DARK MATTER-DARK ENERGY COUPLING BIASING PARAMETER ESTIMATES FROM COSMIC MICROWAVE BACKGROUND DATA

When cosmic microwave background data are used to derive cosmological parameters, their very choice does matter: some parameter values can be biased if the parameter space does not cover the "true" model. This is a problem because of the difficulty in parameterizing dark energy (DE) physics. We test this risk through numerical experiments. We create artificial data for dynamical or coupled DE models and then use Monte Carlo Markov Chain techniques to recover model parameters by assuming a constant DE state parameter w and no DM-DE coupling. For the DE potential considered, no serious bias arises when coupling is absent. In contrast, omega(o,c) and thence H-o and Omega(o,m) suffer a serious bias when the "true" cosmology includes even just a mild DM-DE coupling. Until the nature of the dark components remains unknown, it can be important to allow for a degree of freedom accounting for DM-DE coupling, even more than increasing the number of parameters accounting for the w(a) behavior.