Voids in coupled scalar field cosmology

We study the properties of voids in two different types of coupled scalar field theories. Due to the fifth force produced by the scalar field coupling, the matter particles feel stronger attraction amongst each other and cluster more quickly than they do in the standard ΛCDM model. Consequently, voids in the coupled scalar field theories start to develop earlier and end up bigger, which is confirmed by our numerical simulations. We find that a significantly larger portion of the whole space is underdensed in the coupled scalar field theories and there are more voids whose sizes exceed given thresholds. This is more prominent in early times because at later times the underdense regions have already been evacuated in coupled scalar field theories and there is time for the ΛCDM model to catch up. The coupled scalar field theories also predict a sharper transition between voids and high-density regions. All in all, the qualitative behaviour is different not only from the ΛCDM result, but also amongst specific coupled scalar field models, making voids a potential candidate to test alternative ideas about the cosmic structure formation.