Poynting–Robertson (P–R) drag is the relativistic effect by which orbiting dust grains absorb and re-emit stellar radiation, lose angular momentum, and slowly spiral inward toward the star (Robertson 1937; Wyatt & Whipple 1950). In planetary systems, detailed modeling shows that P–R drag, radiation pressure, solar wind, and collisions must all be balanced to explain observed dust belts and zodiacal clouds, and small mismatches between predicted and observed dust distributions or inspiral timescales can expose uncertainties in grain properties and dynamical histories rather than cleanly testing ΛCDM itself (Klacka 2004; Lamy et al. 1998).