Gravitational waves offer a powerful probe into the early universe, providing insights into its dynamics and fundamental physics. This seminar will explore the stochastic gravitational wave background (SGWB) and its potential origins, including signals detected in the NANOGrav 15-year dataset. The speaker will discuss how hybrid inflation can generate topological defects, such as cosmic strings, and contribute to the SGWB, highlighting model-building approaches that link theory to observation. Cosmic Strings may have an imprint on the CMB. The researcher will briefly talk about how people can see strings’ footprints in the CMB and constrain the cosmic string tension. In addition, she will examine the possible formation of primordial black holes. This presentation aims to bridge theoretical insights and observational data, showing the connections between inflation, gravitational waves, and cosmic structure formation.