We extend the theory of cosmological perturbations to the case when the “matter” Lagrangian is an arbitrary function of the scalar field and its first derivatives. In particular, this extension provides a unified description of known cases such as the usual scalar field and the hydrodynamical perfect fluid. In addition, it applies to the recently proposed k-inflation, which is driven by non-min...

We exhibit a fundamental relationship between measures of dynamical and structural stability of linear dynamical systems-e.g. linearized models in the vicinity of equilibria. We show that dynamical stability, quantified via the response to external perturbations (i.e. perturbation of dynamical variables), coincides with the minimal internal perturbation (i.e. perturbations of interactions betwe...

We determine optimal inflow boundary perturbations to steady flow through a straight inflexible tube with a smooth axisymmetric stenosis at a bulk-flow Reynolds number Re = 400, for which the flow is asymptotically stable. The perturbations computed produce an optimal gain, i.e. kinetic energy in the domain at a given time horizon normalized by a measure of time-integrated energy on the inflow ...

The intergalactic medium (IGM) prior to the epoch of reionization consists mostly of neutral hydrogen gas. Lyman-α (Lyα) photons produced by early stars resonantly scatter off hydrogen atoms, causing energy exchange between the radiation field and the gas. This interaction results in moderate heating of the gas due to the recoil of the atoms upon scattering, which is of great interest for futur...

Considering the growing interest in sending probes to natural satellite Titan, our work aims investigate and map orbits around this moon. For that, we use mathematical models with forces that have symmetry/asymmetry phenomena, depending on force, applied Titan. We evaluated effects due gravitational attraction of Saturn, together perturbative coming from non-sphericity Titan (the coefficient J2...

In the radial flow of gas into a black hole (i.e. Bondi accretion), the infall of any entropy or vorticity perturbation produces acoustic waves propagating outward. The dependence of this acoustic flux on the shape of the perturbation is investigated in detail. This is the key process in the mechanism of the entropic-acoustic instability proposed by Foglizzo& Tagger (2000) to explain the instab...

We investigate the cosmological perturbations of the brane-induced (Dvali-GabadadzePorrati) model which exhibits a van Dam-Veltman-Zakharov (vDVZ) discontinuity when linearized over a Minkowski background. We show that the linear brane scalar cosmological perturbations over an arbitrary Friedmann-Lemâıtre-Robertson-Walker (FLRW) spacetime have a well defined limit when the radius of transition ...

A new theory of the motion of Phobos – the moon of Mars – is created. The accuracy of this theory is to within one meter of the mean motion of Phobos over one year which is the best result obtained so far for the analytical approach. The perturbational analysis is of main interest. The elements of the physical interactions which are the most essential for the motion of Phobos are specified. The...