Probing the coupling between dark components of the universe

We place observational constraints on a coupling between dark energy and dark matter by using 71 Type Ia supernovae (SNe Ia) from the first year of the five-year Supernova Legacy Survey (SNLS), the cosmic microwave background (CMB) shift parameter from the three-year Wilkinson Microwave Anisotropy Probe (WMAP), and the baryon acoustic oscillation (BAO) peak found in the Sloan Digital Sky Survey (SDSS). The interactions we study are (i) constant coupling δ and (ii) varying coupling δ(z) that depends on a redshift z, both of which have simple parametrizations of the Hubble parameter to confront with observational data. We find that the combination of the three databases marginalized over a present dark energy density gives stringent constraints on the coupling, -0.08<δ<0.03 (95% C.L.) in the constant coupling model and -0.4<δ0<0.1 (95% C.L.) in the varying coupling model, where δ0 is a present value. The uncoupled ΛCDM model (wX=-1 and δ=0) still remains a good fit to the data, but the negative coupling (δ<0) with the equation of state of dark energy wX<-1 is slightly favored over the ΛCDM model.