The expansion of the universe has been a known phenomenon for about a century. However, determining the rate at which celestial objects are moving away from each other remains a topic of debate.
Measuring the rate of cosmic expansion across vast distances is a challenging task. Over time, scientists have refined their measurements, but there is still a discrepancy among the values. The latest measurements range from 67.4 to 76.5 kilometers per second per megaparsec. This discrepancy, known as the “Hubble tension,” has caused some to view it as a crisis in cosmology.
However, for Tejaswi Venumadhav Nerella, a theoretical astrophysicist at UC Santa Barbara, and his colleagues from the Tata Institute of Fundamental Research and the Inter-University Center for Astronomy and Astrophysics in India, this is an exciting period. With advancements in gravitational wave detectors since their first detection in 2015, these scientists believe they can utilize these signals to measure the expansion of the universe and potentially resolve the debate.
Future detectors are expected to provide a wealth of gravitational wave signals in the coming years. Nerella and his colleagues have devised a method to leverage these signals for measuring cosmic expansion—a novel application of this remarkable dataset. “A major scientific goal of future detectors is to deliver a comprehensive catalog of gravitational wave events, and this will be a completely novel use of the remarkable dataset,” said Nerella, who co-authored a paper on the subject published in Physical Review Letters.
Measuring the cosmic expansion rate primarily involves determining velocity and distance. Astronomers employ two methods to measure distances: the first utilizes “standard rulers,” which are objects with a known length. Scientists observe how large these objects appear in the sky. These objects can be features in cosmic background radiation or the distribution of galaxies in the universe.
The second method relies on “standard candles,” which are objects with known luminosity. Scientists measure their distances from Earth by assessing their apparent brightness. By connecting these distances with those of farther, brighter objects, astronomers establish a measurement chain known as the “cosmic distance ladder.” Interestingly, gravitational waves themselves can contribute to measuring cosmic expansion. The energy released during the collision of neutron stars or black holes can be used to estimate the distance to these objects.
Nerella and his co-authors propose a method that falls into the second class of distance measurement techniques, but with a unique twist involving gravitational lensing. Gravitational lensing occurs when massive objects distort the fabric of spacetime, causing waves passing near them to bend. In rare cases, this lensing effect can create multiple copies of the same gravitational wave signal reaching Earth at different times. By studying the time delays between these multiple imaged events, the researchers believe they can calculate the expansion rate of the universe.
The proposed method takes advantage of binary black holes as the sources of these gravitational wave signals. Binary black holes are systems in which two black holes orbit each other and eventually merge, releasing tremendous amounts of gravitational wave energy. While strongly lensed examples of these signals have not been detected yet, the upcoming generation of ground-based detectors is expected to possess the required sensitivity.
The researchers anticipate that the advanced detectors, which are expected to be operational in the next decade, will detect signals from millions of binary black hole pairs. Among these, around 10,000 pairs are estimated to appear multiple times in the same detector due to gravitational lensing. By analyzing the distribution of time delays between these repeated appearances, the Hubble expansion rate can be derived.
Unlike other measurement methods, this approach does not rely on precisely knowing the locations or distances to these binary black holes. The key requirement is accurately identifying a significant number of these lensed signals. Additionally, the observations of lensed gravitational waves could offer insights into other cosmological questions, including the nature of dark matter, which constitutes a significant portion of the universe’s energy content.
Lead author Souvik Jana emphasizes that this method provides complementary sources of error compared to existing techniques, making it a valuable discriminator. The researchers are hopeful that within the next few years, the first observations of lensed gravitational waves will be made, thanks to the increased sensitivity of future detectors. These advancements are expected to extend our ability to explore the universe and detect weaker signals from farther distances.