# Quantum Fluctuations at the Planck Scale

## TL;DR

This paper proposes that the observed cutoff in the cosmic microwave background fluctuation spectrum is due to quantum fluctuations emerging from the Planck scale, involving a non-inflationary exponential scalar field potential.

## Contribution

It introduces a novel interpretation linking the cutoff to Planck-scale quantum fluctuations and identifies a specific scalar potential consistent with grand unified theories.

## Key findings

- The cutoff corresponds to the first mode emerging from the Planck domain.
- Quantum fluctuations in the scalar field classicalize at ~3.5x10^15 GeV.
- The scalar potential is exponential and satisfies the zero active mass condition.

## Abstract

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, rho_phi+3p_phi=0. Quite revealingly, the observed amplitude of the temperature anisotropies requires the quantum fluctuations in phi to have classicalized at ~3.5x10^15 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.