Lighting Dark Ages with Tomographic ISW Effect

The integrated Sachs-Wolfe effect (ISW) describes how CMB photons pick up a net blue or redshift when traversing the time-varying gravitational potentials between the last scattering surface and us. Deviations from its standard amplitude could hint new physics. We show that reconstructing the amplitude of the ISW effect as a function of the redshift may provide a unique tool to probe the gravity sector during the era of dark ages, inaccessible via other cosmological observables. Exploiting Planck CMB temperature, polarization and lensing observations, we find a $2\sigma$ deviation from the standard ISW amplitude at redshift $z=500$. Barrying a systematic origin, our findings could point to either possibly new physics or a departure from the standard picture of structure formation under the General Relativity framework. Assuming the simplest two-redshift-bin scenario, we ensure $38\sigma$ and $2\sigma$ evidences of the early and late ISW effects, respectively, despite a priori possible degeneracy with the CMB lensing amplitude. Using a multiple tomographic method, we present the first complete characterization of the ISW effect over space and time. Future tomographic ISW analyses are therefore crucial to probe the dark ages at redshifts otherwise unreachable via other probes.