Abstract

It has been observed that there are definite cut-offs at which the radial velocities of particles near galaxies/clusters are exactly zero. This boundary can be interpreted as the edge of the galaxy/cluster, where the gravitational attraction of the central object is exactly balanced by the cosmological pull due to the universe’s expansion. One may begin to explain this phenomenon using a non-zero cosmological constant, but of course the current universe is not purely de-Sitter. In this talk I will present a new theoretical framework for describing a mass particle in a realistically expanding background. I will show how this leads to a fully general relativistic invariant expression for the radial force required to hold a test particle at rest relative to the central mass, and use it to successfully predict the edges of large-scale objects and also the locations of maximum stable orbits. Finally I will present some extensions of this work, which include the implications for a universe whose expansion is driven by phantom energy, which leads to a “Big Rip” at a finite time in the future, and the solutions for spatially curved cosmologies.