1,442 research outputs found
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 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
Resonance reactions and enhancement of weak interactions in collisions of cold molecules
With the creation of ultracold atoms and molecules, a new type of chemistry -
"resonance" chemistry - emerges: chemical reactions can occur when the energy
of colliding atoms and molecules matches a bound state of the combined molecule
(Feshbach resonance). This chemistry is rather similar to reactions that take
place in nuclei at low energies. In this paper we suggest some problems for
future experimental and theoretical work related to the resonance chemistry of
ultracold molecules. Molecular Bose-Einstein condensates are particularly
interesting because in this system collisions and chemical reactions are
extremely sensitive to weak fields; also, a preferred reaction channel may be
enhanced due to a finite number of final states. The sensitivity to weak fields
arises due to the high density of narrow compound resonances and the
macroscopic number of molecules with kinetic energy E=0 (in the ground state of
a mean-field potential). The high sensitivity to the magnetic field may be used
to measure the distribution of energy intervals, widths, and magnetic moments
of compound resonances and study the onset of quantum chaos. A difference in
the production rate of right-handed and left-handed chiral molecules may be
produced by external electric and magnetic fields and the finite width of the
resonance. The same effect may be produced by the parity-violating energy
difference in chiral molecules.Comment: 5 pages. Included discussion of expected size of effect
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