Why can a line on a galaxy map measure the properties of the entire universe? It's all about geometry. When astronomers count how often pairs of galaxies occur at different distances, they obtain a field of pluses and minuses: places where galaxies cluster more often than random (plus) and less often (minus). The boundary between these regions, the nodal line, lies where the correlation crosses zero. Its position depends only on the structure of spacetime (parameters $Ω_m$ and h), not on the overall brightness of galaxies or telescope calibration. Imagine multiplying all numbers on the map by two: pluses stay pluses, minuses stay minuses, and the line doesn't move.

Using data from the DESI DR1 survey, scientists constructed such a line for several galaxy samples. They found that its slope and shift are determined by the combination $Ω_m$ $h^{3/2}$, and adding information from baryon acoustic oscillations, a 'frozen' sound wave from the early universe, allows direct extraction of the Hubble constant. The value obtained (H0 = 0,719 ± 0,032) does not rely on early universe calibration and thus helps relieve a long-standing tension in cosmology. A thin black line becomes a geometric compass resistant to systematic shifts.