# How is the Hubble constant determined from gravitational waves?

We know there is a discrepancy between measurements of the Hubble constant, $$H_0$$. On one side there is the method of the Planck mission, where they use the CMB and the $$\Lambda$$CDM model to determine the Hubble constant. On the other side, they use standard candles, like Cepheid variables (for example: Riess et al., 2019) and red giants (for example: Freedman et al., 2020). The very precise measurements don’t overlap and thus we need a ‘solution’.

One way to improve our knowledge about $$H_0$$ is by using gravitational waves to determine the Hubble constant (Feeney et al., Arxiv version). They compare the measured ‘strength’ of the gravitational wave with the ‘strength’ of the wave when it was created to determine the distance to the source object. But how do they calculate/determine the ‘strength’ of the wave at the source? I know they also use an EMS observation for calculating the redshift (and that redshift converted to radial velocity divided by the distance gives $$H_0$$), but from where do they calculate the source-‘strength’ of the gravitational wave? How can they otherwise determine how much energy the wave has lost? I can’t seem to find the answer.

• There are statistical methods for determining $H_0$ without having to identify a host galaxy. (Essentially, you end up doing a weighted average over all possible hosts in the localization error box.) – mmeent Jun 2 '20 at 15:15