Violent Collisions of Black Holes and Neutron Stars may Increase Universe’s Expansion Rate

Violent Collisions of Black Holes and Neutron Stars may Increase Universe’s Expansion Rate

A new study simulated 25,000 scenarios of black holes and neutron stars colliding, with the goal of determining how many would be detectable by instruments on Earth in the mid- to late-2020s. The researchers discovered that by 2030, instruments on Earth would be able to detect ripples in space-time caused by up to 3,000 such collisions, and that telescopes would see accompanying light explosions for approximately 100 of these events.

A new simulation study led by UCL researchers suggests that 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 (University College London).

Our two best methods for estimating the rate of expansion of the Universe – measuring the brightness and speed of pulsating and exploding stars and looking at fluctuations in radiation from the early Universe – produce very different results, implying that our theory of the Universe may be incorrect.

A third type of measurement, looking at light explosions and ripples in space caused by black hole-neutron star collisions, should help to resolve this disagreement and clarify whether our theory of the Universe needs to be rewritten.

Studying the violent collisions of black holes and neutron stars may soon provide a new measurement of the Universe’s expansion rate.

They concluded that this amount of data would be sufficient to provide a new, completely independent measurement of the rate of expansion of the Universe, 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 10 miles across but with a mass up to twice that of our Sun,” said lead author Dr Stephen Feeney (UCL Physics & Astronomy). Its collision with a black hole is a cataclysmic event that causes ripples in space-time known as gravitational waves, which we can now detect on Earth using observatories such as LIGO and Virgo.

“We haven’t seen any light from these collisions yet. However, advances in the sensitivity of gravitational wave detection equipment, combined with new detectors in India and Japan, will result in a huge leap forward in terms of how many of these types of events we can detect. It’s extremely exciting and could usher in a new era of astrophysics.”

Astrophysicists need to know the distance of astronomical objects from Earth as well as the speed at which they are moving away to calculate the rate of expansion of the Universe, known as the Hubble constant. Gravitational wave analysis tells us how far away a collision is, leaving only the speed to be determined.

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

The “redshift” of light – that is, how the wavelength of light produced by a source has been stretched by its motion – tells us how fast the galaxy hosting a collision is moving away. Light explosions that may occur as a result of these collisions would help us pinpoint the galaxy where the collision occurred, allowing researchers to combine distance and redshift measurements in that galaxy.

Dr. Feeney stated: “The computer models of these cataclysmic events are currently insufficient, and this study should provide additional motivation to improve them. If our assumptions are correct, many of these collisions will not result in detectable explosions – the black hole will devour the star without leaving a trace. However, a smaller black hole may rip apart a neutron star before swallowing it, potentially leaving matter outside the hole that emits electromagnetic radiation.”

Professor Hiranya Peiris (UCL Physics & Astronomy and Stockholm University) stated as a co-author: “One of the most perplexing issues in cosmology is the disagreement over the Hubble constant. The spacetime ripples from these cataclysmic events not only help us solve this puzzle, but also open a new window on the universe. Many exciting discoveries are expected in the coming decade.”

Gravitational waves have been detected at two US observatories (the LIGO Labs), one in Italy (Virgo), and one in Japan (KAGRA). LIGO-India, the fifth observatory, is currently under construction.

Our best current estimates of the Universe’s expansion are 67 kilometers per second per megaparsec (3.26 million light-years) and 74 kilometers per second per megaparsec (3.26 million light-years). The first is derived from studying the cosmic microwave background, which is the radiation left over from the Big Bang, while the second is derived from comparing stars at different distances from Earth, specifically Cepheids, which have variable brightness, and exploding stars known as type Ia supernovae.

Dr. Feeney elaborated: “Because the microwave background measurement requires a complete theory of the Universe to be made, but the stellar method does not, the discrepancy suggests new physics beyond our current understanding. However, before we can make such claims, we need independent confirmation of the disagreement, which we believe can be provided by black hole-neutron star collisions.”

Researchers from UCL, Imperial College London, Stockholm University, and the University of Amsterdam conducted the study. The Royal Society, the Swedish Research Council (VR), the Knut and Alice Wallenberg Foundation, and the Netherlands Organization for Scientific Research all provided funding (NWO).

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