massdominated

Massdominated is a term used in theoretical physics to describe a regime in which the dynamics of a system are governed primarily by mass-related terms, such as gravitational effects from mass density, rather than by other forms of energy or pressure. In this sense, massdensity-dominated dynamics mean that gravity sourced by mass dominates over radiation, magnetic fields, or pressure forces.

In cosmology, the concept is reflected in the idea of a matter-dominated (or mass-dominated) epoch. When the energy density of non-relativistic matter exceeds that of radiation and dark energy for a substantial period, the expansion and structure growth are governed mainly by matter. In a simplified, matter-dominated Friedmann model, the scale factor evolves roughly as a(t) ∝ t2/3, highlighting the central role of mass density in the dynamics.

In astrophysical systems, massdominated dynamics occur when the gravitational potential is set primarily by the mass distribution, such as stars and dark matter halos governing galaxy rotation curves, or when accretion flows near compact objects are governed mainly by gravity rather than pressure gradients. In these contexts, the equations of motion and gravitational potential simplify because mass terms overwhelm other energy or stress contributions.

Mathematically, a massdominated regime can be described by comparing energy densities or forces: if ρm is much larger than contributions from radiation, magnetic energy, or pressure, then gravity-to-pressure ratios are high and Poisson’s equation for the gravitational potential provides the leading-order description. The precise boundary of “massdominated” is not fixed and can vary by field and problem.

See also: matter-dominated era, gravity-dominated dynamics, cosmology, astrophysical accretion.