On the Effects of Coupled Scalar Fields on Structure Formation

A coupling between a scalar field (representing the dark energy) and dark matter could produce rich phenomena in cosmology. It affects cosmic structure formation mainly through the fifth force, a velocity-dependent force that acts parallel to particle's direction of motion and proportional to its speed, an effective rescaling of the particle masses, and a modified background expansion rate. In many cases these effects entangle and it is difficult to see which is the dominant one. Here we perform N-body simulations to study their qualitative behaviour and relative importance in affecting the key structure formation observables, for a model with exponential scalar field coupling. We find that the fifth force, a prominent example of the scalar-coupling effects, is far less important than the rescaling of particle mass or the modified expansion rate. In particular, the rescaling of particle masses is shown to be the key factor leading to less concentration of particles in halos than in LCDM, a pattern which is also observed in previous independent coupled scalar field simulations.