  If the voids that we see today in the distribution of galaxies existed at
recombination, they will leave an imprint on the cosmic microwave background
(CMB). On the other hand, if these voids formed much later, their effect on the
CMB will be negligible and will not be observed with the current generation of
experiments. In this paper presented at the 2004 Annual Scientific Meeting of
the Astronomical Society of Australia, we discuss our ongoing investigations
into voids of primordial origin. We show that if voids in the cold dark matter
distribution existed at the epoch of decoupling, they could contribute
significantly to the apparent rise in CMB power on small scales detected by the
Cosmic Background Imager (CBI) Deep Field. Here we present our improved method
for predicting the effects of primordial voids on the CMB in which we treat a
void as an external source in the cold dark matter (CDM) distribution employing
a Boltzmann solver. Our improved predictions include the effects of a
cosmological constant (Lambda) and acoustic oscillations generated by voids at
early times. We find that models with relatively large voids on the last
scattering surface predict too much CMB power in an Einstein--de Sitter
background cosmology but could be consistent with the current CMB observations
in a Lambda-CDM universe.

Astrophysics