Studies of the cosmological backgrounds are approaching the very technological limits of our present day instrumentation capabilities. But already today the systematic uncertainties exceed the statiscal ones. Simply, because our actual knowledge of the Milky Way foregrounds is not sufficient. We need to quantify precisely the essential quantity, the density distribution of hydrogen nucleons along the line of sight. Independent of its physical state of ionized, neutral or molecular.
Cross correlation studies of full sky surveys comprising soft X-ray (E < 0.4 keV), far-infrared and HI 21-cm line measurements provide a powerful tool to disentangle the contribution of these different gaseous phases of the hydrogen nuclei. In this context the eROSITA mission is becoming a crucial role for multi-frequency studies of the early universe. Here, we demonstrate the quantitative modelling to the hydrogen nuclei density towards the high galactic latitude sky, allowing to probe the cosmological background at an unsurpassed accuracy level.