Black hole-neutron star collisions may help settle dispute over expansion of Universe – sciencedaily

Studying the violent collisions of black holes and neutron stars may soon provide a new measure of the Universe’s rate of expansion, helping to resolve a long-standing dispute, suggests a new simulation study led by researchers from UCL (University College London).

Our current two best ways of estimating the rate of expansion of the Universe – measuring the brightness and speed of pulsating and explosive stars, and examining the radiation fluctuations of the first Universe – yield very different answers, suggesting that 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 resolve this disagreement and clarify whether our theory of the Universe should be rewritten.

The new study, published in Physical examination letters, simulated 25,000 black hole and neutron star collision scenarios, aiming to see how many would likely be detected by instruments on Earth between the mid to late 2020s.

Researchers found that by 2030, instruments on Earth could detect ripples in space-time caused by up to 3,000 of these collisions, and that for about 100 of these events, telescopes would also see explosions. associated light.

They concluded that this data would be sufficient to provide a completely independent new measure 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 – usually 10 miles in diameter but with a mass up to twice that of our Sun. Its collision with a black hole is a cataclysmic event, causing ripples in space-time, called gravitational waves, which we can now detect on Earth with observatories like LIGO and Virgo.

“We have not yet detected the light of these collisions. But advances in the sensitivity of gravitational wave detection equipment, as well as new detectors in India and Japan, will allow for a huge leap forward in terms of many of these types of events that we can detect. It’s incredibly exciting and should usher in a new era for astrophysics. “

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

To find out how fast the colliding galaxy is moving away, we look at the “redshift” of light – that is, how the wavelength of light produced by a source has been stretched. by its movement. Explosions of light that could accompany these collisions would help us locate the galaxy where the collision occurred, allowing researchers to combine distance measurements and redshift measurements in that galaxy.

Dr Feeney said: “Computer models of these cataclysmic events are incomplete and this study should provide additional motivation to improve them. If our assumptions are correct, many of these collisions will not produce explosions that we can detect – the black hole will swallow the star.without leaving a trace. But in some cases, a smaller black hole can first tear apart a neutron star before swallowing it, potentially leaving material outside the emitting hole. electromagnetic radiation. “

Co-author Professor Hiranya Peiris (UCL Physics & Astronomy and Stockholm University) said: “The disagreement over the Hubble constant is one of the greatest mysteries in cosmology. Besides helping us unravel this puzzle, the spatiotemporal waves of these cataclysmic events open a new window on the universe. We can expect many exciting discoveries in the decade to come. “

Gravitational waves are detected in two observatories in the United States (LIGO Labs), one in Italy (Virgo) and one in Japan (KAGRA). A fifth observatory, LIGO-India, is currently under construction.

Our two best current estimates of the expansion of the Universe are 67 kilometers per second per megaparsec (3.26 million light years) and 74 kilometers per second per megaparsec. The first is derived from the analysis of the cosmic background of microwaves, the radiation left behind by the Big Bang, while the second is derived from the comparison of stars at different distances from Earth – in particular the Cepheids, which have a variable brightness, and exploding stars called type Ia supernovae.

Dr Feeney explained: “Since microwave background measurement requires a full theory of the Universe, but not the stellar method, the disagreement offers tantalizing evidence of new physics beyond our current understanding. Before we can make such claims, however, we need confirmation of the disagreement from completely independent observations – we believe these may be provided by black hole-neutron star collisions. “

The study was carried out by researchers from UCL, Imperial College London, Stockholm University and University of Amsterdam. It was supported by the Royal Society, the Swedish Research Council (VR), the Knut and Alice Wallenberg Foundation and the Netherlands Organization for Scientific Research (NWO).

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