT STATION RESULTS(4), 1468-1480 DOI: 10.1088/0004-6256/139/4/1468

Jacobs, D., Aguirre, J., Parsons, A., Pober, J., Bradley, R., Carilli, C., Gugliucci, N., Manley, J., van der Merwe, C., Moore, D., & Parashare, C. (2011). NEW 145 MHz SOURCE MEASUREMENTS BY PAPER IN THE SOUTHERN SKY The Astrophysical Journal, 734 (2) DOI: 10.1088/2041-8205/734/2/L34