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 \pm 0.020 ( 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 .
When the CMB lensing likelihood is included in the analysis the statistical significance for MG simply vanishes , indicating also the possibility of a systematic effect for this MG signal .