  Fluctuations in sky maps of the galaxy redshifts, dubbed as angular redshift
fluctuations (ARF), contain precise information about the growth rate of
structures and the nature of gravity in the Universe. Specifically, ARF
constrain the combination of cosmological parameters $H/H_0\,f\sigma_8(z)$,
while being an intrinsically tomographic probe and largely insensitive to many
observational systematic errors, all this without requiring the assumption of
any redshift-to-distance relation under a given fiducial cosmology. We present
the first cosmological constraints derived from ARF by using BOSS LOWZ+CMASS
DR12 galaxy samples, obtaining 7\%-accurate constraints on $H/H_0 f\sigma_8(z)$
at more than 20 redshifts over the range $z \in [0.26,0.72]$. Our best-fitting
value is $10\%$ larger, but compatible at the $1.4\sigma$ level, than the
$\Lambda$CDM expectation set by {\it Planck} observations of the Cosmic
Microwave Background (CMB) radiation. Our tomographic measurements, combined
with these CMB data, provides one of the strongest constraints on the gravity
index $\gamma$, $\gamma=0.44^{+0.09}_{-0.07}$, which lies within $2\sigma$ from
the prediction of General Relativity ($\gamma_{\rm GR}\simeq 0.55$).
