[Paper Review] How can gravitational-wave standard sirens and 21 cm intensity mapping jointly provide a precise late-universe cosmological probe?

Shang-Jie Jin, Ling-Feng Wang+3 more|arXiv (Cornell University)|Jun 3, 2021

Cosmology and Gravitation Theories118 references52 citations

TL;DR

This paper proposes a joint cosmological probe using gravitational-wave standard sirens from the Einstein Telescope and Taiji, combined with 21-cm intensity mapping from the SKA Phase I mid-frequency array, to break parameter degeneracies in late-universe cosmology. It forecasts that this synergy achieves σ(H₀) = 0.28 km s⁻¹ Mpc⁻¹ in the ΛCDM model—better than Planck+BAO+SNe—demonstrating a powerful, precise method to resolve the Hubble tension.

ABSTRACT

In the next decades, the gravitational-wave (GW) standard siren observations and the neutral hydrogen 21-cm intensity mapping (IM) surveys, as two promising cosmological probes, will play an important role in precisely measuring cosmological parameters. In this work, we make a forecast for cosmological parameter estimation with the synergy between the GW standard siren observations and the 21-cm IM surveys. We choose the Einstein Telescope (ET) and the Taiji observatory as the representatives of the GW detection projects and choose the Square Kilometre Array (SKA) phase I mid-frequency array as the representative of the 21-cm IM experiments. In the simulation of the 21-cm IM data, we assume perfect foreground removal and calibration. We find that the synergy of the GW standard siren observations and the 21-cm IM survey could break the cosmological parameter degeneracies. The joint ET+Taiji+SKA data give $\sigma(H_0)=0.28\ { m km\ s^{-1}\ Mpc^{-1}}$ in the $\Lambda$CDM model, $\sigma(w)=0.028$ in the $w$CDM model, which are better than the results of $Planck$+BAO+SNe, and $\sigma(w_0)=0.077$ and $\sigma(w_a)=0.295$ in the CPL model, which are comparable with the results of $Planck$+BAO+SNe. In the $\Lambda$CDM model, the constraint precision of $H_0$ and $\Omega_{ m m}$ is less than or rather close to 1%, indicating that the magnificent prospects for precision cosmology with these two promising cosmological probes are worth expecting.

Motivation & Objective

• To investigate the synergistic potential of gravitational-wave standard sirens and 21-cm intensity mapping for improving late-universe cosmological parameter estimation.
• To address the Hubble tension by combining two independent, next-generation cosmological probes with complementary systematics.
• To forecast cosmological constraints under realistic assumptions, including perfect foreground removal and calibration in 21-cm data.
• To evaluate the performance of the joint probe in the ΛCDM, wCDM, and CPL models, focusing on H₀, w, w₀, and wₐ.
• To demonstrate that the combination breaks cosmological parameter degeneracies more effectively than individual probes or Planck+BAO+SNe.

Proposed method

• Simulates gravitational-wave standard siren events from the Einstein Telescope (ET) targeting binary neutron stars (BNS) and from Taiji targeting massive black hole binaries (MBHB).
• Models the 21-cm intensity mapping power spectrum using the Square Kilometre Array (SKA) Phase I mid-frequency array, assuming perfect foreground removal and calibration.
• Uses a Fisher matrix analysis to forecast cosmological parameter constraints under the ΛCDM, wCDM, and CPL models.
• Sets the fiducial cosmological parameters to Planck 2018 TT,TE,EE+lowE results for mock data generation.
• Combines the Fisher matrices of ET, Taiji, and SKA to compute joint constraints on H₀, Ωₘ, w, w₀, and wₐ.
• Evaluates the precision of H₀ and dark energy equation of state parameters, comparing results to the Planck+BAO+SNe baseline.

Experimental results

Research questions

• RQ1 Can the synergy between GW standard sirens and 21-cm intensity mapping break cosmological parameter degeneracies more effectively than individual probes?
• RQ2 What level of precision can be achieved for H₀, w, w₀, and wₐ using combined ET, Taiji, and SKA data in the ΛCDM, wCDM, and CPL models?
• RQ3 How does the joint constraint precision compare to the Planck+BAO+SNe combination, especially in resolving the Hubble tension?
• RQ4 To what extent does the inclusion of both high-redshift (MBHB) and low-redshift (BNS) standard sirens improve cosmological constraints?
• RQ5 Can 21-cm IM data provide competitive constraints on w(z) compared to distance–redshift measurements, especially when combined with standard sirens?

Key findings

• The joint ET+Taiji+SKA data forecast σ(H₀) = 0.28 km s⁻¹ Mpc⁻¹ in the ΛCDM model, surpassing the precision of Planck+BAO+SNe.
• In the wCDM model, the joint probe achieves σ(w) = 0.028, which is tighter than the Planck+BAO+SNe result.
• For the CPL model, σ(w₀) = 0.077 and σ(wₐ) = 0.295, which are comparable to the Planck+BAO+SNe constraints.
• The constraint on H₀ and Ωₘ in the ΛCDM model is less than or very close to 1%, indicating sub-percent-level precision is achievable.
• The synergy effectively breaks degeneracies between cosmological parameters, particularly enhancing constraints on H₀ and w(z).
• The results demonstrate that combining GW standard sirens and 21-cm IM offers a powerful, independent, and precise probe for late-universe cosmology, with strong potential to resolve the Hubble tension.