Infrared observations explore the hidden Universe. Cosmic dust particles effectively obscure parts of the visible Universe and block our view of many critical astronomical environments. Infrared observations access a wealth of spectral features. Emission and absorption bands of virtually all molecules and solids lie in the infrared, where they can be used to probe conditions in cool celestial environments. Many atoms and ions have spectral features in the infrared which can be used for diagnostic studies of stellar atmospheres and interstellar gas, exploring regions which are too cool or too dust-enshrouded to be reached with optical observations.
Infrared observations reach back to the early life of the cosmos.
The cosmic redshift which results from the general expansion of the Universe inexorably shifts energy to longer wavelengths in an amount proportional to the object's distance. Because of the finite speed of light, objects at high redshift are observed as they were when the Universe and the objects were much younger. As a result of the expansion of the Universe, most of the optical and ultraviolet radiation emitted from stars, galaxies, and quasars since the beginning of time now lies in the infrared. How and when the first objects in the Universe formed will be learned in large part from infrared observations. Apart from a few narrow windows at the shortest wavelengths, all of the infrared radiation emitted by the above objects is absorbed by Earth's atmosphere.