Cosmological constraints from a joint analysis of cosmic growth and
  expansion
  Combining measurements on the expansion history of the Universe and on the
growth rate of cosmic structures is key to discriminate between alternative
cosmological frameworks and to test gravity. Recently, Linder (2017) proposed a
new diagram to investigate the joint evolutionary track of these two
quantities. In this letter, we collect the most recent cosmic growth and
expansion rate datasets to provide the state-of-the-art observational
constraints on this diagram. By performing a joint statistical analysis of both
probes, we test the standard $\Lambda$CDM model, confirming a mild tension
between cosmic microwave background predictions from Planck mission and cosmic
growth measurements at low redshift ($z<2$). Then we test alternative models
allowing the variation of one single cosmological parameter at a time. In
particular, we find a larger growth index than the one predicted by general
relativity $\gamma=0.65^{+0.05}_{-0.04}$). However, also a standard model with
total neutrino mass of $0.26\pm0.10$ eV provides a similarly accurate
description of the current data. By simulating an additional dataset consistent
with next-generation dark-energy mission forecasts, we show that growth rate
constraints at $z>1$ will be crucial to discriminate between alternative
models.
