Light speed variation with brane/string-inspired space-time foam

Abstract

Recently a series of studies on high energy gamma-ray burst (GRB) photons suggest a light speed variation with linear energy dependence at the Lorentz violation scale of 3.6 × 10 17 GeV, with subluminal propagation of high energy photons in cosmological space. We propose stringy space-time foam as a possible interpretation for this light speed variation. In such a string-inspired scenario, bosonic photon open-string travels in vacuo at an infraluminal speed with an energy dependence suppressed by a single power of the string mass scale, due to the foamy structure of space-time at small scales, as described by D-brane objects in string theory. We present a derivation of this deformed propagation speed of the photon field in the infrared (IR) regime. We show that the light speed variation, revealed in the previous studies on GRBs time-delay data, can be well described within such a string approach towards space-time foam. We also derive the value of the effective quantum-gravity mass in this framework, and give a qualitative study on the theory-dependent coefficients. We comment that stringent constraints on Lorentz violation in the photon sector from complementary astrophysical observations can also be explained and understood in the space-time foam context.

Ultrahigh-energy photons from LHAASO as probes of Lorentz symmetry violations

“…where M QG-D-foam denotes the stringy quantum-gravity scale for photons in the D-particle foam. It has been noted [40,41] that this stringy QG framework is able to explain the observed light speed variation [4][5][6] from GRB multi-GeV photons, while being compatible with many other astrophysical results available to date, such as severe limits from γ-decays and/or birefringent effects. Detailed discussions can be found in the recent works [40,41].…”

“…It has been noted [40,41] that this stringy QG framework is able to explain the observed light speed variation [4][5][6] from GRB multi-GeV photons, while being compatible with many other astrophysical results available to date, such as severe limits from γ-decays and/or birefringent effects. Detailed discussions can be found in the recent works [40,41]. Here, we want to further indicate that this D-brane foam model is also consistent with the LHAASO experiment.…”

“…Meanwhile, we indicate that the UHE γ-ray observations with LHAASO could be a signal of the existence of subluminal deformation of Lorentz symmetry at the Planckscale level, in agreement with the light speed variation with the determined scale E (sub) LV = 3.6 × 10 17 GeV as observed from Fermi -GRBs. We further comment that the stringy space-time foam, a fully developed quantum gravity framework able to accounting for the light speed variation [40,41], can also explain consistently the LHAASO observation of PeV photons.…”

“…Hence, we conclude that the D-brane string foam explanation of light speed variation, proposed initially in Refs. [40,41], is also capable of explaining the data on the LHAASO UHE γ-ray.…”

“…The vacuum behaves like a dispersive gravitational medium, due to the foamy structure of space-time, thus has a tendency to slow down the energetic photons. QG effects therefore motivate subluminal corrections to the standard dispersion relation of photons that depends linearly on the photon energies [40,41]…”