Astrophysical spectral lines offer two important insights into the workings of our Universe. First, they are probes of the fundamental (QM) nature of matter because they originate from subatomic, atomic and molecular systems. Second, they provide, via the Doppler effect, critical dynamical information on astrophysical systems ranging in scale from planetary systems to superclusters of galaxies. Examples of major contemporary problems in astrophysics that can be addressed through spectral line studies and the associated quantum mechanics include.

Missing mass and the halos of galaxies The most common element in the Universe is hydrogen and much of it is in a cold state. Given the 10 eV gap between the ground state and the first excited state of the simple Bohr atom, we should have little direct knowledge of this gas, yet it is the best studied gaseous component of the Universe. The reason is the 21 cm line corresponding to the hyperfine splitting of the ground state. The extremely low transition probability of this transition and the consequently narrow width of this line have led to its widespread use in measuring galaxy dynamics and kinematics. Studies of galaxy rotation have shown evidence for missing matter and point to the possibility of dark-matter halos. The nature of the dark matter and the implication on the long-term fate of the Universe remain contentious issues in astrophysics. The nature of this line and its use in these studies is discussed.