FRWL models...
______________

In similar way, the time interval can be computed:



Look-back time is:

The age of the universe is:

Again, in general this has to be evaluated numerically, and 
even for the               , the formulae are rather cumbersome.

For              , and              :


For              , and              :


Note also that the cosmological parameters change in time.  
For example:







The theologically prefered                ,               model is also 
called the Einstein - De Sitter model.