The concept of simultaneity, and the associated problems of synchronization of moving clocks and establishment of a universal time scale, were given little or no attention prior to the twentieth century. In 1905, however, Einstein analyzed the concept of simultaneity on the basis of two principal postulates: Absolute velocity is meaningless,The velocity of light in a vacuum is the same for all unaccelerated observers.

Of the many cosmological models developed during the past forty years, none is completely satisfactory; so work continues in the development of new models and in the checking of their predictions against the growing mass of experimental evidence. The validity of a model rests on its agreement with experimental data, particularly the data on distribution of galaxies and on redshifts.

Recession of the galaxies indicates a repulsive force and suggests that Newton's formulation of gravitation is not complete. A possible modification is proposed, and this Neo-Newtonian equation allows a quantitative treatment of Mach's principle. It also limits the velocity of matter to c and gives a correct prediction for the perihelion of Mercury.