Triangulating the Universe’s Contents, Laws, and Origin with Galaxy Correlation

The Universe is now in an epoch of accelerated expansion of space-time itself, presumed due to dark energy. 3D maps of the distribution of galaxies such as the Sloan Digital Sky Survey (SDSS) have yielded percent-level constraints on dark energy using the imprint of Baryon Acoustic Oscillations (BAO) as a standard ruler to map the Universe’s expansion history. Dark Energy Spectroscopic Instrument (DESI; 2019-2024) will map 30 million galaxies (30X SDSS) to improve dark energy constraints by an order of magnitude.

The standard analysis approach for these surveys uses the clustering of galaxy pairs. However, fully exploiting the powerful DESI dataset will demand new ideas. The clustering of galaxy triplets—triangles—measured through the 3-Point Correlation Function (3PCF), offers a powerful new window on cosmic acceleration and dark energy. First, the BAO method can be used in the 3PCF exactly as it is already used with pairs, to sharpen our history of the Universe’s expansion. Second, the 3PCF offers information on the fidelity with which galaxies trace the underlying matter (galaxy biasing), enhancing their utility for cosmology. Third, the direction-dependent 3PCF enables probing modified gravity, another possible explanation for accelerated cosmic expansion. I will present a transformatively fast 3PCF algorithm, and demonstrate its use to make the first application of the BAO method to the 3PCF and place high-precision constraints on galaxy biasing. I will then show how the direction-dependent 3PCF can provide a complementary probe of modified gravity. I will close with further exciting 3PCF cosmology opportunities, such as constraining inflation and the neutrino mass, as well as a novel application to understand the physics of the turbulent interstellar medium as relevant for star formation.