underdense

Underdense is an adjective describing something that has density lower than a reference density, typically the surrounding medium or a standard or mean density. The term is used across disciplines to denote regions, materials, or plasmas where density is below that reference.

In cosmology and astrophysics, underdense regions of the universe are called voids. These voids have matter densities well below the cosmic mean and occupy a large fraction of the volume, even as they contain a smaller fraction of its mass. They arise from the gravitational evolution of initial density fluctuations: matter evacuates from underdense zones into surrounding filaments and walls, producing a sponge-like network. Observationally, voids are identified in galaxy redshift surveys and studied via their effects on light, such as the integrated Sachs-Wolfe effect, and through weak gravitational lensing.

In plasma physics and laser–matter interaction, underdense plasma refers to a plasma whose electron density is below the critical density for a given electromagnetic wave frequency. In such plasmas, laser radiation can propagate and drive phenomena like laser wakefield acceleration. In contrast, overdense plasmas reflect incident radiation and short-circuit propagation. The concept is central to high-intensity laser experiments and inertial confinement fusion research.

Outside physics, underdense describes any region or material with density lower than a reference, including porous or foam-like structures within solids, where porosity reduces effective density relative to the bulk material.