DESI DR2 Results IV: Alcock-Paczy'nski Measurements from the Lyman Alpha Forest and Cosmological Constraints

arXiv:2607.27410 · astro-ph.CO · Submitted 2026-07-29 · Read on arXiv

DESI Collaboration, A. G. Adame, J. Aguilar, S. Ahlen, O. Alves, A. Anand, U. Andrade, E. Armengaud, S. Avila, A. Aviles, P. Bansal, A. Bault, J. R. Bermejo-Climent, F. Beutler, D. Bianchi, C. Blake, S. Blasby, M. Bonici, S. Brieden, A. Brodzeller, D. Brooks, A. Carnero Rosell, K. Carrion, L. Casas, F. J. Castander, E. Chaussidon, J. Chaves-Montero, D. Chebat, X. Chen, Z. Chen, Y. Cho, T. Claybaugh, A. Cuceu, T. M. Davis, K. S. Dawson, R. de Belsunce, A. de la Macorra, J. Della Costa, A. Dey, M. Doshi, H. Ebina, D. J. Eisenstein, W. Elbers, G. Farren, V. A. Fawcett, E. Fernández-García, S. Ferraro, A. Font-Ribera, D. Forero-Sánchez, J. E. Forero-Romero, C. S. Frenk, G. Gambardella, C. Garcia-Quintero, L. H. Garrison, H. Gil-Marín, S. Gontcho A Gontcho, A. X. Gonzalez-Morales, C. Gordon, D. Green, R. Gsponer, G. Gutierrez, J. Guy, B. Hadzhiyska, C. Hahn, S. He, M. Herbold, H. K. Herrera-Alcantar, M. -F. Ho, K. Honscheid, J. Hou, D. Huterer, V. Iršič, M. Ishak, J. -Q. Jiang, S. Jos, S. Juneau, N. V. Kamble, N. G. Karaçaylı, T. Karim, D. Kirkby, A. Kremin, A. Krolewski, O. Lahav, C. Lamman, M. Landriau, J. Lasker, J. M. Le Goff, L. Le Guillou, A. Leauthaud, Q. Li, W. Liu, K. Lodha, Y. Luo, O. Manasoiu, M. Manera, P. Martini, M. Maus, A. Meisner, R. Miquel, J. Morawetz, J. Moustakas, E. Mueller, P. Mukherjee, A. Muñoz-Gutiérrez, A. D. Myers, S. Nadathur, J. Najita, G. Niz, H. E. Noriega, E. Paillas, N. Palanque-Delabrouille, J. Pan, M. P. Ibanez, W. J. Percival, A. Porredon, F. Prada, H. Pulido-Hernández, A. Pérez-Fernández, I. Pérez-Ràfols, A. Raichoor, M. Rashkovetskyi, J. Ratajczak, C. Ravoux, A. Robertson, A. Rocher, J. Rohlf, A. J. Ross, G. Rossi, R. Ruggeri, M. F. Ruiz-Herrera Bernal, L. Samushia, E. Sanchez, C. Saulder, D. Schlegel, H. Seo, A. Shafieloo, R. Sharples, J. Silber, T. Simon, F. Sinigaglia, M. Siudek, T. Tan, G. Tarlé, W. Turner, R. Vaisakh, M. Vargas-Magaña, B. A. Weaver, M. Wolfson, H. Yang, J. Yu, C. Yèche, H. Zhang, Y. Zhang, R. Zhao, R. Zhou

astro-ph.CO

Submitted: 2026-07-29

Code: https://github.com/andreicuceu/vega

License: http://creativecommons.org/licenses/by/4.0/

The gist: We present Alcock-Paczy'nski (AP) measurements from the full shape of Lyman- alpha (Ly alpha) forest correlation functions measured from the second data release (DR2) of the Dark Energy Spectroscopic

Terminology

Abstract

We present Alcock-Paczy'nski (AP) measurements from the full shape of Lyman- alpha (Ly alpha) forest correlation functions measured from the second data release (DR2) of the Dark Energy Spectroscopic Instrument (DESI). Our measurements include information from the Ly alpha forest auto-correlation and its cross-correlation with quasars. We constrain the AP effect with 1% precision at an effective redshift z eff=2.33, which is twice as tight as the Baryon Acoustic Oscillation (BAO) constraint from the same data. When using the joint Ly alpha AP and BAO results, we measure the ratios D H(z eff)/r d=8.600 plus or minus 0.066 and D M(z eff)/r d=39.32 plus or minus 0.33, where D M is the transverse comoving distance, D H is the Hubble distance, and r d is the sound horizon at the drag epoch. Assuming CDM, Ly alpha forest measurements combined with a nucleosynthesis prior produce a constraint on the Hubble constant H 0=66.5 plus or minus1.3, km,s-1,Mpc-1. The Ly alpha AP result corresponds to a matter fraction constraint m=0.325 plus or minus0.018 in CDM, which is 1.4 sigma higher than DESI BAO. This impacts the DESI results relative to the Cosmic Microwave Background (CMB), slightly reducing their discrepancy from 2.4 sigma to 2.2 sigma. We present updated constraints on extended models using the joint DESI DR2 BAO and Ly alpha forest full shape data, together with external data sets. When considering a time-evolving dark energy equation of state parametrized by w 0 and w a, we find it is preferred over CDM at 2.7 sigma for the combination of DESI and CMB data, and at 3.2 sigma when also including supernovae. With the new Ly alpha AP measurement, DESI provides its most precise anchor for the expansion history at z > 1 in the matter-dominated Universe.

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