Primordial Clocks within Stochastic Gravitational Wave Anisotropies
  A first-order phase transition in the early universe can give an observable
stochastic gravitational background (SGWB), which will necessarily have
primordial anisotropies across the sky. In multi-field inflationary scenarios,
these anisotropies may have a significant isocurvature component very different
from adiabatic fluctuations, providing an alternate discovery channel for high
energy physics at inflationary scales. Here, we consider classically
oscillating heavy fields during inflation that can imprint distinctive
scale-invariance-breaking features in the power spectrum of primordial
anisotropies. While such features are highly constrained in the cosmic
microwave background, we show that their amplitude can be observably large in
isocurvature SGWB, despite both probing a similar period of inflation.
Measuring SGWB multipoles at the required level, $\ell \sim {\cal O}(10-100)$,
will be technologically challenging. However, we expect that early detection of
a strong isotropic SGWB, and the guarantee of anisotropies, would spur
development of next-generation detectors with sufficient sensitivity, angular
resolution, and foreground discrimination.
