Effects Of Spacetime Foam Research Articles

Astrophysical effects of space-time foam

During the quantum stage, the space-time had a foamlike structure. As the Universe cools down, the foam structure tempers and does not disappear, therefore the Friedmann model represents only an idealization. We show that local inhomogeneity of the foamy structure should be observed as a Dark Matter (DM) halo around point sources. Such a halo extends up to the scales where the foamy structure averages to homogeneity. We also show that scattering of photons from a point source on the foamlike structure gives rise to the origin of a diffuse component of radiation whose distribution traces that of DM.

Supersymmetric D-brane model of space-time foam

We present a supersymmetric model of space-time foam with two stacks of eight D8-branes with equal string tensions, separated by a single bulk dimension containing D0-brane particles that represent quantum fluctuations in the space-time foam. The ground-state configuration with static D-branes has zero vacuum energy. However, gravitons and other closed-string states propagating through the bulk may interact with the D0-particles, causing them to recoil and the vacuum energy to become non-zero. This provides a possible origin of dark energy. Recoil also distorts the background metric felt by energetic massless string states, which travel at less than the usual (low-energy) velocity of light. On the other hand, the propagation of chiral matter fields anchored on the D8-branes is not affected by such space-time foam effects.

Space-time foam effects on particle interactions and the Greisen-Zatsepin-Kuzmin cutoff

Modelling space-time foam using a non-critical Liouville-string model for the quantum fluctuations of D branes with recoil, we discuss the issues of momentum and energy conservation in particle propagation and interactions. We argue that momentum should be conserved exactly during propagation and on the average during interactions, but that energy is conserved only on the average during propagation and is in general not conserved during particle interactions, because of changes in the background metric. We discuss the possible modification of the GZK cutoff on high-energy cosmic rays, in the light of this energy non-conservation as well as the possible modification of the usual relativistic momentum-energy relation.

Search for quantum spacetime foam

The possibility that quantum fluctuations in the structure of spacetime at the Planck scale might be subject to experimental probes in discussed. The effects of spacetime foam in an approach inspired by string theory, in which solitonic D-brane excitations are taken into account when considering the ground state, are studied. The properties of this medium are described by analyzing the recoil of a D particle which is induced by the scattering of a closed-string state. This recoil causes an energy-dependent perturbation of the background metric, which in turn induces an energy-dependent refractive index in vacuo, and stochastic fluctuations of the light cone. Distant astrophysical sources such as Gamma-Ray Bursters (GRBs) may be used to test this possibility, and an illustrative analysis of GRBs whose redshifts have been measured is presented. The propagation of massive particles through such a quantum spacetime foam is also discussed.

Distance Measurement and Wave Dispersion in a Liouville-String Approach to Quantum Gravity

Within a Liouville approach to noncritical string theory, we discuss space–time foam effects on the propagation of low-energy particles. We find an induced frequency-dependent dispersion in the propagation of a wave packet, and observe that this would affect the outcome of measurements involving low-energy particles as probes. In particular, the maximum possible order of magnitude of the space–time foam effects would give rise to an error in the measurement of distance comparable to that independently obtained in some recent heuristic quantum-gravity analyses. We also briefly compare these error estimates with the precision of astrophysical measurements.

Is quantum gravity unpredictable?

An investigation of Hawking's proposal that the inclusion to topologically non-trivial manifolds in the functional integral of quantum gravity leads to the loss of quantum coherence is carried out. We discuss some of the problems associated with Hawking's $-matrix theory, including the breakdown of the connection between symmetry principles and conservation laws. It is proposed to use Kaluza-Klein theories to study this issue, since these theories contain well-defined euclidean instantons. These can be used to perform explicit semiclassical calculations of the effects of space-time foam. A general method is presented for constructing Kaluza-Klein instantons based on solutions of ordinary Yang-Mills theory. It is argued that none of these will lead to a breakdown of quantum mechanics. The physical effects of space-time foam are discussed in some detail using explicit instantons of a four-dimensional Kaluza-Klein theory.