The direct detections of gravitational waves (GWs) by Advanced LIGO/Virgo have opened a new era in astronomy. The discovery of an unexpected population of massive binary BHs (BBHs) constraints their formation pathway and requires massive star formation in low-metallicity environments. In this talk, I discuss low-metallicity massive star formation in the early universe, so called Population III stars (PopIII stars), and binary evolution of massive PopIII stars into colliding BBHs. Intriguingly, PopIII binary model can explain the natures of coalescing BBHs observed by LIGO/Virgo, in terms of the merger event rate and BH mass distribution. A majority of the first binary BHs form and merge in the early universe. We have suggested that such individually unresolved GWs develop a GW background, which would be detectable by O5 LIGO/Virgo observation. More interestingly, the BH population producing the GW background will also be probed by the observations of the cosmic microwave background, on which stellar activities of the first stars are directly imprinted.
