Chiral gravitational wave background in millihertz from axion-like fields
The first detection of gravitational waves has opened a new window to probe fundamental problems in astrophysics and cosmology.In this paper,we briefly introduce the chiral gravitational waves originating from the early universe,focusing on those within the millihertz-frequency range.Millihertz frequencies are within the detection capabilities of space-based gravitational wave detectors,including LISA,Taiji,and Tianqin.We mainly compare the gravitational wave spectra generated by a rolling axion field during the radiation-dominated era,examining two similar models:axion-dark-photon coupling and axion-graviton coupling.Furthermore,we introduce a mechanism of axion inflation with gravitational Chern-Simons coupling.We numerically calculate the shape of the stochastic gravitational wave spectrum generated in this scenario,paying special attention to its peak frequency range as it would be observed today.Additionally,we discuss the parity-violating effect of the gravitational wave signal induced by the Chem-Simons topological term.
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