The Hubble Constant From Catastrophic Collisions (Cosmology / Astronomy)

According to a new study from University College London, observing violent collisions of black holes and neutron stars could soon provide a new independent measure of the universe’s rate of expansion, helping to resolve the long-standing controversy over estimating the constant of Hubble. All the details on Physical Review Letters

Currently, the two best methods to estimate the expansion rate of the universe – the one using Cepheid variables and supernova explosions , and the measurement of the cosmic microwave background radiation – give very different values, suggesting that our theory describes the universe itself could be wrong. A third type of measurement, which exploits the potential of multi-message astronomy and which is based on the observation of electromagnetic emissions and gravitational waves generated by the merger of black holes and neutron stars, should help resolve the disagreement and clarify whether our theory of the universe actually needs to be rewritten.

A new study published in Physical Review Letters presents the results of a simulation of 25,000 merger scenarios between black holes and neutron stars , the aim of which was to understand how many of these mergers could potentially be detected by instruments on Earth. The researchers found that, by 2030, Earth instruments could perceive ripples in spacetime caused by up to 3,000 such mergers and that for about 100of these events the telescopes will also be able to pick up radiation emissions. Scientists concluded that these data would be sufficient to provide a completely independent new measure of the expansion rate of the universe, accurate and reliable enough to confirm or deny the need for new physics. The idea that the gravitational waves generated by these catastrophic events could put an end to the dispute over the expansion of the universe is not new: the same authors had proposed it a couple of years ago .

“A neutron star is a dead star, generated when a very large star explodes and then collapses,” explains Stephen Feeney of the Department of Physics and Astronomy at UCL ( University College London ), first author of the study. “It is incredibly dense – typically about 10 miles in diameter and up to twice the mass of the Sun. Its collision with a black hole is a catastrophic event, causing ripples in spacetime known as gravitational waves, which we can detect on the Earth with observers like Ligo and Virgo ».

Gravitational waves are detected by two observatories in the United States ( Ligo ), one in Italy ( Virgo ) and one in Japan ( Kagra ). A fifth observatory, Ligo-India , is currently under construction. “Advances in the sensitivity of equipment that detect gravitational waves,” continues Feeney, “together with the new detectors in India and Japan, will lead to a huge leap forward in terms of the number of events of this type that we will be able to detect.”

To calculate the expansion rate of the universe – the Hubble constant – astrophysicists need to know the distance of astronomical objects from Earth and the speed at which they are moving away. Gravitational wave analysis tells us how far away the collision is, leaving only the speed to be determined. To understand how fast the galaxy hosting a collision is moving away, we observe the redshift , or how the wavelength of the light produced by a source is increased due to its movement. The radiation emissions that may accompany these collisions help us pinpoint the galaxy where the collision occurred, allowing researchers to combine distance and redshift measurements.

“The computer models of these catastrophic events are incomplete and this study should provide additional motivation for improving them. If our assumptions are correct, many of these collisions will produce no detectable radiation emissions: the black hole will swallow the star without a trace. But in some cases a smaller black hole could destroy the neutron star before engulfing it, leaving matter outside the hole itself that emits electromagnetic radiation, ”Feeney says.

Currently, the two best estimates of the universe’s rate of expansion are 67 kilometers per second per megaparsec and 74kilometers per second per megaparsec. The first derives from the analysis of the cosmic microwave background while the second comes from the comparison of stars at different distances from the Earth, in particular the Cepheid variables and type Ia supernovae. “Since the microwave background measurement requires a complete theory of the universe, as opposed to the stellar method, the disagreement offers tantalizing evidence of a new physics, which is beyond our current understanding. However, before we can make such claims we need confirmation of disagreement by completely independent observations. We believe these may be provided by black hole-neutron star collisions, ”Feeney confidently concludes.

Featured image: Observing violent collisions of black holes and neutron stars could soon provide a new measure of the universe’s rate of expansion, helping to resolve a long-standing controversy, according to a new UCL study, published in Physical Review Letters. Credits: University College London

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