Testing the Weak Equivalence Principle with Cosmological Gamma Ray
  Bursts
  Gamma Ray Bursts (GRBs) with rapid variations at cosmological distances are
used to place new limits on violations of the gravitational weak equivalence
principle (WEP). These limits track intrinsic timing deviations between GRB
photons of different energies as they cross the universe, in particular in the
KeV to GeV energy range. Previous limits in this energy range have involved
only the gravitational potential of local sources and utilized temporal
variability on the order of 0.1 seconds. Here WEP violation limits are derived
from sources with greater distance, faster variability, and larger intervening
mass. Specifically, GRB sources with redshifts as high as 6.5 are considered,
with variability as fast 0.2 milliseconds, and passing the gravitational
potentials of inferred clusters of galaxies distributed randomly around the
line of sight. WEP violation limits are derived from data from GRB 910711, GRB
920229, GRB 021206, GRB 051221, GRB 090429, and GRB 090510. The strongest
constraint in the very early universe comes from GRB 090429 which limits
$\gamma(500$ keV$) - \gamma(250$ keV$) < 1.2\times10^{-13}$. The strongest
overall constraint comes from GRB 090510 which yields a WEP violation limit of
$\gamma(30$ GeV$) - \gamma (1$ GeV$) < 6.6\times10^{-16}$. This strongest
constraint is not only a new record for WEP violation limit for gamma-ray
photons and in the early universe, but the strongest upper bound for $\Delta
\gamma$ that has ever been recorded between any two energy bands.
