New Constraints on Quantum Gravity from X-ray and Gamma-Ray Observations

TL;DR

This paper uses X-ray and gamma-ray observations to place new constraints on models of quantum spacetime foam, ruling out certain theoretical models based on observational limits on path-length fluctuations.

Contribution

It reassesses previous astronomical tests of spacetime foam models and provides tighter constraints on the accumulation power parameter $oldsymbol{\alpha}$ using multi-energy observations.

Findings

Chandra X-ray data constrains $oldsymbol{\alpha extgreater 0.58}$

Fermi gamma-ray data constrains $oldsymbol{\alpha extgreater 0.67}$

Cherenkov telescope data constrains $oldsymbol{\alpha extgreater 0.72}$

Abstract

One aspect of the quantum nature of spacetime is its "foaminess" at very small scales. Many models for spacetime foam are defined by the accumulation power $\alpha$, which parameterizes the rate at which Planck-scale spatial uncertainties (and thephase shifts they produce) may accumulate over large path-lengths. Here $\alpha$ is defined by theexpression for the path-length fluctuations, $\delta \ell$, of a source at distance $\ell$, wherein $\delta \ell \simeq \ell^{1 - \alpha} \ell_P^{\alpha}$, with $\ell_P$ being the Planck length. We reassess previous proposals to use astronomical observations ofdistant quasars and AGN to test models of spacetime foam. We show explicitly how wavefront distortions on small scales cause the image intensity to decay to the point where distant objects become undetectable when the path-length fluctuations become comparable to the wavelength of the radiation. We use X-ray observations from {\em Chandra} to set the constraint $\alpha \gtrsim 0.58$, which rules out the random walk model (with $\alpha = 1/2$). Much firmer constraints canbe set utilizing detections of quasars at GeV energies with {\em Fermi}, and at TeV energies with ground-based Cherenkovtelescopes: $\alpha \gtrsim 0.67$ and $\alpha \gtrsim 0.72$, respectively. These limits on $\alpha$ seem to rule out $\alpha = 2/3$, the model of some physical interest.