Universe Expansion - Terminal Velocity?

[Efar]

If one looks at the expansion(increasing in speed) of the universe as a "falling away" from The Beginning(not religious,I just hate the Big Bang term)in all directions,and Time as an expansion rather than a linear progression,What is the terminal velocity of the universe?

 

[bcrowell]

Velocity is relative, so you'd have to specify the velocity of some object relative to some other object.

General relativity does not have a uniquely defined notion of the velocity of one object relative to another distant object.

Suppose that you pick as your measure of relative velocity the apparent velocity of galaxy B as determined by its redshift measured by an observer A, who crunches the numbers as if it were a special-relativistic redshift. Then because of the cosmological constant, this velocity will eventually surpass c for any given pair of galaxies A and B, after which B will no longer be observable to A.

On the other hand, there are other measures of relative velocity according to which B could have *always* had a velocity greater than c relative to A, and yet be observable to A: Davis and Lineweaver, Publications of the Astronomical Society of Australia, 21 (2004) 97, msowww.anu.edu.au/~charley/papers/DavisLineweaver04.pdf

 

[Chronos]

The Hubble sphere determines expansion rates. A photon that originated in a superluminal patch of spacetime can eventually reach a subluminal patch of spacetime. This is discussed in the Lineweaver paper bcrowell linked.

 

[Radrook]

Space expansion doesn't have a terminal velocity that we know of since the speed of light limit doesn't apply to it but only to matter moving within space itself.