A corporate bond bubble?

The FT has the story this morning that corporate bonds are going through the roof, raising fears of a bubble. The rally was driven by a number of positive factors, coupon payments, abundant liquidity, and little new issuance. The lack of willing sellers in the secondary market has forced investors into the primary market. The boom also extends to riskier bonds. Credit Suisse's US high yield bond index yield has fallen to about 7.2%. The article quotes an example of lowly rated HeidelbergCement, which earlier this month sold a €300m bond with a yield of just 4%.