The question the Fed has to answer on slow growth is: Why is it happening?

The assumptions behind the Fed's growth model have crumbled

The Washington Post has a great recap of how the Fed got to this point and what it means for today.

The main thrust is that for years after the crisis, the Fed was waiting for certain headwinds to dissipate; like homebuilding, credit or household debt. But every 'temporary' problem they cited has been fixed and yet growth still hasn't come.

So the Fed is being forced to recognize that it was wrong. So its long-held reason for 'why' slow growth was happening is invalid.

Maybe they put and say there's a new headwind; lately it's been the dollar. That would leave the Fed with a hawkish outlook.

The alternative is that they take the view that slow growth is the new normal. That's an idea that's been growing on the fringes of the Fed and has partly been accepted by the core. If they fully accept it; that would imply lower rates for much longer and fewer hikes.

Yellen had flatly rejected a New Normal in 2014 but last month said this: "I think all of us are involved in a process of constantly re-evaluating where is that neutral rate going," .... "Maybe more of what's causing this neutral rate to be low are factors that are not going to be rapidly disappearing but will be part of the new normal."

But it's a hard theory for the Fed to accept until they find an answer to the question of why it's happening at all.

Here is more on today's FOMC decision: