Modern Physics Letters A

The observation of gravitational waves using space-borne instrumentation allows the detection of astrophysical sources in a much longer wavelength part of the spectrum than ground-based instruments can reach. This opens up the prospect of a completely different view of the Universe than can be achieved in any other way. This article summarises the scientific goals of the LISA mission in its search for sources within the 10^-4 to 10^-1 Hz window and describes the current status of the overall project.

Cosmological data reveal a dark energy with a negative equation of state w=P/ρ<0. We discuss its stability under nucleation from Λ>0 to Λ=0 in an assumed first-order phase transition. From arguments about stimulated nucleation it is shown that the critical nucleation radius must be at least of galactic size. The spontaneous nucleation rate is infinitesimal thus explaining the dark energy's extreme stability. Dark energy detection at subgalactic scales will never be possible.

We consider a massless scalar field, conformally coupled to the Ricci scalar curvature, in the pre-inflation era of a closed FLRW Universe. The scalar field potential can be of the form of the Coleman–Weinberg one-loop potential, which is flat at the origin and drives the inflationary evolution. For positive values of the conformal parameter ξ, less than the critical value ξ_c=(1/6), the model admits exact solutions with nonzero minimum scale factor and zero initial Hubble parameter. Thus these solutions can be matched smoothly to the so-called Pre-Big-Bang models. At the end of this pre-inflation era one can match inflationary solutions by specifying the form of the potential and the whole solution is of the class C⁽¹⁾.

Massive gravity models in (2+1) dimensions, such as those obtained by adding to Einstein's gravity the usual Fierz–Pauli, or the more complicated Ricci scalar squared (R²), terms, are tree level unitary. Interesting enough these seemingly harmless systems have their unitarity spoiled when they are augmented by a Chern–Simons term. Furthermore, if the massive topological term is added to gravity, or to gravity (higher-derivative gravity), which are nonunitary at the tree level, the resulting models remain nonunitary. Therefore, unlike the common belief, as well as the claims in the literature, the coexistence between three-dimensional massive gravity models and massive topological terms is conflicting.

For the creation operator a^† and the annihilation operator a of a harmonic oscillator, we consider Weyl ordering expression of (a^†a)ⁿ and obtain a new symmetric expression of Weyl ordering w.r.t. a^†a≡N and aa^†=N+1 where N is the number operator. Moreover, we interpret intertwining formulas of various orderings in view of the difference theory. Then we find that the noncommutative parameter corresponds to the increment of the difference operator w.r.t. variable N. Therefore, quantum (noncommutative) calculations of harmonic oscillators are done by classical (commutative) ones of the number operator by using the difference theory. As a by-product, nontrivial relations including the Stirling number of the first kind are also obtained.

Recent investigations involving the decay of unstable D-branes in string theory suggest that the tree level open string theory which describes the dynamics of the D-brane already knows about the closed string states produced in the decay of the brane. We propose a specific conjecture involving quantum open string field theory to explain this classical result, and show that the recent results in two-dimensional string theory are in exact accordance with this conjecture.

The dielectric function of nuclear matter excited by ρNN tensor coupling has been studied in the framework of finite temperature field theory. The induced current mechanism has been introduced to explain the three extrema on the dielectric function curve, of which one is in the space-like region and the other two are in the time-like region. It points out that the tensor coupling contributes much more large amplitude than the vector coupling and plays a more important role on the time-like region compared with its effect on the space-like region.

Multi-Hamiltonian systems are investigated by using the Hamilton–Jacobi method. Integration of a set of total differential equations which includes the equations of motion and the action integral function is discussed. It is shown that this set is integrable if and only if the total variations of the Hamiltonians vanish. Two examples are studied.