Phases of New Physics in the BAO Spectrum
  We show that the phase of the spectrum of baryon acoustic oscillations (BAO)
is immune to the effects of nonlinear evolution. This suggests that any new
physics that contributes to the initial phase of the BAO spectrum, such as
extra light species in the early universe, can be extracted reliably at late
times. We provide three arguments in support of our claim: First, we point out
that a phase shift of the BAO spectrum maps to a characteristic sign change in
the real space correlation function and that this feature cannot be generated
or modified by nonlinear dynamics. Second, we confirm this intuition through an
explicit computation, valid to all orders in cosmological perturbation theory.
Finally, we provide a nonperturbative argument using general analytic
properties of the linear response to the initial oscillations. Our result
motivates measuring the phase of the BAO spectrum as a robust probe of new
physics.
