Reducing cosmological degeneracies by combining multiple classes of LISA gravitational-wave standard sirens

Abstract

We present the first joint gravitational-wave cosmological inference with LISA extreme mass-ratio inspirals at $z\lesssim1$ (galaxy redshifts) and massive black hole binaries at $z\gtrsim1$ (electromagnetic counterparts). Combining these standard sirens reduces cosmological degeneracies and yields competitive constraints on the Hubble constant $H_0$ and the dark-energy equation-of-state parameter $w_0$. This highlights LISA's potential for late-time cosmology across a broad redshift range with systematics distinct from electromagnetic distance indicators.

Figures (3)

• Figure 1: Reconstructed $d_L$-$z$ regression line for the representative 4-year LISA detection scenario shown in Fig. \ref{['fig:LambdaCDM_2d']} (see main text for more details): we show the median (solid black) and $68\%$ and $90\%$ credible regions in yellow and light gray, respectively. The red dashed line corresponds to the fiducial cosmology. Each data point shows the $1\sigma$ uncertainty of $d_L$ (including LISA instrumental and weak lensing uncertainties), while the redshift uncertainties correspond to the redshift shell over which each single-event likelihood is marginalized. For the MBHBs data points, larger (smaller) redshift error bars correspond to photometric (spectroscopic) follow-ups. The inset shows the most constrained redshift region, while the bottom panel shows the residuals of the inferred regression line and its credible regions.
• Figure 2: Constraints on the two parameters $h$ and $\Omega_m$ in the $\Lambda$CDM model from EMRI dark sirens (blue), MBHB bright sirens (red), and their joint combination (black), for the representative 4-year realization shown in Fig. \ref{['fig:LambdaCDM_regression']}. The contours show the 68% and 90% confidence levels, while the dashed lines show the fiducial cosmology. The top and side panels show 1D marginalized constraints.
• Figure 3: Constraints on the $w_0$ parameter in the DE scenario from EMRI dark sirens (blue), MBHB bright sirens (red), together with their joint combination (black), for a representative 4-year realization. The parameter $w_a$ is unconstrained and is therefore not shown.