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Department of Physics and Astronomy

Colloquium – Julien Guy

Julien Guy, Lawrence Berkeley National Laboratory

The dark energy of spectroscopic instrument: coSmological results from the data release 2

The Dark Energy Spectroscopic Instrument (DESI) collaboration is conducting a 5 year redshift survey of 40 million extra-galactic sources over 14,000 square degrees of the northern sky. One of its primary goals is to measure the cosmic expansion history with baryon acoustic oscillations (BAO). After an overview of the survey and the instrument, I will present the measurement of BAO in galaxy, quasar and Lyman-alpha forest tracers from the 2nd Data Release. With 14 million galaxy and quasar redshifts in the range 0.1 < z < 3.5, including 820,000 Lyman-alpha forest quasars at z>2.1, the aggregate precision on BAO is of 0.3% at z<2.1 and 0.7% at an effective redshift z=2.3, which is a 40% improvement over the first data release. After highlighting some of the numerous validation tests performed with simulations and blinded data, I will discuss the main cosmological results with improved constraints on the dark energy equation of state. I will also present recent results from the full shape analysis of DESI galaxies two point function and the cross-correlation of galaxy positions with CMB lensing.

Bio: Julien Guy is a cosmologist at Lawrence Berkeley National Laboratory, specializing in experimental tests of the accelerated expansion of the universe. Having made significant contributions to measuring cosmological distances using Type Ia supernovae, he now focuses on large-scale galaxy redshift surveys. As co-project scientist for the Dark Energy Spectroscopic Instrument (DESI), he plays a key role in spectrograph calibration, data processing, and R&D instrumentation. His research also extends to the analysis of the Lyman-alpha forest, with a particular emphasis on measuring baryon acoustic oscillations at high redshifts to refine our understanding of cosmic evolution.

October 2, 2025 @ 4:10pm (CST) in 4327 Stevenson Center; light refreshments available at 3:50 PM

Host: S Taylor