The Spectral Index of a source of Electromagnetic Radiation is a scalar measure describing its spectrum, assuming the spectrum forms a Power Law. It is specifically the exponent of the Frequency that produces a term proportional to the Radiant Flux at the same frequency, i.e.,
S ∝ να
Confusingly, sometimes the term is used to mean the negative of this exponent, or equivalently, the analogous exponent of the Wavelength.
In cases where the radiant flux by frequency is not a power law, it can be said that the spectral index varies by frequency. The concept seems less useful in this case.
Lambda-CDM model (ΛCDM)