Alleviating the tension at lowℓthrough axion monodromy

Abstract

There exists some tension on large scales between the Planck data and the LCDM concordance model of the Universe, which has been amplified by the recently claimed discovery of non-zero tensor to scalar ratio $r$. At the same time, the current best-fit value of $r$ suggests large field inflation delta phi>M_p, which requires a UV complete description of inflation. A very promising working example that predicts large tensor modes and can be UV completed is axion monodromy inflation. This realization of inflation naturally produces oscillating features, as consequence of a broken shift symmetry. We analyse a combination of Planck, ACT, SPT, WMAP low l polarization and BICEP2 data, and show a long wavelength feature from a periodic potential can alleviate the tension at low multipoles with an improvement delta chi^2 ~2.5-4 per degree of freedom, depending on the level of foreground subtraction. As with an introduction of running, one expects that any scale dependence should lead to a worsened fit at high multipoles. We show that the logarithmic nature of the axion feature in combination with a tilt n_s~1 allows the fit to be identical to a no feature model at the 2 percent level on scales 100 =< l =<3500, and quite remarkable actually slightly improves the fit at scales l >2000. We also consider possible unremoved dust foregrounds and show that including these hardly changes the best-fit parameters. Our analysis suggests an axion decay constant of f/M_p ~O(.01). We discuss how Planck measurements of the TE and EE spectra can further constrain axion monodromy inflation with such a large feature. A measurement of the large scale structure power spectrum is even more promising, as the effect is much bigger since the tensor modes do not affect the large scales. At the same time, a feature could also lead to a lower sigma8, lifting the tension between CMB and SZ constraints on sigma8.