Decoding the Universe's Enigma: A Fresh Approach to Gauging Cosmic Expansion

Researchers Propose Innovative Approach for Measuring Universe Expansion

A team of scientists at the International Centre for Theoretical Sciences in Bengaluru has put forward a novel method to determine the rate of the universe's expansion by utilizing gravitational waves generated during the merger of black hole pairs. By leveraging sophisticated gravitational wave detectors, the researchers aim to analyze the sequences of black hole mergers and the time intervals between them, enabling them to calculate the expansion rate without relying on specific data about the involved galaxies or black hole pairs.

In 1929, the discovery that galaxies are moving away from each other prompted the understanding that the universe is in a state of expansion. However, the quest to accurately measure the speed of this expansion has yielded conflicting results through various measurement techniques, creating a persistent challenge for understanding the universe's evolving dynamics.

Addressing this issue, a team led by Souvik Jana from the International Centre for Theoretical Sciences in Bengaluru has proposed a potential solution outlined in a recent paper published in the Physical Review Letters, which has garnered recognition as an Editor's suggestion. Their strategy revolves around investigating gravitational waves, the ripples in spacetime first observed by astronomers in 2015. The researchers have delved into the influence of gravity itself on these gravitational waves.

As pairs of black holes engage in a cosmic ballet to merge into a single entity, they emit gravitational waves that travel through space. When these waves reach Earth, they are studied by kilometer-long detectors to ascertain the characteristics of the merging black holes. The gravitational fields of massive galaxies positioned between the black holes and Earth cause deviations in the paths of these spacetime ripples, leading to the detectors detecting multiple instances of the same waves. This phenomenon is known as gravitational lensing. Parameswaran Ajith, a co-author of the study, pointed out that while gravitational lensing of light has been observed for more than a century, the first observation of lensed gravitational waves is anticipated in the coming years.

The gravitational waves produced by distant black hole mergers are influenced by massive celestial objects like galaxies situated between the source of the waves and Earth. This effect, called gravitational lensing, induces time delays in the signal. The researchers have demonstrated that the time lag between such lensed signals can serve as a means to gauge the universe's expansion rate.

Over the next two decades, scientists will initiate operations of advanced gravitational wave detectors to capture signals from merging black holes. Shasvath J. Kapadia, a co-author from the Inter-University Centre for Astronomy and Astrophysics in Pune, explained that these future detectors will have the capability to observe much greater distances compared to current detectors. Another co-author, Tejaswi Venumadhav from the University of California at Santa Barbara, emphasized the potential of these advanced detectors to pick up weaker gravitational wave signals that might be overshadowed by noise in present detectors.

Astronomers predict that the advanced detectors will collect data from millions of black hole pairs merging to create more massive black holes. Among these astronomical events, approximately 10,000 instances of black hole mergers are anticipated to be detected multiple times by the same detector due to gravitational lensing effects. Souvik's team illustrated that by tallying these recurrent black hole mergers and examining the time gaps between them, they can gauge the rate at which the universe is expanding. As data gradually accumulates from advanced gravitational wave detectors during the next couple of decades, their approach has the potential to provide an accurate measurement of the universe's expansion rate.

According to Souvik, their proposed methodology circumvents the necessity of possessing detailed information about individual galaxies that produce duplicate gravitational wave signals, the distances between the black hole pairs, or even the precise positions of these pairs in the sky. Instead, it relies on a reliable technique for identifying lensed signals. Shasvath further notes that efforts are underway to enhance the precision of identifying these recurring signals.

Gravitational lensing requires that the astronomical source be positioned at a considerable distance. Black hole pairs meet this criterion, potentially originating as far back as 13.3 billion years ago, a mere 500 million years after the birth of the universe.

Shasvath emphasizes that their proposed method will yield results only when the advanced detectors capture data from millions of black hole mergers. Currently, the team is examining how this future observation could distinguish between different cosmological models of the universe that have been put forward by cosmologists.

The team clarified that these models aim to unravel the mysteries surrounding elusive dark matter, a form of matter that lacks interaction with light. The hypothesis of dark matter resolves the puzzle of galaxies' observed mass for astronomers. However, the properties of dark matter remain uncertain, leading to a variety of proposed dark matter models.