  During single clock inflation, hidden fields (i.e. fields coupled to the
inflaton only gravitationally) in their adiabatic vacua can ordinarily only
affect observables through virtual effects. After renormalizing background
quantities (fixed by observations at some pivot scale), all that remains are
logarithmic runnings in correlation functions that are both Planck and slow
roll suppressed. In this paper we show how a large number of hidden fields can
partially compensate this suppression and generate a potentially observable
running in the tensor two point function, consistently inferable courtesy of a
large $N$ resummation. We detour to address certain subtleties regarding loop
corrections during inflation, extending the analysis of [1]. Our main result is
that one can extract bounds on the hidden field content of the universe from
bounds on violations of the consistency relation between the tensor spectral
index and the tensor to scalar ratio, were primordial tensors ever detected.
Such bounds are more competitive than the naive bound inferred from requiring
inflation to occur below the strong coupling scale of gravity if deviations
from the consistency relation can be bounded to within the sub-percent level.
We discuss how one can meaningfully constrain the parameter space of various
phenomenological scenarios and constructions that address naturalness with a
large number of species (such as `N-naturalness') with CMB observations up to
cosmic variance limits, and possibly future 21cm and gravitational wave
observations.
