Baryon bias and structure formation in an accelerating universe

In most models of dark energy the structure formation stops after the accelerated expansion begins. In contrast, we show that the coupling of dark energy to dark matter may induce the growth of perturbations even in the accelerated regime. In particular, we show that this occurs in the models proposed to solve the cosmic coincidence problem, in which the ratio of dark energy to dark matter is constant. Depending on the parameters, the growth may be much faster than in a standard matter-dominated era. Moreover, if the dark energy couples only to dark matter and not to baryons, as requested by the constraints imposed by local gravity measurements, the baryon fluctuations develop a constant, scale-independent, large-scale bias which is in principle directly observable. We find that a lower limit to the baryon bias $b>\mathrm{}0.5\mathrm{}$ requires the total effective parameter of state $w_e=1+p/\rho$ to be larger than $0.6$ while a limit $b>\mathrm{}0.73\mathrm{}$ would rule out the model.