ABSTRACT
The terahertz (THz) portion of the electromagnetic spectrum (ν ~ 1-10 THz; λ ~ 30030 μm) provides us with a powerful window into cosmic evolution. THz photons arriving at Earth can yield valuable insights into everything from the birth and death of stars to the cataclysmic events associated with the origin of galaxies and the Universe itself. Most of the THz photons we observe are emitted by the gas and dust between the stars, that is, the interstellar medium (ISM). At THz frequencies we can observe photons associated with the ISM of our own galaxy, the Milky Way, as well as from the ISMs of distant galaxies. In this chapter, we will examine the ISM and introduce some of the THz observational probes that can be used in discerning its nature.
