The early universe was a hot soup of particles and light. In this soup, sound waves, fluctuations in density, spread outward from clumps of matter like ripples on a pond. When the universe cooled, these waves froze, leaving an imprint in the distribution of galaxies: they are slightly more abundant at a certain distance from each other. This distance is a 'standard ruler' for measuring the cosmos.
Astronomers from the DESI survey used a new tool: they looked not only at galaxies but also at quasars — bright cores of distant galaxies. Light from quasars passes through gas clouds, and carbon atoms absorb some of the light. These 'dark lines' in the spectrum reveal where the clouds are. By comparing the positions of quasars and carbon clouds, the scientists found the same acoustic imprint.
The measurement yielded the distance to that era with 3% precision. That's like measuring the length of a room with an error of a couple of centimeters, but for the entire universe. Such precision helps us understand how the expansion of the universe has changed over time and what dark energy is accelerating it.
as written by the authors arXiv:2601.08103
We present a measurement of Baryon Acoustic Oscillations (BAO) in the cross-correlation of triply ionized carbon C IV absorption with the positions of quasars (QSO) and Emission Line Galaxies (ELG). We use quasars and ELGs from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI) survey. Our data sample consists of 2.5 million quasars, 3.1 million ELGs, and the C IV absorption is measured along the line of sight of 1.5 million high redshift quasars with $$z > 1.3$$. We measure the isotropic BAO signal at 4.2$$\sigma$$ for the CIV$$\times$$QSO cross-correlation. This translates into a 3.0% precision measurement of the ratio of the isotropic distance scale, $$D_{\rm V}$$, and the sound horizon at the drag epoch, $$r_{\rm d}$$, with $$D_{\rm V}/r_{\rm d}(z_{\rm eff} = 1.92) = 30.3 \pm 0.9$$. We make the first detection of the BAO feature in the CIV$$\times$$ELG cross-correlation at a significance of 2.5$$\sigma$$ and find $$D_{\rm V}/r_{\rm d}(z_{\rm eff} = 1.47) = 24.6 \pm 1.0$$.
Sound waves in the early universe traveled at more than half the speed of light — about 60% of it.