Observational constraints on phantomlike braneworld cosmologies

We investigate a simple braneworld model in which the universe contains only cold dark matter and a cosmological constant, but the effective dark energy is phantomlike because of extra-dimensional gravity effects. Modified gravity screens the cosmological constant Lambda, allowing for a larger Lambda. In practice, observations do not favor any significant screening. We use supernova data, the cosmic microwave background shift parameter, and the baryon oscillation peak in the galaxy distribution to constrain the model. We find the mean value of Omega(m) with 68% confidence limits, and an upper limit on Omega(Lambda) at the 68% confidence level. The best-fit model is very close to a standard LCDM model, but the LCDM model provides a better fit since it has one less parameter.