Bounds on Spectral Dispersion from Fermi-detected Gamma Ray Bursts

TL;DR

This paper uses data from four Fermi gamma-ray bursts to establish extremely tight limits on spectral dispersion of light over cosmological distances, constraining quantum gravity effects and suggesting spacetime smoothness at high energies.

Contribution

It provides the most stringent bounds to date on photon dispersion from GRB data, improving previous limits by analyzing brief photon bunches and high-energy photons.

Findings

Bound on photon dispersion: Δc/c < 6.94 x 10^{-21}

Limits on dispersion parameters: k_1 and k_2

Implication: spacetime is smooth near the Planck scale

Abstract

Data from four Fermi-detected gamma-ray bursts (GRBs) is used to set limits on spectral dispersion of electromagnetic radiation across the universe. The analysis focuses on photons recorded above 1 GeV for Fermi detected GRB 080916C, GRB 090510A, GRB 090902B, and GRB 090926A because these high-energy photons yield the tightest bounds on light dispersion. It is shown that significant photon bunches in GRB 090510A, possibly classic GRB pulses, are remarkably brief, an order of magnitude shorter in duration than any previously claimed temporal feature in this energy range. Although conceivably a $>3 \sigma$ fluctuation, when taken at face value, these pulses lead to an order of magnitude tightening of prior limits on photon dispersion. Bound of $\Delta c / c < 6.94$ x $10^{-21}$ is thus obtained. Given generic dispersion relations where the time delay is proportional to the photon energy to the first or second power, the most stringent limits on the dispersion strengths were $k_1 <$ 1.61 x $10^{-5}$ sec Gpc$^{-1}$ GeV$^{-1}$ and $k_2 <$ 3.57 x $10^{-7}$ sec Gpc$^{-1}$ GeV$^{-2}$ respectively. Such limits constrain dispersive effects created, for example, by the spacetime foam of quantum gravity. In the context of quantum gravity, our bounds set $M_1 c^2$ greater than 525 times the Planck mass, suggesting that spacetime is smooth at energies near and slightly above the Planck mass.