The era of tang and song rule is called the golden age because

The Tang and Song rule is described to be the reunification and renaissance in Chinese civilization mostly due to the fact of military advancements, rapid expansion, the Civil Service Exams and taxation methods that developed over time. Li Yuan, Duke of Tang, established the golden age of Tang because of his rapid expansion and strong Chinese bureaucracy. The Civil Service exams created opportunities for families to raise their rank in the social and political hierarchy. The Song dynasty valued scholars over military, making them weak to invaders yet highly cultural and innovative. 

The answer is migration