Recent quotes:

A Rant Against the Use of the Word “Bubble” in the Context of the Bond Market - Washington Center for Equitable Growth

Thus I think clearer thought is obtained by eschewing applying the word “bubble” the bond market. Call them extremely richly-valued. Call them low return. Call them risky for those without the option to hold them to maturity. But if you have to use the word “bubble” apply it to other things. I wouldn’t even say that the U.S. is currently in a monetary policy bubble. For that to be the case, it would have to be a small open economy borrowing heavily in foreign currency, or in some form of currency union with larger economic powers…