Violent Black Hole-Neutron Star Collisions May Help Settle Dispute Over Universe’s Expansion

Neutron Star Black Hole

Conflicting results from current methods of estimating the Universe’s expansion rate suggest possible flaws in our understanding. A study proposes using black hole-neutron star collisions for a more conclusive measurement, potentially reshaping our cosmological theory.

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 University College London.

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 mid- to late-2020s.

A Black Hole Devours a Neutron Star

A still from a NASA animation of a black hole devouring a neutron star. Credit: Dana Berry/NASA

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 (16 kilometers) across but with a mass up to twice that of our Sun. 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.”

To calculate the Universe’s rate of expansion, 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. Analyzing gravitational waves tells us how far away a collision is, leaving only the speed to be determined.

To tell how fast the galaxy hosting a collision 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 motion. Explosions of light that may accompany these collisions would help us pinpoint the galaxy where the collision happened, allowing researchers to combine measurements of distance and measurements of redshift in that galaxy.

Dr. Feeney said: “Computer models of these cataclysmic events are incomplete and this study should provide extra 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 may first rip apart a neutron star before swallowing it, potentially leaving matter outside the hole that emits electromagnetic radiation.”

Co-author Professor Hiranya Peiris (UCL Physics & Astronomy and Stockholm University) said: “The disagreement over the Hubble constant is one of the biggest mysteries in cosmology. 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.”

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

Our two best current estimates of the Universe’s expansion are 67 kilometers (42 miles) per second per megaparsec (3.26 million light years) and 74 kilometers (46 miles) per second per megaparsec. The first is derived from analyzing the cosmic microwave background, the radiation left over from the Big Bang, while the second comes from comparing stars at different distances from Earth – specifically Cepheids, which have variable brightness, and exploding stars called type Ia supernovae.

Dr. Feeney explained: “As the microwave background measurement needs a complete theory of the Universe to be made but the stellar method does not, 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 can be provided through black hole-neutron star collisions.”

Reference: “Prospects for Measuring the Hubble Constant with Neutron-Star–Black-Hole Mergers” by Stephen M. Feeney, Hiranya V. Peiris, Samaya M. Nissanke and Daniel J. Mortlock, 28 April 2021, Physical Review Letters.
DOI: 10.1103/PhysRevLett.126.171102

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

8 Comments on "Violent Black Hole-Neutron Star Collisions May Help Settle Dispute Over Universe’s Expansion"

  1. Bibhutibhusanpatel | May 2, 2021 at 11:21 pm | Reply

    Whiĺe taĺkìnģ abòut acceleration in the expansion ŕàte òf unìverse,we need experimentaĺ data to draw añy conclusion.Many of the gravitational wave based observational results support tword sùch flùction.

  2. Bibhutibhusanpatel | May 3, 2021 at 1:08 am | Reply

    Citing to vary in the vaĺue of Hùbble constant,we speak about the gravitatiònal wave measurement method for different galaxies.But,does Theory of Relativity alòne is sùffìcient to describe the change in valùe compĺetly.

    • Torbjörn Larsson | May 3, 2021 at 8:30 am | Reply

      It suffices if it is complemented with the correct model, lots of work has shown that, and the current most used model is the Lambda-Cold Drak Matter model [“Lambda-CDM model”, Wikipedia].

      However, if the rate tension in mostly local (high expansion rates) and mostly global (low expansion rates) measurements is real and not an artifact of the methods, there may be something we are missing. We don’t need entirely new physics though, recently it was discovered that a mundane but overlooked mechanism may be at play.

      “Astronomers are discovering that magnetic fields permeate much of the cosmos. If these fields date back to the Big Bang, they could solve a major cosmological mystery.”

      “Strikingly, this exact amount of primordial magnetism may be just what’s needed to resolve the Hubble tension — the problem of the universe’s curiously fast expansion.

      That’s what Pogosian realized when he saw recent computer simulations by Karsten Jedamzik of the University of Montpellier in France and a collaborator. The researchers added weak magnetic fields to a simulated, plasma-filled young universe and found that protons and electrons in the plasma flew along the magnetic field lines and accumulated in the regions of weakest field strength. This clumping effect made the protons and electrons combine into hydrogen — an early phase change known as recombination — earlier than they would have otherwise.”

      “Their calculations indicated that, indeed, the amount of primordial magnetism needed to address the Hubble tension also agrees with the blazar observations and the estimated size of initial fields needed to grow the enormous magnetic fields spanning galaxy clusters and filaments. “So it all sort of comes together,” Pogosian said, “if this turns out to be right.””

      [ ]

      We’ll see what comes out of all this fairly soon, I hope.

  3. Torbjörn Larsson | May 3, 2021 at 8:20 am | Reply

    Nice! A lot of these alternative methods for measuring expansion rate are studied now.

    There is another promising redshift method using fast radio bursts that already gives results and promise them faster.

    As an FRB signal travels through the dust and gas separating galaxies, it becomes scattered in a predictable way that causes some frequencies to arrive slightly later than others. The farther away the FRB, the more dispersed the signal.
    [The expansion rate] it’s tentatively closer to the value from the cosmic microwave background, or CMB.

    With a few hundred FRBs, the team estimates that it could reduce the uncertainties and match the accuracy of other methods such as supernovas. … Many new radio observatories are coming online and larger surveys, such as ones proposed for the Square Kilometre Array, could discover tens to thousands of FRBs every night. Hagstotz expects there will sufficient FRBs with distance estimates in the next year or two to accurately determine the Hubble constant.
    [ ]

  4. David Calvert | May 4, 2021 at 1:22 am | Reply

    I feel to measure the rate at which the universe is growing measure your closes to furthest stars out in all directions. That would give you an outline of our universes shape and then come up with a period of time to allow to pass by and go back and re measure the stars and see how much further they have moved since the time you first measured them it would literally grid out our entire universe and give a good calculation on the rate the universe is growing because as it grows I’m pretty sure the furthest star will have moved away from its last place as everything is moving through space that is.

    • Torbjörn Larsson | May 4, 2021 at 9:56 am | Reply

      That is what they do, except that the yearly change in distance is too small relative to the redshift stretch. So they take a “snapshot” of redshifts and fit them to a set of models of the universe and its expansion.

      “To calculate the Universe’s rate of expansion, 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.”

  5. Dinar Dayanov | May 7, 2021 at 9:24 pm | Reply

    Expansion of space might be different in voids and in cosmic web. The same as time flows. It is possible in future shorter distance would be not to move by straight line but around the corner 😜

    • Torbjörn Larsson | May 9, 2021 at 7:17 am | Reply

      It is possible with non-homogeneities, but the universe starts out homogeneous and isotropic to 1 part in 100,000 as seen in the cosmic background radiation.

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