Let's begin with RISK-FREE RATE 101 class. (Prerequisites: Default-free, Treasury, Safe, Secured)
2 years in the business school and reading all those books of CFA program curriculum. The most repeated term has been- Risk-Free Rate (Rf). It is considered as yield of US Treasury securities. US Government is (was?) considered as default-free thought it fought hard to prevent default recently. Whenever global economy melts or shakes, investors look for Treasury securities. They have been known as the safest investment since there is no volatility or risk included. We employ many risk-return tools (for example, Sharpe Ratio) to calculate excess return achieved by investing in particular assets. This excess return is calculated by considering risk-free rate as benchmark. Many countries with political instability and higher inflation rates do not have their risk-free rates. They add their inflation rate to US risk-free rate to get the benchmark rate for their country. And thus long-term bond yields and so on. This is what the biggest economy stands for.
But in the recent turmoil, I am not able to believe completely that we should still use Treasury yields as risk-free rate or not? I do not want to discuss why S&P has recently downgraded US credit ratings to AA+ from AAA. It may be reversed or may be not. S&P might be wrong in deriving this verdict or they might be late in doing this. I do not know the facts. But few things I surely know that there are few problems in Washington DC. There are serious fiscal challenges we have been facing. Looking at the current scenarios, US may not deserve AAA. But when country has to put efforts to prevent default which US did last week, their risk-free rate should not be considered as default-free rate or benchmark rate. Why can't we change that? Why can't we use risk-free rates from countries like UK, Germany and Canada who still have AAA?
I am not sure about literature we used in the business school, but CFA institute updates its books every year based on recent market conditions. I will not be surprised if I see Canadian rates as risk-free rate in those books next year...