The universe has been expanding for about a century, but the exact rate at which celestial objects are moving apart is still a topic of debate. Different measurements of cosmic expansion have led to a discrepancy known as the "Hubble tension," causing some to see it as a crisis in cosmology.
However, researchers at UC Santa Barbara, along with colleagues in India, see this as an exciting opportunity. They propose a method to use gravitational wave signals, which have been greatly improved since their first detection in 2015, to measure the universe's expansion and potentially settle the debate.
To measure cosmic expansion, astronomers use two methods based on velocity and distance. One method involves using "standard rulers" or known lengths of objects in the cosmic background radiation or galaxy distribution. The other method uses "standard candles," which are objects with known luminosity, to measure their distances from Earth based on their apparent brightness.
The proposed method by the researchers belongs to the second class and involves using gravitational lensing. Gravitational lensing occurs when massive objects warp spacetime and bend waves traveling near them. In rare cases, lensing can create multiple copies of the same gravitational wave signal, and by measuring the delays between these signals, the universe's expansion rate can be calculated.
The signals used in this method come from binary black holes, where two black holes orbit and merge, emitting gravitational waves. The researchers expect advanced ground-based detectors to observe lensed gravitational waves in the next decade, and this will help determine the Hubble expansion rate.
Unlike other measurement methods, this approach does not require knowing the exact locations or distances of the binary black holes. It only needs the accurate identification of a sufficient number of lensed signals.
Furthermore, observations of lensed gravitational waves can provide insights into other cosmological questions, such as the nature of dark matter, which makes up a significant portion of the universe's energy content. With these advancements, the researchers are hopeful that the Hubble tension can be resolved and our understanding of cosmic expansion can be further refined.
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