The discovery of gravitational waves from the coalescence of binary black holes in 2015 has marked the dawn of the gravitational waves astronomy/cosmology. Besides the gravitational wave bursts from black hole/neutron star mergers, our universe is fulfilled by stochastic background of gravitational waves with a large range of frequencies, which could have many physical origins in the early universe, for instance incoherent superposition of binary systems, first order phase transitions, reheating after inflation, topological defects, and primordial gravitational waves. As our universe is transparent to gravitational wave, it is a fossil recording the information of its generation and how our universe evolves. In this talk I will briefly review the stochastic background of the gravitational waves (GWs), especially the secondary gravitational waves induced by scalar perturbations, and discuss their connection to the primordial black holes which may be candidates for dark matter or the seeds for galaxy formation.
