DESI preprint reports 1% measurement of the universe’s expansion at redshift 2.33

A new DESI Collaboration preprint reports a 1% measurement of the universe’s high-redshift expansion geometry from the Lyman-alpha forest, giving the survey its sharpest such probe yet when the cosmos was far younger than it is today.

In the paper, posted to arXiv on July 29, DESI said it used Data Release 2 observations of the Lyman-alpha forest — absorption patterns in quasar light caused by intergalactic hydrogen — to measure the Alcock-Paczynski, or AP, effect at an effective redshift of 2.33. “We constrain the AP effect with 1% precision at an effective redshift z_eff = 2.33,” the abstract said. DESI said that is about twice as tight as the BAO-only, or baryon acoustic oscillation-only, constraint from the same data. Using the joint Lyα AP and BAO results, the collaboration reported D_H(z_eff)/r_d = 8.600 ± 0.066 and D_M(z_eff)/r_d = 39.32 ± 0.33.

That matters because the measurement gives cosmologists a more precise high-redshift anchor for the expansion history, at a time when matter still dominated the universe’s dynamics. In practical terms, a tighter anchor at redshift 2.33 feeds into estimates of today’s expansion rate, known as the Hubble constant, and the total matter content of the universe. As the abstract put it, “With the new Lyα AP measurement, DESI provides its most precise anchor for the expansion history at z > 1 in the matter-dominated Universe.”

Under the standard ΛCDM model, the paper said that combining the new Lyman-alpha forest result with a Big Bang nucleosynthesis prior yields H0 = 66.5 ± 1.3 kilometers per second per megaparsec. It also reported Ω_m = 0.325 ± 0.018, a matter-density estimate that the collaboration said is 1.4 standard deviations higher than DESI’s BAO-only value. The paper added that including the new Lyα AP result slightly reduces the discrepancy between DESI and cosmic microwave background-based results, narrowing it from 2.4 standard deviations to 2.2.

The paper also reported somewhat stronger hints of time-varying dark energy in an extended model that allows the dark energy equation of state to evolve over time through parameters called w0 and wa. In those combined-data fits, DESI found a 2.7-standard-deviation preference for that model over ΛCDM when paired with cosmic microwave background data, and a 3.2-standard-deviation preference when supernova data were added. Those results are model-dependent and come from combining multiple datasets, not a standalone detection of new physics from DESI alone.

DESI, the Dark Energy Spectroscopic Instrument at Kitt Peak, is mapping galaxies and quasars to chart how cosmic expansion has changed over time. The Lyman-alpha forest is especially useful because it lets researchers probe the universe at high redshift by using distant quasars as backlights for intervening hydrogen gas. The advance in the new preprint is its precision: eBOSS, an earlier survey, reported in 2020 much looser measurements at the same redshift, with D_H/r_d = 8.99 ± 0.19 and D_M/r_d = 37.5 ± 1.1. That makes the new DESI result a notable step forward, even as the broader cosmology implications will need the usual scrutiny that comes with any new preprint.