  I will discuss to what degree the cosmic microwave background (CMB) can be
used to constrain primordial non-Gaussianity involving one tensor and two
scalar fluctuations, focusing on the correlation of one $B$-mode polarization
fluctuation with two temperature fluctuations (BTT). In the simplest models of
inflation, the tensor-scalar-scalar primordial bispectrum is non-vanishing and
is of the same order in slow-roll parameters as the scalar-scalar-scalar
bispectrum. I will show that constraints from an experiment like CMB-Stage IV
using this observable are more than an order of magnitude better than those on
the same primordial coupling obtained from temperature measurements alone. I
will argue that $B$-mode non-Gaussianity opens up an as-yet-unexplored window
into the early Universe, demonstrating that significant information on
primordial physics remains to be harvested from CMB anisotropies. BTT presents
a measure of both primordial tensors and primordial non-Gaussianity, two of the
most sought after signatures of the inflationary paradigm.
