Constraints on Scalar Spectral Index from Latest Observational Measurements

Recently, the nine-year data release of the Wilkinson Microwave Anisotropy Probe (WMAP9) found that the inflationary models with the scalar spectral index n_s \geq 1 are excluded at about 5\sigma confidence level. In this paper, we set the new limits on the scalar spectral index in different cosmological models combining the WMAP9 data with the small-scale cosmic microwave background measurement from the South Pole Telescope, baryon acoustic oscillation data, Hubble Telescope measurements of the Hubble constant, and supernovae luminosity distance data. In most of extended cosmological models, e.g. with a dark energy equation of state, the constraints on n_s do not change significantly. The Harrison-Zel'dovich-Peebles (HZ) scale invariant spectrum is still disfavored at more than 4\sigma confidence level. However, when considering the model with a number of relativistic species N_{eff}, we obtain the limit on the spectral index of n_s=0.980\pm0.011 (1\sigma), due to the strong degeneracy between n_s and N_{eff}. The HZ spectrum now is still consistent with the current data at 95% confidence level.