In the aftermath of the Big Bang, the Universe entered a phase known as the “cosmic dark ages,” spanning the first few hundred million years before the formation of stars and galaxies. This era presents a formidable challenge for astronomers, as there were limited luminous sources to illuminate this primordial period.
However, neutral hydrogen became a significant focus during this time. Dominating the cosmos, hydrogen remains the most prevalent element in the Universe. While nearly all hydrogen today exists in an ionized state as a hot plasma, it was neutral and cool prior to the emergence of the first stellar bodies.
Neutral hydrogen comprises a single proton and an electron, each exhibiting a quantum characteristic known as spin, which—while similar to everyday spinning—differs fundamentally at the quantum level. In its lowest energy state, the proton and electron spins align in opposite directions. Occasionally, through spontaneous quantum fluctuations, the electron can flip its orientation. When this happens, the hydrogen atom quickly induces the electron to revert to its original state, releasing energy in the form of a photon with a 21-centimeter wavelength.
This quantum transition, although rare, can produce a significant signal when enough neutral hydrogen is present. Indeed, the detection of 21-cm radiation has been instrumental in astronomical studies, particularly in mapping the distribution of cold gas throughout the Milky Way galaxy.
Thus, despite its name, the cosmic dark ages are not devoid of light; vast clouds of primordial neutral hydrogen were emitting substantial amounts of 21-cm radiation during this time. However, this radiation originates from more than 13 billion years ago. As it traverses the immense distances of the expanding Universe towards Earth, it undergoes redshift, altering its spectral properties.
