The Fermi bubbles (FBs) are two giant, non-thermal, bipolar lobes extending \sim50^o in latitude above and below the Galactic center (GC). It is still unclear if the FBs are driven by starburst activity near the GC or by an outflow from its supermassive black hole. By simulating the Milky Way with different GC activity, we use the phenomenology of the FBs to both constrain their origin and measure the circumgalactic medium (CGM) parameters.