Cosmological wheel of time: A classical perspective of f(R) gravity

It is shown that the structures in the universe can be interpreted to show a closed wheel of time, rather than a straight arrow. An analysis in [Formula: see text] gravity model has been carried out to show that due to local observations, a small arc at any given spacetime point would invariably indicate an arrow of time from past to future, though on a quantum scale it is not a linear flow but a closed loop, a fact that can be examined through future observations.

Covariant diagonalization of the perfect fluid stress–energy tensor

We introduce new tetrads that manifestly and covariantly diagonalize the stress–energy tensor for a perfect fluid with vorticity at every spacetime point. This new tetrad can be applied to introduce simplification in the analysis of astrophysical relativistic problems where vorticity is present through the Carter–Lichnerowicz equation. We also discuss the origin of inertia in this special case from the standpoint of our new local tetrads.

Time-dependent collisional radiative model of ionizing and recombining plasmas

A time-dependent collisional radiative model, based on a five-charge state at any spacetime point in the plasma, is elaborated in order to follow the basic atomic physics processes in a plasma undergoing ionization and recombination. This model can either be used independently or be included in a magnetohydrodynamics code of a transient plasma. In the latter case, the efficiency of the model makes it particularly useful in saving computing time of hydrodynamic codes developed in connection with ion-driven fusion.