The cosmic microwave background radiation (CMB) is the remnant radiation from the early universe, which has been redshifted and cooled down over billions of years. It was first detected in 1965 and is a key piece of evidence for the Big Bang theory.
The wavelengths of the CMB are indeed stretched out due to the expansion of the universe, resulting in the radiation being predominantly in the microwave region of the electromagnetic spectrum. The CMB has been extensively studied, and while it does not exhibit distinct spectral lines like atomic or molecular spectra, it does possess a specific pattern known as the blackbody spectrum.
The blackbody spectrum of the CMB closely matches that of a perfect blackbody radiator at a temperature of approximately 2.7 Kelvin (-270.45 degrees Celsius or -454.81 degrees Fahrenheit). This spectrum represents the thermal radiation emitted by a uniform object at that temperature.
The absence of spectral lines in the CMB is primarily due to its origin and composition. The CMB radiation originated when the universe transitioned from being opaque to transparent, approximately 380,000 years after the Big Bang. At this point, the universe cooled enough for atoms to form, mostly hydrogen and helium. The radiation we observe as the CMB is a relic of this early time, when the universe was a hot, dense plasma.
Since the CMB is the result of a primordial event rather than the emission from specific atomic or molecular transitions, it does not possess spectral lines in the same way as the light from distant stars or galaxies. The CMB is characterized by its thermal spectrum, which provides valuable insights into the early universe's properties, such as its temperature and composition, as well as supporting the Big Bang cosmological model.