Detecting extremely low frequency primordial gravitational wave by gravitational lens system

Abstract: Primordial gravitational waves (PGWs) are predicted to origin from inflation, according to which a period of accelerated expansion exists in the very early Universe. The detection of PGWs would verify the inflationary theory and determine its energy scale. The traditional method of using B-mode polarization to detect extremely low frequency PGW faces challenges due to the contamination from dust in Milky Way. We investigated the feasibility of using gravitational lens system (GLS) with source of high redshift to detect extremely low frequency PGW. With GLS perturbed by extremely low frequency PGWs, we found that the observed time delay in GLS could strongly deviate from the theoretical one, such strong deviation is the evidence of extremely low frequency PGWs.