Wormholes and Child Universes

Evidence to the case that classical gravitation provides the clue to make sense out of quantum gravity is presented. The key observation is the existence in classical gravitation of child universe solutions or almost solutions, almost because of some singularity problems. The difficulties of these child universe solutions due to their generic singularity problems will be very likely be cured by quantum effects, just like for example almost instanton solutions are made relevant in gauge theories with breaking of conformal invariance. Some well motivated modifcations of General Relativity where these singularity problems are absent even at the classical level are discussed. High energy density excitations, responsible for UV divergences in quantum field theories, including quantum gravity, are likely to be the source of child universes which carry them out of the original space time. This decoupling could prevent these high UV excitations from having any influence on physical amplitudes. Child universe production could therefore be responsible for UV regularization in quantum field theories which take into account semiclassically gravitational effects. Child universe production in the last stages of black hole evaporation, the prediction of absence of tranplanckian primordial perturbations, connection to the minimum length hypothesis and in particular the connection to the maximal curvature hypothesis are discussed. Some discussion of superexcited states in the case these states are Kaluza Klein excitations is carried out. Finally, the posibility of obtaining string like effects from the wormholes associated with the child universes is discussed.

Gravitational Theory with a Dynamical Time

A gravitational theory involving a vector field $chi^{mu}$, whose zero component has the properties of a dynamical time, is studied. The variation of the action with respect to $chi^{mu}$ gives the covariant conservation of an energy momentum tensor $ T^{mu u}_{(chi)}$. Studying the theory in a background which has killing vectors and killing tensors we find appropriate shift symmetries of the field $chi^{mu}$ which lead to conservation laws. The energy momentum that is the source of gravity $ T^{mu u}_{(G)}$ is different but related to $ T^{mu u}_{(chi)}$ and the covariant conservation of $ T^{mu u}_{(G)}$ determines in general the vector field $chi^{mu}$. When $ T^{mu u}_{(chi)}$ is chosen to be proportional to the metric, the theory coincides with the Two Measures Theory, which has been studied before in relation to the Cosmological Constant Problem. When the matter model consists of point particles, or strings, the form of $ T^{mu u}_{(G)}$, solutions for $chi^{mu}$ are found. For the case of a string gas cosmology, we find that the Milne Universe can be a solution, where the gas of strings does not curve the spacetime since although $ T^{mu u}_{(chi)} eq 0$, $ T^{mu u}_{(G)}= 0$, as a model for the early universe, this solution is also free of the horizon problem. There may be also an application to the time problem of quantum cosmology.

String Gas Shells, their Dual Radiation and Hedgehog Signature Control

We search for spherically symmetric, stationary solutions with a string gas shell as a source. The requirement of a uniform newtonian potential, or constancy of the 00 component of the metric, implies the existence of a dual radiation, which we argue can be interpreted as representing the virtual quantum fluctuations that stabilize the shell. A string hedgehog can be introduced also into the solution. For zero or small hedgehog strength the string gas shell is of a regular nature, while the dual radiation is of a spacelike nature. For higher hedgehog strengths however the radiation materializes and becomes timelike while the string gas shell becomes space like. The significance of these solutions for the quantum theory is discussed.

Photon and Axion Splitting in an Inhomogeneous Magnetic Field

The axion photon system in an external magnetic field, when the direction of propagation of axions and photons is orthogonal to the direction of the external magnetic field, displays a continuous axion-photon duality symmetry in the limit the axion mass is neglected. The conservation law that follow in this effective 2+1 dimensional theory from this symmetry is obtained. The magnetic field interaction is seen to be equivalent to first order to the interaction of a complex charged field with an external electric potential, where this ficticious electric potential is proportional to the external magnetic field. This allows one to solve for the scattering amplitudes using already known scalar QED results. From the scalar QED analog the axion and the photon are symmetric and antisymmetric combinations of particle and antiparticle. If one considers therefore scattering experiments in which the two spatial dimensions of the effective theory are involved non trivially, one observes that both particle and antiparticle components of photons and axions are preferentially scattered in different directions, thus producing the splitting or decomposition of the photon and axion into their particle and antiparticle components in an inhomogeneous magnetic field. This observable in principle effect is of first order in the axion photon coupling, unlike the light shining through a wall phenomena , which is second order.