ROLES OF A COHERENT SCALAR FIELD ON THE EVOLUTION OF COSMIC STRUCTURES

A coherently oscillating scalar field, an axion as an example, is know n to behave as a cold dark matter. The arguments were usually made in the Newtonian context. Ratra proved the case in relativistic context u sing the synchronous gauge. In this Letter we present another proof ba sed on a more suitable gauge condition, the uniform-curvature gauge, w hich fits the problem. By a proper time averaging the perturbed oscill ating scalar field behaves as a cold dark matter on the relevant scale s including the superhorizon scale. (C) 1997 Elsevier Science B.V.