[Paper Review] First measurement of the Hubble parameter from bright binary black hole GW190521

Suvodip Mukherjee, Archisman Ghosh+7 more|arXiv (Cornell University)|Sep 29, 2020

Pulsars and Gravitational Waves Research9 references36 citations

TL;DR

The paper measures H0 and constrains Omega_m and w0 using a bright BBH event GW190521 with a candidate EM counterpart ZTF19abanrhr, combining GW distance with EM redshift, and also updates H0 with GW170817 and VLBI inclination information.

ABSTRACT

The Zwicky Transient Facility (ZTF) reported the event "ZTF19abanrhr" as a candidate electromagnetic (EM) counterpart at a redshift $z=0.438$ to the gravitational wave (GW) emission from the binary black hole merger GW190521. Assuming that ZTF19abanrhr is the {\it bona fide} EM counterpart to GW190521, and using the GW luminosity distance estimate from three different waveforms NRSur7dq4, SEOBNRv4PHM, and IMRPhenomPv3HM, we report a measurement of the Hubble constant $H_0= 50.4_{-19.5}^{+28.1}$ km/s/Mpc, $ 62.2_{-19.7}^{+29.5}$ km/s/Mpc, and $ 43.1_{-11.4}^{+24.6}$ km/s/Mpc (median along with $68\%$ credible interval) respectively after marginalizing over matter density $Ω_m$ (or dark energy equation of state $w_0$) assuming the flat LCDM (or wCDM) model. Combining our results with the binary neutron star event GW170817 with its redshift measurement alone, as well as with its inclination angle inferred from Very Large Baseline Interferometry (VLBI), we find $H_0= 67.6_{-4.2}^{+4.3}$ km/s/Mpc, $Ω_m= 0.47_{-0.27}^{+0.34}$, and $w_0= -1.17_{-0.57}^{+0.68}$ (median along with $68\%$ credible interval) providing the most stringent measurement on $H_0$ and the first estimation on $Ω_m$ and $w_0$ from bright standard siren. In the future, $1.3\%$ measurement of $H_0=68$ km/s/Mpc and $28\%$ measurement of $w_0=-1$ is possible from about $200$ GW190521-like sources.

Motivation & Objective

• Motivate and evaluate the use of gravitational-wave standard sirens from BBH mergers with EM counterparts to measure cosmological parameters.
• Compute H0, Omega_m, and w0 using Bayesian inference by combining GW luminosity distance with EM redshift data under LCDM and wCDM models.
• Assess improvements by including multiple GW waveforms and additional events (GW170817, VLBI inclination) to constrain cosmology.
• Explore prospects and forecast how future BBH+EM standard sirens could tighten cosmological constraints.

Proposed method

• Use Bayes theorem to infer cosmological parameters Theta_c from GW luminosity distance and EM redshift data.
• Marginalize over d_l with priors and over Omega_m or w0 depending on LCDM or wCDM models.
• Analyze three data set combinations (D1, D2, D3) with three GW waveforms (SEOBNRv4PHM, NRSur7dq4, IMRPhenomPv3HM).
• Incorporate GW170817 and VLBI inclination to refine H0 estimates.
• Present posterior distributions for H0, Omega_m, and w0 under different model/data combinations.

Experimental results

Research questions

• RQ1 Can a bright BBH event with an EM counterpart constrain the Hubble constant using standard siren methods?
• RQ2 How do different GW waveforms affect the inferred H0 and cosmological parameters from GW190521+ZTF19abanrhr?
• RQ3 What is the impact of including GW170817 and VLBI inclination on joint cosmological constraints?
• RQ4 What level of precision can be achieved on H0, Omega_m, and w0 with future GW190521-like events?
• RQ5 Do the results from BBH standard sirens align with Planck or SH0ES measurements within uncertainties?

Key findings

• H0 estimates from GW190521+ZTF19abanrhr vary by waveform: 62.2+29.5-19.7, 50.4+28.1-19.5, and 43.1+24.6-11.4 km/s/Mpc (D1a, D1b, D1c).
• Combining with GW170817 yields H0 = 69.1-5.8+9.4 km/s/Mpc for D2b under NRSur7dq4; VLBI inclination further tightens to H0 = 67.6+4.3-4.2 km/s/Mpc (D3b).
• Joint constraints from D3b give Omega_m = 0.47-0.27+0.34 and w0 = -1.17-0.57+0.68 for the first standard-siren-era limits on these parameters.
• The measured H0 is consistent with Planck within about 1.6 sigma of SH0ES, and the work provides the first constraints on Omega_m and w0 from bright standard sirens.
• Forecasts suggest about 1.3% precision on H0 and ~28% on w0 with ~200 GW190521-like sources with EM counterparts.
• The analysis demonstrates the potential of bright BBH mergers with EM counterparts to probe cosmology up to high redshift.