subPlanckian

SubPlanckian is a term used in physics to denote scales, energies, or field values that lie below the Planck scale. The Planck length is about 1.616 × 10^-35 meters and the Planck energy is approximately 1.22 × 10^19 GeV. Because the Planck scale is where quantum gravity is expected to become important, sub-Planckian regimes are often described using conventional theories such as quantum field theory in curved spacetime or general relativity, with the caveat that these theories may lose predictive power as one approaches the Planck boundary.

In cosmology, sub-Planckian commonly refers to wavelengths or fluctuations that were smaller than the Planck length in the early universe. This raises questions about whether predictions of inflationary perturbations are sensitive to unknown Planck-scale physics and how such effects might imprint on observable quantities like the cosmic microwave background.

In particle physics and inflationary model building, sub-Planckian can describe scalar field excursions that stay below the Planck mass. Sub-Planckian field values are often discussed as a way to minimize reliance on quantum-gravity corrections, whereas super-Planckian (or trans-Planckian) excursions imply greater sensitivity to Planck-scale physics. The term is therefore a descriptive shorthand rather than a precise boundary, reflecting scale relationships rather than a strict division.

Limitations: The Planck scale is a conjectured frontier where current theories are expected to falter, so “sub-Planckian” lacks a universally fixed operational meaning. It remains a heuristic way to discuss regimes where known physics may be applicable versus regimes requiring new quantum gravity insights.