Cosmological Hints of Modified Gravity ?

TL;DR

Recent Planck data shows hints of modified gravity affecting structure formation, improving consistency with weak lensing and reionization data, and potentially resolving anomalies in the CMB spectrum.

Contribution

This study confirms a preference for modified gravity from Planck 2015 data and demonstrates its ability to resolve the $A_{lens}$ anomaly and better align with other cosmological measurements.

Findings

Modified gravity scenario is preferred at 95% confidence level.

It resolves the $A_{lens}$ anomaly in the CMB spectrum.

It yields a lower reionization optical depth, consistent with recent data.

Abstract

The recent measurements of Cosmic Microwave Background temperature and polarization anisotropies made by the Planck satellite have provided impressive confirmation of the $\Lambda$CDM cosmological model. However interesting hints of slight deviations from $\Lambda$CDM have been found, including a $95 \%$ c.l. preference for a "modified gravity" structure formation scenario. In this paper we confirm the preference for a modified gravity scenario from Planck 2015 data, find that modified gravity solves the so-called $A_{lens}$ anomaly in the CMB angular spectrum, and constrains the amplitude of matter density fluctuations to $\sigma_8=0.815_{-0.048}^{+0.032}$, in better agreement with weak lensing constraints. Moreover, we find a lower value for the reionization optical depth of $\tau=0.059\pm0.020$ (to be compared with the value of $\tau= 0.079 \pm 0.017$ obtained in the standard scenario), more consistent with recent optical and UV data. We check the stability of this result by considering possible degeneracies with other parameters, including the neutrino effective number, the running of the spectral index and the amount of primordial helium. The indication for modified gravity is still present at about $95\%$ c.l., and could become more significant if lower values of $\tau$ were to be further confirmed by future cosmological and astrophysical data.