The Universe is the vast combination of space, time, energy and matter where we live. Even though we can interact barely with 5% of it, there is still much that we still do not understand about this small fraction. For centuries, we were limited by the only existence of optical observatories that allowed us to peer into the visible universe, but that small 5% still held a lot of invisible mysteries for us to find.
Radio-telescopes opened a new window for us to look into the most basic building pieces of the universe. The neutral hydrogen atom, also known as HI, was the first chemical element to exist in the Universe, and remains the most abundant. However, it was not until 1951 that astronomers Harold Ewen and Edward Purcell observed it for the first time, using a radio-antenna sensitive to its energy emission.
The reason why the most common element in the universe is invisible lies in its structure. The HI atom is made of a proton and an electron orbiting around it. Much as the Moon around the Earth, the electron and the proton also rotate around themselves. The direction of this inner rotation is known as spin. If the spin of proton and electron is the same, the HI atom can store more energy than when the spins are opposite. When they spontaneously unalign, the HI loses one photon of energy, an elemental light particle with a wavelength of 21cm and a frequency of 1.42GHz, can traverse through the dusty regions that obscure galaxies such our Milky Way, through our atmosphere, and be detected by radioastronomers
employing antennas during night and day.
HI makes up for most of the material inside galaxies, and is the raw component needed to form stars. These stars will later reprocess the hydrogen to transform it into other chemical elements, so in a way neutral hydrogen can be considered as the parent of the rest of chemical elements in the universe. Since then, this emission has become the backbone of radioastronomy. It has allowed us to reveal the spiral structure of the Milky Way and how it exchanges gas with its neighbours. It was even employed back in 1959 as the first modern SETI study to search for advanced extraterrestrial civilizations. In the next decade, new radio telescopes will be able to explore the neutral hydrogen present in bigger volumes of the Universe and retrace the evolution of matter in the cosmos back to earlier in its history.
MSc in Astrophysics and Particle Physics.
PhD candidate working in the field of Galaxy Evolution and Large Scale Structure.