Is it necessary for the Product Owner to have a complete and exhaustive product backlog before the first Sprint can start?

It is not necessary for the Product Owner to have a complete and exhaustive product backlog before the first Sprint begins. In Scrum, the product backlog is a living artifact that evolves and changes over time as new information is gained and priorities shift. The focus of the initial backlog should be on having enough items defined to provide clarity and direction for the first Sprint. Items that are high priority should be clearly defined, but there is no requirement for the entire scope of the product to be detailed from the outset. This allows the team to start delivering value early and to adapt based on feedback and learning that occurs during each Sprint.

The nature of agile development emphasizes iterative progress and flexibility, meaning that the backlog is continuously refined and prioritized as work progresses. This practice supports the Scrum principles of responding to change and delivering increments of usable product as early as possible.