Inflation explains the largest structures in the universe by stretching the smallest lengths in physics, and that bargain is the trans-Planckian problem. For inflation to imprint the observed CMB fluctuations, the comoving modes we measure today must have begun, at inflation's onset, with physical wavelengths shorter than the Planck length for any inflationary history longer than minimal: the calculation that yields the celebrated scale-invariant spectrum runs its quantum field theory through a regime where quantum gravity, unknown by definition, governs, and assumes the modes emerge in the standard vacuum regardless. Either the predictions are sensitive to trans-Planckian physics, in which case inflation's successes are accidents of an unjustified vacuum choice, or they are insensitive, which itself requires an unproven decoupling assumption.
The problem has been formalized as a conjecture against the theory: the Trans-Planckian Censorship Conjecture (Bedroya and Vafa 2019) holds that no consistent theory permits trans-Planckian modes to cross the Hubble horizon and classicalize, which, if true, bounds inflation's duration and energy scale so severely that most viable inflationary models are excluded, including essentially all those producing observable tensor modes. Inflationary theorists respond by constructing models threading the censorship window or by rejecting the conjecture, but the structural point stands independent of the conjecture: the standard model of cosmology derives its initial conditions from a calculation whose ultraviolet foundation it cannot exhibit.
The standing is a foundational dispute rather than a data tension: no observation yet adjudicates it, but it shapes which inflationary models survive, and any confirmed primordial tensor signal would force the issue by certifying a high inflationary scale deep in censored territory.