Radio Astronomy: Travel back in time
Astrophysics as we know is the area of Science focusing on studying the objects and processes outside our planet Earth. It deals with Stars, Galaxies, how they are formed, how they evolve and how they eventually die. Perhaps a bigger question is how did we come into existence? To answer this, we have to step back in time and start with what we think is the beginning of the Universe: The big bang.
As the matter started forming, there were only fundamental particles such as quarks, which then came together to form protons, neutrons and electrons. With these charged particles, the Universe was ionized. These particles came together to form neutral hydrogen atoms. With more and more hydrogen coming together, the gravity started acting on the matter eventually leading to formation of stars. When these first stars started forming, they gave out heat and radiation which re-ionized the matter. This era in the evolution of the Universe is what we call Epoch of Re-ionization. Studying this era will help us answer many of those unanswered questions about the the evolution and history of the Universe.
So how do we actually study this era? Well, it’s a simple concept in the nutshell. When the heat and radiation was re-ionizing the matter, there was a particular interaction, that’s very interesting. We call it hydrogen spin-flip interaction, where electron in the neutral hydrogen atom jumped from one spin state to the other leading to the emission of photon at 1420MHz frequency or 21cm wavelength. Because this happened in the early Universe, and light takes finite amount of time to travel, studying this would actually mean looking at the past.
This coupled with the expansion of the Universe, the signal is actually red-shifted to a lower frequency in the radio frequency range of about 50–200 MHz. To put things in perspective, think of how a train horn gets louder or quieter depending on whether the train is moving toward or away from us. Redshift is the same idea of doppler-shift but extending to cosmic scales. The light rays is expanded by a factor of 6–20 by the time it travels across the Universe from when it was emitted in the early Epoch to when it gets detected by our telescope. The challenge however is that, this falls in the radio frequency range, which means it is dominated by the terrestrial FM stations and also by our own milky-way, so much so that our signal of interest is about 100,000 times fainter compared to other signals dominating this frequency range. So it is an engineering challenge to detect and study this signal.
There are multiple on-going projects aimed at measuring this signal from the early era of Epoch of re-ionization, but the motivation and the crux of the idea remains the same. More about these instruments soon, in another article!
