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# A Hubble constant measurement from superluminal motion of the jet in GW170817

## Abstract

The Hubble constant (H0) measures the current expansion rate of the Universe, and plays a fundamental role in cosmology. Tremendous effort has been dedicated over the past decades to measure H0 (refs. 1,2,3,4,5,6,7,8,9,10). Gravitational wave (GW) sources accompanied by electromagnetic (EM) counterparts offer an independent standard siren measurement of H0 (refs. 11,12,13), as demonstrated following the discovery of the neutron star merger, GW170817 (refs. 14,15,16). This measurement does not assume a cosmological model and is independent of a cosmic distance ladder. The first joint analysis of the GW signal from GW170817 and its EM localization led to a measurement of $$H_0 = 74_{ - 8}^{ + 16}\,{\mathrm{km}}\,{\mathrm{s}}^{-1}\,{\mathrm{Mpc}}^{-1}$$ (median and symmetric 68% credible interval)13. In this analysis, the degeneracy in the GW signal between the source distance and the observing angle dominated the H0 measurement uncertainty. Recently, tight constraints on the observing angle using high angular resolution imaging of the radio counterpart of GW170817 have been obtained17. Here, we report an improved measurement $$H_0 = 70.3_{ - 5.0}^{ + 5.3}\,{\mathrm{km}}\,{\mathrm{s}}^{-1}\,{\mathrm{Mpc}}^{-1}$$ by using these new radio observations, combined with the previous GW and EM data. We estimate that 15 more GW170817-like events, having radio images and light curve data, as compared with 50–100 GW events without such data18,19, will potentially resolve the tension between the Planck and Cepheid–supernova measurements.

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## Data availability

MCMC samples are available from the corresponding author on request.

## Code availability

The codes used for generating the synthetic light curves are currently being readied for public release. Markov chain Monte Carlo Ensemble sampler: emcee.

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## Acknowledgements

We are grateful to D. Brown, C. Hirata, V. Scowcroft, P. Shawhan, D. Spergel and H. Peiris for useful discussions. We thank the LIGO Scientific and Virgo Collaborations for public access to their data products. K.H. is supported by the Lyman Spitzer Jr. Fellowship at the Department of Astrophysical Sciences, Princeton University. E.N. and O.G. are supported by the I-Core center of excellence of the CHE-ISF. S.N. is grateful for support from NWO VIDI and TOP Grants of the Innovational Research Incentives Scheme (Vernieuwingsimpuls) financed by the Netherlands Organization for Scientific Research (NWO). The work of K.M. is supported by NASA through the Sagan Fellowship Program executed by the NASA Exoplanet Science Institute, under contract with the California Institute of Technology (Caltech)/Jet Propulsion Laboratory (JPL). G.H. acknowledges the support of NSF award AST-1654815. A.T.D. is the recipient of an Australian Research Council Future Fellowship (FT150100415).

## Author information

Authors

### Contributions

K.H. carried out MCMC simulations with the synthetic models. E.N. and O.G. derived an analytic model and carried out hydrodynamic simulations to derive constraints on the observing angle. K.H. and K.M. analysed the posterior samples and calculated H0. G.H., K.P.M. and A.T.D. provided the input observational data. K.H., E.N., S.N. and G.H. wrote the paper. All co-authors discussed the results and provided comments on the manuscript.

### Corresponding authors

Correspondence to K. Hotokezaka or E. Nakar.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information: Nature Astronomy thanks Peter Nugent and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–6 and Supplementary ref. 1.

## Rights and permissions

Reprints and Permissions

Hotokezaka, K., Nakar, E., Gottlieb, O. et al. A Hubble constant measurement from superluminal motion of the jet in GW170817. Nat Astron 3, 940–944 (2019). https://doi.org/10.1038/s41550-019-0820-1

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• DOI: https://doi.org/10.1038/s41550-019-0820-1

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