According to a recent simulation study performed by experts at UCL (University College London), studying the catastrophic collisions of black holes and neutron stars may soon offer a fresh measurement of the Universe’s expansion rate, potentially resolving a long-standing debate.
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.
The third sort of measurement, which looks at the light bursts and ripples in the fabric of space generated by black hole-neutron star collisions, might assist to resolve this debate and explain if our theory of the Universe has to be rewritten.
The latest study, which was published in Physical Review Letters, simulated 25,000 scenarios of black holes and neutron stars colliding to determine how many might be detected by Earth-based detectors in the mid to late-2020s. The researchers discovered that by 2030, equipment on Earth will be able to detect ripples in space-time generated by up to 3,000 such collisions, with about 100 of these occurrences accompanied by light bursts.
They came to the conclusion that this data would be sufficient to produce a new, fully independent measurement of the rate of expansion of the Universe, accurate and trustworthy enough to validate or refute the necessity 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. 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. However, improvements in the sensitivity of gravitational wave detection equipment, as well as new detectors in India and Japan, will result in a significant increase in the number of such occurrences that can be detected. It’s really exciting, and it may usher in a new age in astrophysics.”
Astrophysicists need to know the distance of celestial objects from Earth, as well as the speed at which they are traveling away, to calculate the Hubble constant, which is the rate at which the Universe expands. The distance to a collision may be estimated by analyzing gravitational waves, leaving only the speed to be computed.
The “redshift” of light, or how the wavelength of light generated by a source has been stretched by its velocity, is used to determine how rapidly the galaxy hosting a collision is moving away. Light explosions that may follow these collisions might enable researchers to locate the galaxy where the collision occurred, allowing them to combine distance and redshift data 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.”
Two observatories in the United States (LIGO Labs), one in Italy (Virgo), and one in Japan have all discovered gravitational waves (KAGRA). LIGO-India, the fifth observatory, is currently under development.
Our 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 (3.26 million light-years). The first comes from comparing stars at various distances from Earth especially Cepheids, which have fluctuating brightness, and exploding stars known as type Ia supernovae and the second comes from analyzing the cosmic microwave background, which is radiation left over from the Big Bang.
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.”
Researchers from UCL, Imperial College London, Stockholm University, and the University of Amsterdam collaborated on the study. The Royal Society, the Swedish Research Council (VR), the Knut and Alice Wallenberg Foundation, and the Netherlands Organisation for Scientific Research all contributed to the project.
