We describe the first observations of the same celestial object with gravitational waves and light. ▪ GW170817 was the first detection of a neutron star merger with gravitational waves. ▪ The detec...

First Multimessenger Observations of a Neutron Star Merger

Values of the Hubble constant between the direct measurements from various independent local observations and that inferred from the cosmic microwave background with the $\mathrm{\ensuremath{\Lambda}}$-cold-dark-matter model are in tension with persistent significance We propose a late-time inhomogeneous resolution suggesting that a chameleon field coupled to a local overdensity of matter could be trapped at a higher potential energy density as an effective cosmological constant driving the local expansion rate faster than that of the background with lower matter density We illustrate this mechanism in a toy model in which a region with only 20% overdensity of matter is sufficient to resolve the Hubble tension, and the Hubble constant measured by the local distance ladders could be accommodated by the chameleon coupled to the observed overdensities from the large-scale structure surveys

Chameleon dark energy can resolve the Hubble tension

The Hubble tension, if not caused by any systematics, could be relieved or even resolved from modifying either the early-time or late-time Universe. The early-time modifications are usually in tension with either galaxy clustering or galaxy lensing constraints. The late-time modifications are also in conflict with the constraint from the inverse distance ladder, which, however, is weakened by the dependence on a sound-horizon prior and some particular approximation for the late-time expansion history. To achieve a more general no-go argument for the late-time scenarios, we propose to use a global parametrizationbased on the cosmic age (PAge) to consistently use the cosmic chronometers data beyond the Taylor expansion domain and without the input of a sound-horizon prior. Both the early-time and late-time scenarios are therefore largely ruled out, indicating the possible ways out of the Hubble tension from either exotic modifications of our concordance Universe or some unaccounted systematics. 

/pdf/no-go-guide-for-the-hubble-tension-late-time-solutions-396j9kuq.pdf

No-go guide for the Hubble tension: Late-time solutions

The multimessenger data of neutron star merger events are promising for constraining the Hubble constant. So far, GW170817 is still the unique gravitational wave event with multiwavelength electromagnetic counterparts. In particular, its radio and X-ray emission have been measured in the past ∼3–5 yr. In this work, we fit the long-lasting X-ray, optical, and radio afterglow light curves of GW170817/GRB 170817A, including the forward shock radiation from both the decelerating relativistic gamma-ray burst outflow and the subrelativistic kilonova outflow (though whether the second component contributes significantly is still uncertain), and find out a relatively large viewing angle (∼0.5 rad). Such a viewing angle has been taken as a prior in the gravitational wave data analysis, and the degeneracy between the viewing angle and the luminosity distance is broken. Finally, we have a Hubble constant H0=72.57−4.17+4.09kms−1Mpc−1 , which is more consistent with that obtained by other local measurements. If rather similar values are inferred from multimessenger data of future neutron star merger events, it will provide critical support to the existence of the Hubble tension.

The Late Afterglow of GW170817/GRB 170817A: A Large Viewing Angle and the Shift of the Hubble Constant to a Value More Consistent with the Local Measurements

The multi-messenger detection of the gravitational-wave signal the corresponding kilonova AT2017gfo and the short gamma-ray burst GRB170817A, as well as the observed afterglow has delivered a scientiﬁc breakthrough. For an accurate interpretation of all these different messengers, one requires robust theoretical models that describe the emitted gravitational-wave, the electromagnetic emission, and dense matter reliably. In addition, one needs efﬁcient and accurate computational tools to ensure a correct cross-correlation between the models and the observational data. For this purpose, we have developed the NMMA (Nuclear-physics and Multi-Messenger Astrophysics) framework. The code allows incorporation of nuclear-physics constraints at low densities as well as X-ray and radio observations of isolated neutron stars. It also enables us to classify electromagnetic observations, e.g., to dis-tinguish between supernovae and kilonovae. In previous works, the NMMA code has allowed us to constrain the equation of state of supranuclear dense matter, to measure the Hubble constant, and to compare dense-matter physics probed in neutron-star mergers and in heavy-ion collisions. The extension of the NMMA code presented here is the ﬁrst attempt of analysing the gravitational-wave signal, the kilonovae, and the GRB afterglow simultaneously, which re-duces the uncertainty of our constraints. Incorporating all available information, we estimate the radius of a 1 . 4 M (cid:12) neutron star to be R =11 . 98 +0 . 35 − 0 . 40 km.

pdf/nmma-a-nuclear-physics-and-multi-messenger-astrophysics-33rrijy2.pdf

NMMA: A nuclear-physics and multi-messenger astrophysics framework to analyze binary neutron star mergers

The electromagnetic radiation that followed the neutron star merger event GW170817 revealed that gamma-ray burst afterglows from jets misaligned with our line of sight exhibit a light curve with slowly rising flux. The slope of the rising light curve depends sensitively on the angle of the observer with respect to the jet axis, which is likely to be perpendicular to the merger plane of the neutron star binary. Therefore, the afterglow emission can be used to constrain the inclination of the merging system. Here, we calculate the gamma-ray burst afterglow emission based on the realistic jet structure derived from general-relativistic magnetohydrodynamical simulations of a black hole torus system for the central engine of the gamma-ray burst. Combined with gravitational wave parameter estimation, we fit the multi-epoch afterglow emission of GW170817. We show that with such a jet model, the observing angle can be tightly constrained by multimessenger observations. The best fit observing angle of GW170817 is θv = 0.38 ± 0.02 rad. With such a constraint, we can break the degeneracy between inclination angle and luminosity distance in gravitational wave parameter estimation, and substantially increase the precision with which the Hubble constant is constrained by the standard siren method. Our estimation of the distance is DL = 43.4 ± 1 Mpc and the Hubble constant constraint is 69.5 ± 4 km s−1 Mpc−1. As a result, multimessenger observations of short-duration gamma-ray bursts, combined with a good theoretical understanding of the jet structure, can be powerful probes of cosmological parameters.

Multimessenger Parameter Estimation of GW170817: From Jet Structure to the Hubble Constant

The electromagnetic radiation that followed the neutron-star merger event GW170817 revealed that gamma-ray burst afterglows from jets misaligned with our line of sight exhibit a light curve with slowly rising flux. The slope of the rising light curve depends sensitively on the angle of the observer with respect to the jet axis, which is likely to be perpendicular to the merger plane of the neutron star binary. Therefore, the afterglow emission can be used to constrain the inclination of the merging system. Here, we calculate the GRB afterglow emission based on the realistic jet structure derived from general-relativistic magnetohydrodynamical simulations of black hole-torus system for the central engine of the gamma-ray burst. Combined with gravitational wave parameter estimation, we fit the multi-epoch afterglow emission of GW170817. We show that with such a jet model, the observing angle can be tightly constrained by multi-messenger observations. The best fit observing angle of GW170817 is $\theta_{\rm v} = 0.38\pm 0.02$ rad. With such a constraint, we can break the degeneracy between inclination angle and luminosity distance in gravitational wave parameter estimation, and substantially increase the precision with which the Hubble constant is constrained by the standard siren method. Our estimation of the distance is $D_{\rm L}=43.4\pm 1\ \rm Mpc$ and the Hubble constant constraint is $69.5\pm 4\ \mathrm{km\ s^{-1}Mpc^{-1}}$. As a result, multi-messenger observations of short-duration gamma-ray bursts, combined with a good theoretical understanding of the jet structure, can be powerful probes of cosmological parameters.

Multi-messenger parameter estimation of GW170817: from jet structure to the Hubble constant

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Multimessenger Parameter Estimation of GW170817: From Jet Structure to the Hubble Constant

Multi-messenger parameter estimation of GW170817: from jet structure to the Hubble constant