Combined observations of neutron stars constrain their equation of state and the Hubble constant


Combining signals from multiple observations of neutron stars has allowed researchers to better understand the properties of ultra-dense matter and constrain the Hubble constant, which describes how fast the Universe is expanding, according to a new study. Neutron stars are the collapsed cores of massive stars and have greater densities than an atomic nucleus. However, little is known about the properties of matter under such conditions, which cannot be reached in Earth-bound laboratories. To study matter at these extremes, researchers turn to cosmic collisions – binary neutron star mergers. When neutron stars collide, they release both electromagnetic radiation and gravitational waves. Observations of these distinct signals from the same event, known as multi-messenger astronomy, can be used to study the state of immensely dense neutron star material and the expansion rate of the Universe. Tim Dietrich and colleagues developed an analytical framework that combined messengers from two neutron star mergers – the gravitational wave event GW170817 and its accompanying electromagnetic signals, and the gravitational wave-only event GW1904215. Combining these events with independent electromagnetic measurements of isolated neutron stars and calculations from nuclear physics theory, Dietrich et al. constrained the neutron star equation of state, which relates the mass and radius of each neutron star. The approach also provides a measurement of the Hubble constant; they find a value which is most consistent with previous measurements of the cosmic microwave background.


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