The universe’s expansion rate has been a topic of debate for scientists for many years. While it is known that the universe is expanding, the exact rate at which celestial objects are moving away from each other is still uncertain.
To measure the rate of cosmic expansion, scientists have used various methods over the years. The most recent measurements have ranged from 67.4 to 76.5 kilometers per second per megaparsec. This discrepancy in measurements is known as the “Hubble tension” and has caused some concern in the field of cosmology.
However, researchers at UC Santa Barbara, the Tata Institute of Fundamental Research in Bangalore, India, and the Inter-University Center for Astronomy and Astrophysics in Pune, India, see this as an exciting opportunity. They have developed a method to use lensed gravitational waves to measure the universe’s expansion and potentially resolve the debate.
Since the first detection of gravitational waves in 2015, detectors have been significantly improved and are expected to detect a greater number of signals in the coming years. The researchers propose using these signals to measure cosmic expansion, which would be a novel use of the data collected by gravitational wave detectors.
Measuring cosmic expansion involves determining both velocity and distance. Astronomers typically use two methods to measure distances: standard rulers and standard candles. Standard rulers involve measuring the size of known objects in the sky, such as features in cosmic background radiation or the distribution of galaxies. Standard candles involve measuring the apparent brightness of objects with known luminosity and using that information to calculate their distances from Earth.
The method proposed by the researchers falls into the second category and utilizes gravitational lensing. Gravitational lensing occurs when massive objects warp spacetime, causing waves to bend as they pass by. In rare cases, lensing can produce multiple copies of the same gravitational wave signal that reach Earth at different times. By analyzing the delays between these signals, the researchers can calculate the universe’s expansion rate.
The researchers believe that this method has the potential to complement existing measurement methods and provide valuable insights into cosmic expansion. They anticipate that the upcoming generation of ground-based detectors will have the necessary sensitivity to observe lensed gravitational waves in the next few years.
The sources of these signals would be binary black holes, which are systems of two black holes orbiting each other and eventually merging. While strongly lensed examples of these signals have not been detected yet, the researchers expect that future detectors will be able to detect them.
The researchers estimate that these advanced detectors will record signals from a few million black hole pairs, with about 10,000 of them appearing multiple times in the same detector due to gravitational lensing. By analyzing the distribution of delays between these repeat appearances, they can determine the Hubble expansion rate.
One advantage of this method is that it does not rely on knowing the exact locations or distances to the binary black holes. The only requirement is accurately identifying a sufficient number of lensed signals.
In addition to measuring cosmic expansion, observations of lensed gravitational waves could also provide insights into other cosmological questions, such as the nature of dark matter. Dark matter is an invisible substance that makes up a significant portion of the energy content of the universe.
Overall, the researchers are optimistic about the potential of using lensed gravitational waves to measure cosmic expansion and contribute to our understanding of the universe. They believe that future observations with advanced detectors will provide valuable data and help settle the debate surrounding the rate of cosmic expansion.
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