Parity Violation of Gravitons in the CMB Bispectrum

Do the CMB Temperature Fluctuations Conserve Parity?

Observations of the cosmic microwave background (CMB) have cemented the notion that the large-scale Universe is both statistically homogeneous and isotropic. But is it invariant also under reflections? To probe this we require parity-sensitive statistics: for scalar observables, the simplest is the trispectrum. We make the first measurements of the parity-odd scalar CMB, focusing on the large-scale (2<ℓ<510) temperature anisotropies measured by Planck. This is facilitated by new quasi-maximum-likelihood estimators for binned correlators, which account for mask convolution and leakage between even- and odd-parity components, and achieve ideal variances within ≈20%. We perform a blind test for parity violation by comparing a χ^{2} statistic from Planck to theoretical expectations, using two suites of simulations to account for the possible likelihood non-Gaussianity and residual foregrounds. We find consistency at the ≈0.4σ level, yielding no evidence for novel early-Universe phenomena. The measured trispectra allow for a wealth of new physics to be constrained; here, we use them to constrain eight primordial models, including ghost inflation, cosmological collider scenarios, and Chern-Simons gauge fields. We find no signatures of new physics, with a maximal detection significance of 2.0σ. Our results also indicate that the recent parity excesses seen in the BOSS galaxy survey are not primordial in origin, given that the CMB dataset contains roughly 250× more primordial modes, and is far easier to interpret, given the linear physics, Gaussian statistics, and accurate mocks. Tighter CMB constraints can be wrought by including smaller scales (though rotational invariance washes out the flat-sky limit) and adding polarization data.

Mild quasilocal non-Gaussianity as a signature of modified gravity during inflation

We show that modifications of Einstein gravity during inflation could leave potentially measurable imprints on cosmological observables in the form of non-Gaussian perturbations. This is due to the fact that these modifications appear in the form of an extra field that could have nontrivial interactions with the inflaton. We show it explicitly for the case R+αR(2), where nearly scale-invariant non-Gaussianity at the level of f(NL) ≈ - (1 to 30) can be obtained, in a quasilocal configuration.