Using quasars as standardizable candles provides a critical probe of cosmic expansion at high redshifts (z >1.5), bridging the observational gap beyond the reach of Type Ia supernovae. This seminar presents a novel cosmological framework based on a highly homogeneous sample of ~2,000 quasars spanning z = 0.7-5.0, constructed by cross-matching the SDSS-DR16 quasar catalogue with XMM-Newton (4XMM-DR14) observations. Strict selection criteria are applied to eliminate dust reddening, gas obscuration, BAL and radio-bright quasars, as well as host-galaxy UV contamination. Rejection of shallow X-ray observations resolves the severe flux-limit bias caused by mismatched SDSS and XMM-Newton sensitivity thresholds. The standardized L_X-L_UV relation demonstrates remarkable stability across cosmic time, exhibiting a slope of 0.58 and a tight dispersion of 0.15 dex. In the overlap region, quasars and Type Ia supernova Hubble diagrams show complete consistency, supporting a unified distance scale. However, the combined supernova and quasar Hubble diagram reveals a strong (>4 sigma) tension with the standard flat LCDM model at z>1.5. These cosmological constraints show robust agreement with independent quasar analyses based on variability-derived distances, confirming that high-redshift departures from flat LCDM stem from underlying physical mechanisms rather than sample-specific systematics.