The MAXIMUM size of the universe

[stevemclaren]

hey guys,
anyone know what the scale factor and density would be when the universe is at maximum size. just getting into cosmology recently and this one is puzzling me.

cheers all

steve-o

 

[Wallace]

Our current theories that describe the Universe assume that it is infinite. This is not neccessarily the case, but is certainly very big. This is a separate issue though.

The important point to realize is that the scale factor 'a' is dimensionless, so it doesn't describe some real physical size, it is simply a ratio of the relative scales between two different times. The scale factor is always defined relative to some fixed point when doing a calculation.

Therefore there is no upper bound on the scale factor. Current theory suggests the Universe will continue to expand forever and therefore the scale factor will simply increase and there is no maximum size of the Universe.

With different values of some of the parameters describing cosmology to what we get from experimental data the models do predict the universe would at some stage turnaround from expanding and start contracting. If this was the case then you could calculate the scale factor at this turnaround, although again it only has meaning relative to the scale factor at some other time, and this would be a dimensionless 'maximum size' of the universe. As I say however, current theory does not predict that this turnaround will occur.

 

[Chronos]

In a cyclical universe, you have problems with entropy. Each 'cycle' appears to demand a tiny [but real] toll of energy. But, such a universe would shrink exponential with each cycle. This begs the question of why it has not already shrunk to extinction. While it is logical the current universe appears to be almost exactly flat, it remains unexplained.