BAO, the Cosmic Ruler: How DSI Maps Expansion to Test Dark Energy

In this episode, we unpack baryon acoustic oscillations—the standard ruler carved into the distribution of galaxies. We’ll explain how the Dark Energy Spectroscopic Instrument uses 5,000 robotic fibers on the Mayall telescope to build a 3D map of tens of millions of galaxies, measure the BAO scale at different redshifts, and translate that into the expansion history of the universe. By comparing the transverse (D_A) and radial (H(z)) BAO signals, we test whether dark energy has remained constant or evolved over billions of years. Plus, what this means for the Lambda-CDM model and future surprises.

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