The bond market believes the economy is gaining momentum. Long before Federal Reserve policymakers begin gathering in Washington today to debate what to do with interest rates, mortgage rates throughout the country have been heating up. Borrowing rates on 30-year fixed-rate mortgages have climbed to 4.21%. That’s up from 4.09% in January and 3.54% in November, a month before the Fed last hiked short-term rates. The Fed is all-but-certain to hike short-term rates once again — by a quarter of a percentage point after its two-meeting ends Wednesday afternoon. This should keep the central bank on track for three rate increases by year end, which is what policymakers have recently been hinting at.