Dynamics of voids and their shapes in redshift space

We investigate the dynamics of a single spherical void embedded in a Friedmann- Lema^ tre universe, and analyze the void shape in the redshift space. We nd that the void in the redshift space appears as an ellipse shape elongated in the direction of the line of sight (i.e., an opposite deformation to the Kaiser eect). Applying this result to observed void candidates at the redshift z 1-2, it may provide us with a new method to evaluate the cosmological parameters, in particular the value of a cosmological constant.