When astronomers speak of distributions like the distribution of hydrogen atoms "in space", what mathematical model of space are they using?

For example, astrological data based on receiving electromagnetic waves is out of date by the time it reaches earth. So if I an astronomer wishes to do statistics about the distribution of something "in space", what convention is used to account for this? Does the astronomer predict what conditions in the rest of the universe will be at the current local time on earth and used 3-D Euclidean space as the model? Or does the astronomer consider a distribution of something "in space" to include things that existed at times in our past? Or is something fancier done, based on the model of 4-D spacetime?