Cosmic Ruler: Inflection Point Instead of Peak express
There's no tool to measure distances to faraway galaxies, but nature left a clue: after the Big Bang, sound waves created rhythmic clumps of matter. Today, this is noticeable as a slight preference for galaxies to be at a certain distance from each other — a built-in ruler. Over cosmic time, the pattern blurs, like a sand dune in the wind: the sharp peak on the distance graph becomes a gentle slope, but its steepest part barely moves. This inflection point — the linear point — more reliably preserves the imprint of the early universe.
Data from the DESI project, mapping millions of galaxies, confirmed its advantages. After reconstruction, which partially cleans the signal from late-time distortions, measurement precision improved by 15–60%. A small correction for the galaxy sample composition, accounting for dark matter and gravity, yielded perfect agreement with traditional measurements. Thus, the linear point becomes a new tool for hunting dark energy and understanding the future of the universe's expansion.
Original abstract
as written by the authors arXiv:2601.05967v1
The linear point, a purely geometric feature in the monopole of the two-point correlation function, has been proposed as an alternative standard ruler. Compared to the peak in the correlation function, it is more robust to late-time nonlinear effects at the percent level. In light of improved simulations and high quality data, we revisit the robustness of the linear point and use it as an alternative to template-based fitting approaches typically used in BAO analyses. We present the linear point measurements on galaxy samples from the first and second data releases (DR1 and DR2) of the DESI survey. We convert the linear point into a dimensionless parameter $$\alpha_{iso,LP}$$, defined as the ratio of the linear point in the fiducial cosmology and the observed value, analogous to the isotropic BAO scaling parameter $$\alpha_{iso}$$ used in previous BAO measurements. Using the 2nd generation of AbacusSummit mock catalogs, we find that linear point measurements are more precise when calculated in the post-reconstruction regime with 15-60% smaller uncertainties than those pre-reconstruction. We find a systematic shift in the linear point measurements compared against the isotropic BAO measurements in mocks; we attribute this to the isotropic damping parameter responsible for smearing the linear point in the nonlinear regime. We propose a sample-dependent correction that mitigates the impact of late-time nonlinear effects. While this introduces a cosmology dependence in an otherwise model-independent measurement, this is necessary given the sub-percent precision dictated by current cosmological surveys. Comparing $$\alpha_{iso,LP}$$ with isotropic BAO measurements made on the DESI DR1 and DR2 galaxy samples, we find excellent agreement after applying this correction, particularly post-reconstruction. We discuss future scope regarding cosmological inference with linear point measurements.
The theoretical idea has been around since the 2010s, but only DESI's unprecedented maps allowed it to be used with sub-percent precision — and it turned out that the inflection point is more informative than the blurred peak.