A weather forecaster draws a thick black line across Europe: north of it, frost; south, a thaw. That boundary, the zero isotherm, won't tell you the temperature in Stockholm or Rome, but it honestly marks where winter ends. In recent cosmology, a map has emerged where a similar frontier can be drawn. Only instead of temperature, it's the strength of gravitational ties between galaxies, and instead of a weather front, it's the zero-correlation contour — a nodal line in the distribution of star systems.

By measuring where this statistic changes sign, researchers obtain a geometric tool nearly indifferent to the whims of instrumental calibration. If all galaxies suddenly became twice as bright or dim, the correlation function would multiply by a constant factor, but its zero would not budge a pixel. The nodal line proves to be a witness immune to the lure of systematic errors. Such robustness is exactly what's needed today, when local and global measurements of the Hubble constant diverge by several percent, creating the 'Hubble tension'.

The method's authors processed data from the first release of DESI DR1 — a spectroscopic survey covering five galaxy samples at redshifts from 0,4 to 2,1. For each sample, they constructed a sign map of the correlation function in transverse and line-of-sight separation coordinates and described the zero contour as a line in Hessian normal form: tilt angle and offset from the origin. Comparison with predictions from model simulations allowed them to assess how the line's position depends on cosmological parameters.

The nodal line clearly appears in all samples. Its position is primarily controlled by the combination $Ω_m$ $h^{3/2}$—a kind of cosmological thermometer, where $Ω_m$ is the matter fraction of the Universe's total density, and h is the dimensionless Hubble constant. With fixed galaxy–dark matter connection parameters, the constraint on this combination reaches 3% accuracy. But the most impressive result comes when combined with baryon acoustic oscillations (BAO)—a standard ruler frozen in the primordial plasma. BAO fix the geometry via distance ratios at different redshifts, while the nodal line provides an independent condition. Their intersection allows computing h without invoking early-Universe physics. The obtained value H0 = 0,719 ± 0,032 (accuracy 4,45%) is free from calibration by the sound horizon — the simplification that long fueled debates.

The weather map doesn't end there. Just as a forecaster, having drawn the zero isotherm, can refine the forecast, cosmologists gain a compass that doesn't get thrown off by a change in scale. The nodal line won't replace BAO, but it will serve as insurance against self-deception: as survey statistical power grows and systematic errors threaten to swamp the true signal, robust geometric tests become a mainstay. The line dividing regions of clumps and voids turns out to be the most honest witness of cosmic geometry — simply because it cares only about shape, not brightness.