Supposedly, the 3 gamma photons came from a gamma-ray burst, and were detected by the Fermi telescope within a millisecond of each other after traveling all that distance. The implication here is that if space isn't smooth, but rather quantized at the Planck scale, this "foam" would have affected how quickly photons can travel over some distance, and will be more apparent as the distance goes larger. The closeness of the time of arrival for these 3 photons appears to set the graininess of space, if any, at a scale lower than the Planck scale, which would ruffle the feathers of a lot of theorists.
Robert Nemiroff, an astrophysicist at Michigan Technological University, and colleagues recently took a look at data from a gamma-ray burst detected by the Fermi telescope in May 2009.Oh, here's the reference to the PRL paper:
"Originally we were looking for something else, but were struck when two of the highest energy photons from this detected gamma-ray burst appeared within a single millisecond," Nemiroff told Life's Little Mysteries. When the physicists looked at the data more closely, they found a third gamma ray photon within a millisecond of the other two.
Computer models showed it was very unlikely that the photons would have been emitted by different gamma ray bursts, or the same burst at different times. Consequently, "it seemed very likely to us that these three photons traveled across much of the universe together without dispersing," Nemiroff said. Despite having slightly different energies (and thus, different wavelengths), the three photons stayed in extremely close company for the duration of their marathon trek to Earth.
Many things — e.g. stars, interstellar dust — could have dispersed the photons. "But nothing that we know can undisperse gamma-ray photons," Nemiroff said. "So we then conclude that these photons were not dispersed. So if they were not dispersed, then the universe left them alone. So if the universe was made of Planck-scale quantum foam, according to some theories, it would not have left these photons alone. So those types of Planck-scale quantum foams don't exist."
R.J. Nemiroff et al., Phys. Rev. Lett. 108, 231103 (2012).
Now, I could have sworn that a while back, I read a theoretical paper somewhere which indicated that photons traveling through such quantum foam may not show any change in travel time. Since the slowing down and speeding up over a Planck scale is random, after a while, the randomness washes out and the speed remains the same over very large distances. I can't seem to find that paper right now, but essentially, the result reported in this observation doesn't really rule out the existence of such quantum foam.... at least, not yet anyway.