Quantum fluctuations at the Planck scale

The recently measured cutoff, k_min= 4.34 ± 0.50 / r_cmb (with r_cmb the comoving distance to the last scattering surface), in the fluctuation spectrum of the cosmic microwave background, appears to disfavor slow-roll inflation and the associated transition of modes across the horizon. We show in this Letter that k_min instead corresponds to the first mode emerging out of the Planck domain into the semi-classical universe. The required scalar-field potential is exponential, though not inflationary, and satisfies the zero active mass condition, ρ_ϕ+ 3 p_ϕ= 0. Quite revealingly, the observed amplitude of the temperature anisotropies requires the quantum fluctuations in ϕ to have classicalized at ∼ 3.5 × 10 ¹⁵ GeV, consistent with the energy scale in grand unified theories. Such scalar-field potentials are often associated with Kaluza–Klein cosmologies, string theory and even supergravity.