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Abstract

The simultaneous detection of gravitational waves and light from the binary
neutron star merger GW170817 led to independent measurements of distance and
redshift, providing a direct estimate of the Hubble constant $H_0$ that does
not rely on a cosmic distance ladder nor assumes a specific cosmological model.
By using gravitational waves as ''standard sirens'', this approach holds
promise to arbitrate the existing tension between the $H_0$ value inferred from
the cosmic microwave background and those obtained from local measurements.
However, the known degeneracy in the gravitational-wave analysis between
distance and inclination of the source lead to a $H_0$ value from GW170817 that
was not precise enough to resolve the existing tension. In this review, we
summarize recent works exploiting the viewing-angle dependence of the
electromagnetic signal, namely the associated short gamma-ray burst and
kilonova, to constrain the system inclination and improve on $H_0$. We outline
the key ingredients of the different methods, summarize the results obtained in
the aftermath of GW170817 and discuss the possible systematics introduced by
each of these methods.