Cold neutral hydrogen can be detected because a. it emits light when electrons drop through energy levels. b. it blocks the light from more distant stars. c. it is always hot enough to glow in the radio and infrared wavelengths. d. the atoms change spin states.

21 cm radiation is a type of radio wave that is produced by cold neutral hydrogen atoms. These radio waves have a wavelength of 21 centimeters, which corresponds to a frequency of around 1420 MHz.
This radiation is very important in astronomy because it allows us to map hydrogen distribution in the Milky Way and other galaxies. It specifically helps to identify regions of cold neutral hydrogen, which is not easily visible by other means.
When astronomers detect 21 cm radiation, they can infer the presence of hydrogen gas in space, as well as measure its velocity and distribution. This helps to understand galactic structures and the mechanics of star formation.

The hyperfine transition is a specific energy change within an atom. For hydrogen, this occurs when the electron's spin state aligns or misaligns with the proton's spin state. This tiny energy difference leads to the emission or absorption of 21 cm radiation.
When an electron in a hydrogen atom flips its spin state, the atom undergoes a hyperfine transition, releasing energy in the form of a photon. This photon has the characteristic wavelength of 21 cm. Because this transition happens very rarely (around once every 10 million years for a single atom), large clouds of hydrogen are needed to produce detectable levels of 21 cm radiation.
The hyperfine transition is significant in detecting cold neutral hydrogen because it does not require high temperatures or higher energy levels.

In the context of hydrogen atoms, spin states refer to the orientations of the spins of the proton and the electron within the atom. An atom's proton and electron can have spins that are either aligned (parallel) or opposed (antiparallel).
When the proton and electron spins are aligned (parallel), the atom is in a slightly higher energy state compared to when the spins are opposed (antiparallel). The transition from a higher energy spin state to a lower energy spin state results in the emission of a photon with the 21 cm wavelength.
Cold neutral hydrogen is typically in this lower energy state. Detection of the 21 cm radiation thus involves observing the transitions between these spin states, allowing astronomers to map areas rich in neutral hydrogen in the universe.