Is the expansion rate slowing?
Look “into the past” to see if expansion rate was faster in
early history.
To “look into the past”  look very far away:
Find “Ho” for very distant objects, compare that to “Ho” for
closer objects
Remember – we found Ho by plotting velocity (vr) vs.
distance
We found velocity vr from the red shift (z)
We found distance by measuring apparent magnitude (mv)
of known brightness objects
We can test for changing Ho by measuring mv vs. z