  We report the discovery of a luminosity distance estimator using Active
Galactic Nuclei (AGN). We combine the correlation between the X-ray variability
amplitude and the Black Hole (BH) mass with the single epoch spectra BH mass
estimates which depend on the AGN luminosity and the line width emitted by the
broad line region. We demonstrate that significant correlations do exist which
allows one to predict the AGN (optical or X-ray) luminosity as a function of
the AGN X-ray variability and either the HBeta or the PaBeta line widths. In
the best case, when the PaBeta is used, the relationship has an intrinsic
dispersion of ~0.6 dex. Although intrinsically more disperse than Supernovae
Ia, this relation constitutes an alternative distance indicator potentially
able to probe, in an independent way, the expansion history of the Universe.
With this respect, we show that the new mission concept Athena should be able
to measure the X-ray variability of hundreds of AGN and then constrain the
distance modulus with uncertainties of 0.1 mag up to z~0.6. We also discuss
how, using a new dedicated wide field X-ray telescope able to measure the
variability of thousands of AGNs, our estimator has the prospect to become a
cosmological probe even more sensitive than current Supernovae Ia samples.
