Probing the primordial Universe from the low-multipole CMB data

Since the temperature fluctuations in the cosmic microwave background (CMB) on large-angular scales probe length scales that were super-horizon sized at photon decoupling and hence insensitive to microphysical processes, the low-multipole CMB data are supposed to be a good probe to the physics of the primordial Universe. In this letter we will constrain the cosmological parameters in the base ΛCDM model with tensor perturbations by only using the low-multipole CMB data, including Background Imaging of Cosmic Extragalactic Polarization (B2), Planck data released in 2013 (P13) and Wilkinson Microwaves Anisotropy Probe 9-year data (W9). We find that either sign of the index of the tensor power spectrum is compatible with the data, but a blue-tilted power spectrum of scalar perturbations on large scales is preferred at around 2σ confidence level.