Nonlinear Structure Formation at Two Scales: from Bispectrum Baryon Acoustic Oscillations to Evolution of Halo Profiles

The “cosmic web” of dark matter halos forms via the collapse of post-inflation density fluctuations. While linear perturbation theory describes this process well at large scales and low densities, it fails at small scales and high densities. I explore two facets of nonlinear structure formation that constrain cosmology: at mildly nonlinear scales, measuring the baryon acoustic oscillation (BAO) distance scale in the bispectrum, and at deeply nonlinear scales, tracking the evolution of simulated dark matter halo profiles.

At mildly nonlinear scales, the BAO distance scale constrains the expansion history of the universe and dark energy. The BAO feature has been detected in both the three-point correlation function and the bispectrum, but challenges remain in fully exploiting three-point data. I present a strategy to select triangle configurations that maximize the amplitude of the BAO signal in the bispectrum. A relatively small set of bispectrum measurements can improve constraints on the BAO length scale over power spectrum measurements alone.

I next turn to the internal structure of dark matter halos, whose formation is highly nonlinear. The spherically-averaged density of a halo is well described by the Navarro-Frenk-White (NFW) profile, a function of two parameters: concentration, which describes the density of the central region of the halo, and halo mass. These two parameters are correlated, and the resulting concentration-mass (c-M) relation is sensitive to cosmological parameters. I present a robust measurement of the c-M relation from the largest ever sample of simulated halos (~30 million) and track the evolution of individual halos to connect halo formation time and concentration.

Speaker: Hillary Child, Univ. of Chicago