The 21 cm hydrogen line is electromagnetic radiation emitted by neutral hydrogen atoms that have their electron’s spin aligned with the proton’s spin. This spectral line falls within the radio frequency range and has a wavelength of approximately 21 centimetres (hence the name).
The origin of the 21 cm hydrogen line can be traced back to the properties of the hydrogen atom. The electron in a hydrogen atom can be in one of two possible energy states: spin-up or spin-down. The proton in the nucleus also has a spin, and when the electron and proton have their spins aligned, the atom is said to be in the “parallel” state. Conversely, when the electron and proton spins are opposite, the atom is said to be in the “anti-parallel” state.
When a hydrogen atom transitions from the parallel to the anti-parallel state (or vice versa), it emits a photon with a wavelength of 21 cm. This transition is relatively rare, but since hydrogen is the most abundant element in the universe, the 21 cm line can be detected from any direction in the sky.
The significance of the 21 cm hydrogen line lies in its usefulness for studying the structure and evolution of the universe. For example, astronomers can use this line to map out the distribution of neutral hydrogen gas in space, which provides insight into the formation and evolution of galaxies. The 21 cm line can also be used to measure the temperature and density of interstellar gas clouds, which are crucial for understanding the processes of star formation.
Additionally, the 21 cm line can be used to detect the effects of dark matter, as it provides a way to measure the gravitational forces acting on neutral hydrogen gas.
Source of Images – Hyperphysics