n0

n0 is a symbolic label used across disciplines to denote a reference value or a quantity with a zero subscript. Its exact meaning is not fixed and depends on the context of the field or model.

In nuclear and condensed-matter physics, n0 often denotes a reference or saturation density. In nuclear physics, nuclear saturation density is commonly given as n0 ≈ 0.16 fm^-3, used as a standard scale in equations describing nuclear matter. In many models, the total number density is written as n = n0 + δn, separating a reference density from fluctuations.

In the study of Bose-Einstein condensation and superfluidity, n0 frequently represents the condensate density—the density of particles occupying the zero-momentum ground state. The total particle density is then the sum of the condensate density n0 and the density of non-condensed particles.

In cosmology and astrophysics, n0 commonly denotes the present-day number density of a particular species. For example, one often writes n_b0 for the current baryon number density or n_gamma0 for the current photon number density (n_gamma0 is about 410 cm^-3).

In other contexts, n0 may mark a starting index, a threshold, or a baseline value in equations and models. Because the meaning of n0 is field- and model-dependent, it is important to consult the specific source to determine what n0 represents in a given case.