US debt downgrade drives up borrowing costs

AFP/GettyA sign showing the US national debt in April. It has since risen to more than $36tnInvestors might have hoped the turmoil that hit financial markets last month was over. But another bout of volatility in US borrowing costs is a sign that worries continue to bubble. The interest rate on the US government's long-term debt surpassed the 5% on Monday, before retreating slightly. That was the highest level…