Cosmological evolution of a ghost scalar field

We consider a scalar field with a negative kinetic term minimally coupled to gravity. We obtain an exact non-static spherically symmetric solution which describes a wormhole in cosmological setting. The wormhole is shown to connect two homogeneous spatially flat universes expanding with acceleration. Depending on the wormhole's mass parameter $m$ the acceleration can be constant (the de Sitter case) or infinitely growing.Comment: 8 page

Giant wormholes in ghost-free bigravity theory

We study Lorentzian wormholes in the ghost-free bigravity theory described by two metrics, g and f. Wormholes can exist if only the null energy condition is violated, which happens naturally in the bigravity theory since the graviton energy-momentum tensors do not apriori fulfill any energy conditions. As a result, the field equations admit solutions describing wormholes whose throat size is typically of the order of the inverse graviton mass. Hence, they are as large as the universe, so that in principle we might all live in a giant wormhole. The wormholes can be of two different types that we call W1 and W2. The W1 wormholes interpolate between the AdS spaces and have Killing horizons shielding the throat. The Fierz-Pauli graviton mass for these solutions becomes imaginary in the AdS zone, hence the gravitons behave as tachyons, but since the Breitenlohner-Freedman bound is fulfilled, there should be no tachyon instability. For the W2 wormholes the g-geometry is globally regular and in the far field zone it becomes the AdS up to subleading terms, its throat can be traversed by timelike geodesics, while the f-geometry has a completely different structure and is not geodesically complete. There is no evidence of tachyons for these solutions, although a detailed stability analysis remains an open issue. It is possible that the solutions may admit a holographic interpretation.Comment: 26 pages, 6 figures, section 8.2 describing the W1b wormhole geometry is considerably modifie

The radiative potential method for calculations of QED radiative corrections to energy levels and electromagnetic amplitudes in many-electron atoms

We derive an approximate expression for a "radiative potential" which can be used to calculate QED strong Coulomb field radiative corrections to energies and electric dipole (E1) transition amplitudes in many-electron atoms with an accuracy of a few percent. The expectation value of the radiative potential gives radiative corrections to the energies. Radiative corrections to E1 amplitudes can be expressed in terms of the radiative potential and its energy derivative (the low-energy theorem): the relative magnitude of the radiative potential contribution is ~alpha^3 Z^2 ln(1/(alpha^2 Z^2)), while the sum of other QED contributions is ~alpha^3 (Z_i+1)^2, where Z_i is the ion charge; that is, for neutral atoms (Z_i=0) the radiative potential contribution exceeds other contributions ~Z^2 times. The advantage of the radiative potential method is that it is very simple and can be easily incorporated into many-body theory approaches: relativistic Hartree-Fock, configuration interaction, many-body perturbation theory, etc. As an application we have calculated the radiative corrections to the energy levels and E1 amplitudes as well as their contributions (-0.33% and 0.42%, respectively) to the parity non-conserving (PNC) 6s-7s amplitude in neutral cesium (Z=55). Combining these results with the QED correction to the weak matrix elements (-0.41%) we obtain the total QED correction to the PNC 6s-7s amplitude, (-0.32 +/- 0.03)%. The cesium weak charge Q_W=-72.66(29)_{exp}(36)_{theor} agrees with the Standard Model value Q_W^{SM}=-73.19(13), the difference is 0.53(48).Comment: 29 pages, 2 figure