Narrow Results

In the first part of the review, I discuss ways of obtaining Lorentz-Invariance Violating (LIV) space–time foam in the modern context of string theory, involving brane world scenarios. The foamy structures are provided by lower-dimensional background brane defects in a D3-brane universe, whose density is a free parameter to be constrained phenomenologically. Such constraining can be provided by high energy gamma ray photon tests, including ultra-high energy/infrared photon–photon scattering. In the second part, I analyze the currently available data from MAGIC and FERMI telescopes on delayed cosmic photon arrivals in this context. It is understood of course that conventional astrophysics source effects, which currently are far from being understood, might be the dominant reason for the observed delayed arrivals. I also discuss how the stringent constraints from studies of synchrotron-radiation from distant Nebulae, absence of cosmic birefringence and nonobservation of ultra-high energy cosmic photons can be accommodated within the aforementioned stringy space–time foam model. I argue that, at least within the currently available sets of astrophysical data, the stringy foam model can avoid all theses constraints in a natural range of the string coupling and mass scale. The key features are: (i) transparency of the foam to electrons and charged probes in general, (ii) absence of birefringence effects and (iii) a breakdown of the local effective Lagrangian formalism. However, in order to accommodate, in this theoretical framework, the data of the FERMI satellite on the delayed arrival of photons from the short intense Gamma Ray Burst GRB 090510, in a way consistent with the findings of the MAGIC telescope, a nonuniform density of brane foam defects must be invoked.

In a D-brane model of cosmology and space–time foam, there are contributions to the dark energy that depend on the D-brane 10-velocities and on the density of D-particle defects in the ten-dimensional bulk. The latter may also reduce the speeds of energetic photons, establishing a phenomenological connection with astrophysical probes of the universality of the velocity of light. Specifically, the cosmological dark energy density measured at the present epoch may be linked to the apparent retardation of energetic photons propagating from nearby AGN's. However, this nascent field of "D-foam phenomenology" may be complicated by variations in the D-particle density encountered at different cosmological epochs. A reduced density of D-particles encountered at redshifts z ~ 1 — a "D-void" — would increase the dark energy while suppressing the vacuum refractive index, and thereby might reconcile the AGN measurements with the relatively small retardation seen for the energetic photons propagating from GRB 090510, as measured by the Fermi satellite.