Zusammenfassung
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 mirror reflections? To probe this we
require parity-sensitive statistics: for scalar observables, the simplest is
the four-point function. We make the first measurements of the parity-odd CMB
trispectrum, focusing on the large-scale ($2<\ell<510$) temperature
anisotropies measured by Planck. This is facilitated by new maximum-likelihood
estimators for binned correlators, which account for mask convolution and
leakage between even- and odd-parity components, and achieve optimal variances
within $20\%$. We perform a blind test for parity violation by
comparing a $\chi^2$ 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 $\approx
0.5\sigma$ level, yielding no evidence for parity violation, with roughly
$250\times$ the squared sensitivity of large scale structure measurements
(according to mode-counting arguments), and with the advantage of linear
physics, Gaussian statistics, and accurate mocks. The measured trispectra can
be used to constrain physical models of inflationary parity violation,
including Ghost Inflation, Cosmological Collider scenarios, and Chern-Simons
gauge fields. Considering eight such models, we find no evidence for new
physics, with a maximal detection significance of $2.0\sigma$. These results
suggest that the recent parity excesses seen in the BOSS galaxy survey are not
primordial in origin. Tighter constraints can be wrought by including smaller
scales (though rotational invariance washes out the flat-sky limit) and adding
polarization data.
Nutzer