The Baryon-Dark Matter Relative Velocity and a New Approach to the 3-Point Correlation Function

Zack Slepian (Harvard)

January 22, 2016

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Abstract: Due to their different behaviors before decoupling (z~1020), at high redshift (z~50) there is a relative velocity between baryons and dark matter that is coherent on very large scales. If this relative velocity couples to galaxies today, it can shift the BAO peak in the galaxy-galaxy 2-point correlation function. Such a shift would systematically bias the expansion history inferred using the BAO method. I will give a configuration space picture of the relative velocity and show that the 3-point correlation function can be used to remove this systematic bias from the BAO method. I will then present a reformulation of the 3PCF that: 1. permits computing the 3PCF in N^2 not N^3 time, N the number of objects; 2. allows analytic calculation of a highly accurate covariance matrix; 3. gives perturbation theory predictions in excellent agreement with both mock catalogs and data. Using this approach on ~800,000 galaxies in SDSS DR13 constrains linear and non-linear bias and demonstrates a ~2.8 sigma BAO feature in the 3PCF. I will conclude with where we are going: modeling the 3PCF including RSD and using the BAO feature in it as a cosmological standard ruler just as the 2PCF already does.