LAUSR

Gravitational waves (GWs) from merging black holes and neutron stars directly measure the luminosity distance to the merger, which, when combined with an independent measurement of the source’s redshift, provides a novel probe of cosmology. The proposed next generation of groundbased GW detectors, Einstein Telescope and Cosmic Explorer, will detect tens of thousands of binary neutron stars (BNSs) out to cosmological distances (z > 2), beyond the peak of the star formation rate (SFR), or “cosmic noon.” At these distances, it will be challenging to measure the sources’ redshifts by observing electromagnetic (EM) counterparts or statistically marginalizing over a galaxy catalog. In the absence of an EM counterpart or galaxy catalog, Ding et al. [1] showed that theoretical priors on the merger redshift distribution can be used to infer parameters in a wCDM cosmology. We argue that in the BNS case, the redshift distribution will be measured by independent observations of short gamma ray bursts (GRBs), kilonovae, and known BNS host galaxies. In particular, the peak redshift will provide a clear feature to compare against the peak distance of the GW source distribution and reveal the underlying redshift-distance relation. We show that, in addition to measuring the background cosmology, this method can constrain the effects of dark energy on modified GW propagation. As a simple example, we consider the case in which the BNS rate is a priori known to follow the SFR. If the SFR is perfectly known, O(10, 000) events (to be expected within a year of observation with Cosmic Explorer) would yield a sub-tenth percent measurement of the combination H 0 ΩM in a flat ΛCDM model. Meanwhile, fixing H0 and ΩM to independently-inferred values, this method may enable a 5% measurement of the dark energy equation of state parameter w in a wCDM model. Fixing the background cosmology and instead probing modified GW propagation, the running of the Planck mass parameter cM may be measured to ±0.02. Although realistically, the redshift evolution of the merger rate will be uncertain, prior knowledge of the peak redshift will provide valuable information for standard siren analyses.