We study the nonequilibrium relaxational dynamics of a probe particle
linearly coupled to a thermally fluctuating scalar field and subject to
a harmonic potential, which provides a cartoon for an optically trapped
colloid immersed in a fluid close to its bulk critical point. The
average position of the particle initially displaced from the position
of mechanical equilibrium is shown to feature long-time algebraic tails
as the critical point of the field is approached, the universal
exponents of which are determined in arbitrary spatial dimensions. As
expected, this behavior cannot be captured by adiabatic approaches which
assumes fast field relaxation. The predictions of the analytic,
perturbative approach are qualitatively confirmed by numerical
simulations.
