Anisotropies of Gravitational-Wave Standard Sirens as a New Cosmological Probe without Redshift Information

Gravitational waves (GWs) from compact binary stars at cosmological distances are promising and powerful cosmological probes, referred to as the GW standard sirens. With future GW detectors, we will be able to precisely measure source luminosity distances out to a redshift $z\sim 5$. To extract cosmological information, previously proposed cosmological studies using the GW standard sirens rely on source redshift information obtained through an extensive electromagnetic follow-up campaign. However, the redshift identification is typically time consuming and rather challenging. Here, we propose a novel method for cosmology with the GW standard sirens free from the redshift measurements. Utilizing the anisotropies of the number density and luminosity distances of compact binaries originated from the large-scale structure, we show that, once GW observations will be well established in the future, (i) these anisotropies can be measured even at very high redshifts ($z\ge 2$), where the identification of the electromagnetic counterpart is difficult, (ii) the expected constraints on the primordial non-Gaussianity with the Einstein Telescope would be comparable to or even better than the other large-scale structure probes at the same epoch, and (iii) the cross-correlation with other cosmological observations is found to have high-statistical significance, providing additional cosmological information at very high redshifts.

Figure 1

Angular auto-power spectra with expected $1\sigma$ statistical errors (top) and the cumulative SNR of the LSS-induced anisotropies (bottom) with $\mathrm{\Delta }\ell =1$. Assuming the three-year observation with the ET, we divide measured luminosity distances into five contiguous bins with equal number of binary sources. The angular power spectrum and SNR is then computed, and plotted as a function of the maximum multipole, ${\ell }_{\max }$. The dashed lines show the lensing contributions. The vertical axis at $\ell =100$ corresponds to the angular resolution of the ET while the resolution of DECIGO is approximately $\ell =1000$.