Single word for "Expanding and Contracting"
Solution 1:
I suggest you change you sentence slightly, which would give you the flexibility to use lots of different words:
There is a theory that the size of the universe is _____.
Especially with words like oscillate, you have to tell us what is oscillating. There are a few words like pendulate that are more specific about what's going on, but oscillate only tells you that something is changing magnitude, not what is changing magnitude.