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.
