Gravitational-wave and electromagnetic observations of neutron-star-black-hole mergers can provide precise local measurements of the Universe’s rate of expansion. In new research, astrophysicists from Sweden, the United Kingdom and the Netherlands simulated over 25,000 such mergers, aiming to see how many would likely be detected by instruments on Earth.
The first artist’s illustration shows a key part of the process that created this new black hole, as the two neutron stars spin around each other while merging. The purple material depicts debris from the merger. Image credit: NASA / CXC / M.Weiss.
“The current expansion rate of the Universe — the Hubble constant, H0 — is at the heart of a significant cosmological controversy,” said University College London’s Dr. Stephen Feeney and colleagues.
“Direct measurements in the local Universe by the Cepheid-supernova distance ladder find H0 = 74.03 km per second per megaparsec.”
“This is discrepant from the 67.36 km per second per megaparsec value inferred from ESA’s Planck satellite’s observations of the Cosmic Microwave Background, the radiation left over from the Big Bang, 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.”
In the study, the researchers simulated 25,241 scenarios of black holes and neutron stars colliding.
They found that, by 2030, instruments on Earth could sense ripples in space-time caused by up to 3,000 such mergers, 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.
“A neutron star is a dead star, created when a very large star explodes and then collapses, and it is incredibly dense – typically 20 km across but with a mass up to twice that of our Sun,” Dr. Feeney said.
“Its collision with a black hole is a cataclysmic event, causing ripples of space-time, known as gravitational waves, that we can now detect on Earth with observatories like LIGO and Virgo.”
“We have not yet detected light from these collisions. But advances in the sensitivity of equipment detecting gravitational waves, together with new detectors in India and Japan, will lead to a huge leap forward in terms of how many of these types of events we can detect. It is incredibly exciting and should open up a new era for astrophysics.”
“The disagreement over the Hubble constant is one of the biggest mysteries in cosmology,” added Professor Hiranya Peiris, also from University College London.
“In addition to helping us unravel this puzzle, the spacetime ripples from these cataclysmic events open a new window on the Universe. We can anticipate many exciting discoveries in the coming decade.”
The team’s results were published in the journal Physical Review Letters.
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Stephen M. Feeney et al. 2021. Prospects for Measuring the Hubble Constant with Neutron-Star-Black-Hole Mergers. Phys. Rev. Lett 126 (17): 171102; doi: 10.1103/PhysRevLett.126.171102
