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 H \beta or the Pa \beta line widths .
In the best case , when the Pa \beta is used , the relationship has an intrinsic dispersion of \sim 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 \sim 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 .