Gravitational Wave Informed Inference of 21-cm Global Signal Parameters

Understanding how and when the first stars and galaxies formed remains one of the central challenges in modern cosmology. These structures emerged during the transition from the Dark Ages to the Cosmic Dawn, a period that remains observationally unconstrained despite strong theoretical progress. During this epoch, neutral hydrogen absorbed a fraction of cosmic microwave background photons through its 21-cm hyperfine transition, producing a 21-cm absorption signal whose evolution encodes the early Universe’s thermal and ionization history. However, extracting the underlying astrophysical parameters from this signal is limited by severe parameter degeneracies, which cannot be resolved without independent observational probes. The next-generation gravitational wave (GW) detectors, such as Cosmic Explorer (CE), will observe binary black hole (BBH) mergers up to very large redshifts and hence will detect a fraction of them formed within the redshift range $\sim 13-25$. The merger rate of these BBHs will depend on the star formation rate density (SFRD) at these redshifts, together with the BBH formation efficiency and a time delay distribution. Therefore, the merger rate of these BBHs can work as a tracer of the SFRD in the redshift range $\sim 13-25$. In this Letter, we establish a novel multi-messenger framework and present a proof-of-principle concept of how the observations of BBH mergers form next-generation GW detectors can improve the inference of parameters generating the 21-cm cosmic hydrogen signal, and help break degeneracies between them.

💡 Research Summary

The paper tackles one of the most persistent challenges in early‑universe cosmology: disentangling the astrophysical parameters that shape the global 21‑cm signal from the Cosmic Dawn (redshifts ≈ 13–25). The differential brightness temperature δTb(z) depends on three key quantities: the star‑formation‑rate density (SFRD, Ψ(z)), the Lyman‑α coupling efficiency (fα) that drives the spin temperature toward the kinetic temperature, and the X‑ray heating efficiency (fX) that later raises the kinetic temperature above the CMB. Current global‑signal experiments (EDGES, SARAS, REACH, LEDA) suffer from overwhelming foregrounds and instrumental systematics, leaving a strong degeneracy between fα, fX and the underlying SFRD.

The authors propose a novel multi‑messenger strategy: use the merger rate of binary black holes (BBHs) observed by next‑generation ground‑based gravitational‑wave detectors (Cosmic Explorer, CE, and Einstein Telescope, ET) as an independent tracer of the high‑z SFRD. The BBH merger rate R(z) is modeled as the product of the formation rate Rf(z) (proportional to the SFRD, the initial‑mass‑function, and a BBH formation efficiency η(z)), a delay‑time distribution p(td), and the usual (1+z) cosmological factor. For proof‑of‑principle they adopt a highly simplified population: a delta‑function mass distribution at 16–15 M⊙, η(z)=1 for all redshift, and a fixed delay time of 1 Gyr (p(td)=δ(td−1 Gyr)). With these assumptions the expected number of detectable BBH events in the CE horizon for 13 ≲ z ≲ 25 is ≈100 in a 70‑day run and ≈1000 in a two‑year run.

The 21‑cm signal model itself is kept deliberately simple. The SFRD is parametrized as Ψ(z)=Ψ0 exp