Galaxies are moving away from Earth and one another, as was observed by scientists in 1929. They concluded from this finding that the universe is expanding. However, scientists used several techniques to measure how quickly it was expanding and came up with varying results. Their explanation of the expanding cosmos is nevertheless hampered by the difference.
A study team from the International Centre for Theoretical Sciences in Bengaluru led by Souvik Jana has suggested a potential answer. Their most recent publication, which appeared in Physical Review Letters, has been singled out as an Editor’s recommendation.
Studying gravitational waves, vibrations in spacetime that were initially discovered by astronomers in 2015, is crucial to finding the answer. The group looked into how gravitational waves are impacted by gravity.
Gravitational waves are produced as two black holes combine into one in a cosmic dance. Kilometer-long detectors aid in the study of the characteristics of the black hole pairings as they approach the Earth. The detectors capture several instances of the same wave because the pathways of these spacetime ripples are altered by the massive galaxies that fill the region between the black holes and the Earth. This phenomena is known as gravitational lensing by astronomers.
Light has been gravitationally lensed for more than a century, according to research co-author Parameswaran Ajith. “We anticipate the first lensed gravitational wave observation within the next few years.”
Scientists will start using cutting-edge gravitational wave detectors in the next 20 years to look for the merging black holes. According to Shasvath J. Kapadia, a co-author of the study from the Inter-University Centre for Astronomy and Astrophysics in Pune, “future detectors will be able to see out to much larger distances than the existing ones.” Another co-author, Tejaswi Venumadhav of the University of California, Santa Barbara, said they will be able to find weaker gravitational wave signals that get lost in the background noise that affects current detectors.
According to astronomers, the sophisticated detectors will capture signals from a few million black hole pairs that will eventually merge to become mega-black holes. Due to gravitational lensing, around 10,000 of these black hole mergers will show up multiple times in the same detector.
The team led by Souvik showed how to calculate the universe’s expansion rate by counting the frequency of these recurrent black hole mergers and examining the time interval between them. Their approach may allow precise measurement of the universe’s expansion rate over the next two decades as data from improved gravitational wave detectors come in.
According to Souvik, the team’s idea does not require knowledge of the characteristics of the specific galaxies that produce multiple gravitational waves, the distances to the black hole pairs, or even their precise locations in the sky. It merely needs a precise way to determine which signals are lensed. Shasvath continues that scientists are developing their methods for locating the repetitive signals.
In order for gravitational lensing to occur, the astronomical source must be far. This requirement, which can date back as far as 13.3 billion years, or just 500 million years after the universe’s creation, is met by the black hole pairs.
Shasvath warns that their suggested approach won’t be useful until the cutting-edge detectors capture millions of black hole mergers. The group is currently researching how such an observation in the future would be able to distinguish between various cosmological hypotheses.
The simulations, according to the scientists, try to answer questions about elusive dark matter, a type of substance that doesn’t interact with light. The explanation for why galaxies have the measured mass is provided by the dark matter theory. Different dark matter models have been proposed, however scientists are still unaware of the characteristics of dark matter.
According to the team’s ongoing research, lensed gravitational wave observations in the future may be used as a tool to investigate the characteristics 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.