Black hole-neutron star collisions may settle dispute over Universe’s expansion

Studying the violent collisions of black holes and neutron stars may soon provide a new measurement of the Universe's expansion rate, helping to resolve a long-standing dispute, suggests a new simulation study led by researchers at UCL. Our two current best ways of estimating the Universe's rate of expansion - measuring the brightness and speed of pulsating and exploding stars, and looking at fluctuations in radiation from the early Universe - give very different answers, suggesting our theory of the Universe may be wrong. A third type of measurement, looking at the explosions of light and ripples in the fabric of space caused by black hole-neutron star collisions, should help to resolve this disagreement and clarify whether our theory of the Universe needs rewriting. The new study, published in Physical Review Letters , simulated 25,000 scenarios of black holes and neutron stars colliding, aiming to see how many would likely be detected by instruments on Earth in the midto late-2020s. The researchers found that, by 2030, instruments on Earth could sense ripples in space-time caused by up to 3,000 such collisions, and that for around 100 of these events, telescopes would also see accompanying explosions of light. They concluded that this would be enough data to provide a new, completely independent measurement of the Universe's rate of expansion, precise and reliable enough to confirm or deny the need for new physics. Lead author Dr Stephen Feeney (UCL Physics & Astronomy) said: "A neutron star is a dead star, created when a very large star explodes and then collapses, and it is incredibly dense - typically 10 miles across but with a mass up to twice that of our Sun.