Measuring q_0 from the Distortion of Voids in Redshift Space

Because the transformation from distance to redshift is nonlinear, maps in redshift space become increasingly distorted as the redshift z becomes greater. As noted by Alcock \& Paczy\'nski (1979), observed redshift distortions can be used to estimate the deceleration parameter $q_0$. If $q_0$ is greater than -1, voids in redshift space will be elongated along the line of sight. In addition, distant voids will have a greater volume in redshift space than nearby voids. Accurate measurement of the volume and the axis ratio of voids, as a function of their central redshift, will provide an estimate of $q_0$. To test this method of estimating $q_0$, I create a two-dimensional toy universe, free of peculiar velocities, in which the galaxies are located near the walls of Voronoi cells. The galaxies are then mapped into redshift space, adopting different values of $q_0$. In redshift space, I estimate the area and the axis ratio of the voids by fitting ellipses within the voids, using an algorithm which maximizes the area of the empty ellipses and ensures that ellipses do not overlap. The accuracy of the estimated values of $q_0$ is limited by the intrinsic scatter in the size and shape of the voids. In the toy universe, distinguishing between a $q_0 = -1$ universe and a $q_0 = 1/2$ universe requires a survey which goes to a depth z > 0.1 in redshift space. Peculiar velocities will create an additional source of uncertainty for the values of $q_0$ measured in N-body simulations and in the real universe.