Negative Running of the Spectral Index, Hemispherical Asymmetry and the Consistency of Planck with Large $r$

Understand

Planck favours a negative running of the spectral index, with the likelihood being dominated by low multipoles $l \lesssim 50$ and no preference for running at higher $l$.

• A negative spectral index is also necessary for the 2-$\sigma$ Planck upper bound on the tensor-to-scalar ratio $r$ to be consistent with values significantly larger than 0.1.
• Planck has also observed a hemispherical asymmetry of the CMB power spectrum, again mostly at low multipoles.
• Here we consider whether the physics responsible for the hemispherical asymmetry could also account for the negative running of the spectral index and the consistency of Planck with a large value of $r$.