Scientists Propose Method to Measure Universe’s Expansion Rate using Gravitational Waves
Researchers from the International Centre for Theoretical Sciences in Bengaluru have put forward a new method to measure the expansion rate of the universe. By studying gravitational waves from merging black hole pairs, the team plans to utilize advanced gravitational wave detectors to examine repeated black hole mergers and the delay between them. This will allow them to calculate the expansion rate without needing precise information about the galaxies or black hole pairs involved.
The discovery that galaxies are moving away from each other and from us, known as the expansion of the universe, was made by astronomers in 1929. However, different methods used to measure the rate of expansion have produced conflicting results, which has been a persistent challenge in understanding the expanding universe.
A potential solution to this problem has been proposed by a research team led by Souvik Jana from the International Centre for Theoretical Sciences in Bengaluru. Their recent paper, published in the Physical Review Letters, has been highlighted as an Editor’s Suggestion.
The team’s solution centers around the study of gravitational waves, which are ripples in spacetime. Astronomers first detected these gravitational waves in 2015. By studying how gravity affects gravitational waves, the researchers aim to gain insights into the universe’s expansion rate.
As black hole pairs merge into a single black hole, they emit gravitational waves. Kilometer-length detectors on Earth help scientists analyze the properties of these black hole pairs. However, the presence of massive galaxies between the black holes and Earth causes the paths of these spacetime ripples to change, resulting in multiple copies of the same waves being recorded by detectors. This phenomenon is known as gravitational lensing.
“We have been observing the gravitational lensing of light for over a century,” said Parameswaran Ajith, a co-author of the study. “We expect the first observation of lensed gravitational waves in the next few years!”
In the next two decades, advanced gravitational wave detectors will be used to search for merging black holes. These detectors will be able to detect signals from much greater distances than existing detectors. This will enable scientists to detect weaker gravitational wave signals that are currently hidden in the noise.
It is estimated that the advanced detectors will record signals from millions of black hole pairs, with approximately 10,000 black hole mergers appearing more than once due to gravitational lensing.
The team led by Souvik Jana has demonstrated that by counting the number of repeat black hole mergers and analyzing the delay between them, they can accurately measure the expansion rate of the universe. As data from advanced gravitational wave detectors is collected over the next two decades, their method has the potential to provide precise measurements of the universe’s expansion rate.
Importantly, the researchers’ proposed method does not rely on knowing the properties of individual galaxies or the exact location of black hole pairs. Instead, it only requires an accurate method of identifying lensed signals. Scientists are refining their techniques to identify these repeat signals.
Gravitational lensing requires that the astronomical source be far away. This criterion is met by black hole pairs, which could have originated as early as 13.3 billion years ago, just 500 million years after the birth of the universe.
However, the team cautions that their proposed method will be effective only when the advanced detectors record millions of black hole mergers. Currently, the team is studying how this future observation can help differentiate between different models of the universe that have been proposed by cosmologists.
The team explained that these models aim to explain the mysteries of dark matter, a form of matter that does not interact with light. Dark matter is believed to account for the observed mass of galaxies, but its properties are still uncertain, leading to various dark matter models.
The ongoing research by the team suggests that future observations of lensed gravitational waves will serve as a valuable tool for studying the properties of dark matter.
Reference: “Cosmography Using Strongly Lensed Gravitational Waves from Binary Black Holes” by Souvik Jana, Shasvath J. Kapadia, Tejaswi Venumadhav, and Parameswaran Ajith, 30 June 2023, Physical Review Letters.
DOI: 10.1103/PhysRevLett.130.261401