If the result from the GW siren is consistent with that from the early-time measurements, such as the CMB, it would imply that the current understanding of distance ladder systematics is not enough and that the concordance model ΛCDM still works. Resolving this tension will lead to important implications. Hence, it can shed some light on the Hubble tension. Since the GW siren is a completely independent measurement, its result will suffer from different systematics. The former demand fairly good synergies, which are extremely challenging for high redshift CBC events while the latter, which do not rely on transient measurements, ask for a precise sky localization to reduce the number of possible host galaxies. Depending on whether or not the events are associated with electromagnetic (EM) counterparts, GW events can be categorized into bright sirens and dark sirens . With self-calibration by the theory of general relativity, gravitational waves (GWs) from compact binary coalescence (CBC) events open a completely novel observational window for H 0 determination . Once this precision level is achieved, we give priority to understanding the systematics, especially the unknown ones, rather than simply to increasing the sample volume. A new independent H 0 measurement whose accuracy is better than is crucial in order to judge the current discrepancy . Hence, the resulting values might be miscalibrated due to the aforementioned astro-physical issues. However, generally, it has more serious systematics, such as the reddening of the cepheid or red-giant branch stars, metallicity effects, etc. . The distance ladder is a direct H 0 measurement. Although these measurements are more precise compared with the late-time distance ladder , in this way the resulting H 0 is cosmological model dependent. The values obtained from early-time observables such as the cosmic microwave background (CMB) or the big bang nucleosynthesis plus baryon acoustic oscillation are indirect, because to get H 0 from those measurements one has to assume a cosmological model. The measurement of the Hubble parameter has reached a crossroad .
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