Will the real estate bubble be the main problem for central banks?

The very expansionary monetary policy conducted in the United States and the euro zone is obviously driving up residential real estate prices. But there could be a massive real estate bubble if the interest rate on mortgages were permanently lower than the growth in rents. The reason is that the fundamental value of housing (the discounted sum of future rents) becomes infinite: it becomes rational to run up debt without limit to buy housing. This configuration cannot be seen yet, but it is getting dangerously close in the United States and the euro zone.