The basic idea is that, in the present environment, nominal Treasuries have a negative beta. If we go into Depression, the expectation is that this will be accompanied by substantial deflation, so that Treasuries will do well in real terms. In contrast, the typical pattern is different--perhaps bad times are usually accompanied by high inflation or at least average inflation. Therefore, especially at the shorter end, the relation between indexed and conventional Treasuries has shifted--the real rate on indexed bonds now has to be well above the expected real rate on nominal bonds. This observation also means, particularly at the shorter end, that the spread provides little information about expected inflation.
Barro's brain storming regarding correspondence of Treasury and TIPS and he is says don’t worry be happy market has been always thinner healthier
