Colliding Ghosts: Constraining Inflation with the Parity-Odd Galaxy Four-Point Function

Abstract

Could new physics break the mirror symmetry of the Universe? Utilizing recent measurements of the parity-odd four-point correlation function of BOSS galaxies, we probe the physics of inflation by placing constraints on the amplitude of a number of parity-violating models. Within canonical models of (single-field, slow-roll) inflation, no parity-asymmetry can occur; however, it has recently been shown that breaking of the standard assumptions can lead to parity violation within the Effective Field Theory of Inflation (EFTI). In particular, we consider the Ghost Condensate and Cosmological Collider scenarios -the former for the leading and subleading operators in the EFTI and the latter for different values of mass and speed of an exchanged spin-1 particle -for a total of 18 models. Each instance yields a definite prediction for the inflationary trispectrum, which we convert to a late-time galaxy correlator prediction (through a highly non-trivial calculation) and constrain using the observed data. We find no evidence for inflationary parity-violation (with each of the 18 models having significances below 2.4σ), and place the first constraints on the relevant coupling strengths, at a level comparable with the theoretical perturbativity bounds. This is also the first time Cosmological Collider signatures have directly been searched for in observational data. We further show that possible secondary parity-violating signatures in galaxy clustering can be systematically described within the Effective Field Theory of Large-Scale Structure. We argue that these late-time contributions are subdominant compared to the primordial parity-odd signal for a vast region of parameter space. In summary, the results of this paper disfavor the notion that the recent hints of parity-violation observed in the galaxy sample are due to new physics.

Parity-Violating Trispectrum from Chern-Simons Gravity

“…Fourth, we required µ 1 to prevent both the production of ghost-like graviton modes and the overproduction of tensor modes. Ghost-like modes in the context of scalar inflation have indeed been thoroughly investigated [13,32,143]. In addition, the tensor-to-scalar ratio can be made small by lowering the Hubble scale of inflation.…”

“…Given the dCS-inflation dynamics, primordial power spectra, and degree of gravitational circular polarization, Eqs. (13)(14)(15)(16)(17)(18), we now move to our main calculation: the connected primordial scalar trispectrum due to graviton-exchange.…”

“…it has O (1) error) for µ 0.5, although for simplicity we will continue to refer to µ 1 as our limit. Given the current experimental sensitivity to trispectra in general [13], we consider this approximation reasonable. Moreover, while the above discussion may seem like our approximation is possibly not optimal for sub-horizon physics, we emphasize that in the collapsed limit, the ghost mode will not contribute and this approximation is in fact exact.…”

“…In LSS, the first observable that encapsulates parity violation in the Universe is the galaxy four-point function, as first noted in [9], or alternatively its corresponding trispectrum [10] (see Refs. [11][12][13] for applications of these ideas). In addition, LSS parity violation can be probed in any correlator that involves an odd number of galactic shear B modes, such as the galactic shear E/B cross-correlator [14,15] [or a non-zero curl component].…”

“…Coupling between multiple scalars can break scale-invariance via an IR-divergence [10] and give a parity-odd scalar trispectrum. Coupling between the inflaton and a spin-1 field can break parity via a Chern-Simons term in the presence of a homogeneous U (1) background field [19,20,35,36] or with only perturbations if the U (1) field is massive [13,37]. Chern-Simons gravity [38,39] and the Nieh-Yah term in teleparallel gravity [40] can generate parity violation in the spin-2 sector.…”