Does a time estimate equal a promise in Scrum?

Question

If an estimate is not a promise then as a product owner how can I deliver my projects without knowing how long it will take?

Does a Scrum team work more efficiently if we treat time estimates as a promise?

How much research (preparation, effort to understand the problem) in a story is enough to come up the right estimate?

What about unexpected technical problems (problems that can really mess your initial estimates) that come up after you estimate your work?