Exploring future synergies for large-scale structure between gravitational waves and radio sources

Abstract

Future third-generation gravitational wave detectors like the Einstein Telescope (ET) and Cosmic Explorer (CE) are expected to detect millions of binary black hole (BBH) mergers. Alongside these advances, upcoming radio surveys, such as the Square Kilometer Array Observatory (SKAO) will provide new sets of cosmological tracers. These include mapping the large-scale distribution of neutral hydrogen (\hi) using intensity mapping (IM) and \hi\ and radio continuum galaxies. In this work, we will investigate synergies between gravitational waves (GW) and radio tracers through a multi-tracer approach. We first forecast the precision on the clustering bias of GWs by cross-correlating data from an ET-like detector with an SKAO IM survey. Our results indicate that this approach can constrain the GW clustering bias to within $2\%$ up to $z = 2.5$. Additionally, we explore the potential of a triple cross-correlation using GWs, IM, and photometric galaxies from a survey like the Vera Rubin Observatory's Legacy Survey of Space and Time (LSST). This multi-tracer method enhances constraints on the magnification lensing effect, achieving percent-level precision, and allows for a measurement of the Doppler effect with approximately $15\%$ uncertainty. Furthermore we show for the first time that this method could achieve the precision required to measure subdominant gravitational potential contributions to the relativistic corrections, which had thought to be below cosmic variance. Our analysis highlights the potential of cross-correlations between GWs and radio tracers to improve constraints on astrophysical properties of BBHs, measure relativistic effects, and perform null tests of GR in cosmological scales.

Figures (8)

• Figure 1: Measurement of the amplitude of the clustering bias of GW sources ($B$) by cross-correlating different pairs of GWs detectors and radio surveys. Faint lines show results from only $1$ year of observation, whilst bold lines include the (predicted) full length of the experiment. In the case of an ET-like detector we only adopt $5$ years of observation to show the potential with simply half their predicted run time. On top, above the dassh line, we add the result from 2025MNRAS.537.1912Z using an ET$\times$LSST-like as a mean of comparison for the results when one combines GW with radio surveys.
• Figure 2: Left:Projected $1$ and $2\sigma$ contour of the clustering bias $b$ of GWs as a function of redshift for a $1$ year observation period using cross-correlations of GWs from an ET-like detector and an Hi intensity mapping survey like SKA1-MID. Right: Forecasted $1$ and $2\sigma$ measurement errors on the clustering bias of GWs, for both $1$ and $5$ years of observations from an ET-like detector, and $10$k hours of observation for SKA1-MID-like telescope, in blue and red lines respectively. We also add in faint grey lines the results from an ET$\times$LSST-like cross-correlation for comparison.
• Figure 3: Predicted measurements of different relativistic corrections to the observed number counts fluctuation in a cross-correlation of ET-like GW data and Hi intensity mapping from an SKAO-like instrument. From top to bottom we show constraints on the two parameters of the clustering bias of GWs (i.e. $B$ and $\alpha$), then forecasted measurement of the lensing $\epsilon_L$, luminosity distance space distortions $\epsilon_{LSD}$, Doppler effect $\epsilon_D$ and finally further relativistic corrections $\epsilon_P$. We show the $1\sigma$ contours for $1$ and $5$ years worth of GWs data.
• Figure 4: Constraints on the magnification (top) and evolution (bottom) biases of GWs for an ET-like experiment, obtained by cross-correlating GWs with an IM survey such as SKA1-MID.
• Figure 5: Constraints on magnification and evolution biases of GW sources, $s^{GW}$ and $b_e^{GW}$, at $z=2$, together with constraints on clustering bias parameters $B$ and $\alpha$, and the measurements of the relativistic effects $\epsilon_L,\epsilon_{LSD},\epsilon_D,\epsilon_P$. These are obtained through a cross-correlation of GWs from an ET-like detector and IM survey such as SKA1-MID.