Gravitational lensing of waves: A better way to measure the universe's expansion?

The disparity in the measurement of the expanding universe is known as Hubble tension, and some experts say it is a "crisis in cosmology."
Mrigakshi Dixit
The gravitational waves emitted by the merger of black holes.
The gravitational waves emitted by the merger of black holes.

NASA's Goddard Space Flight Center/Scott Noble 

A team of astrophysicists has suggested gravitational lensing of waves as a new way of measuring the expansion of the universe. 

This phenomenon occurs when huge cosmic objects cause deformation in space-time. In this process, the cosmic waves that travel from near the object appear to bend. 

This idea has been proposed by UC Santa Barbara theoretical astrophysicist Tejaswi Venumadhav Nerella and colleagues at the Tata Institute of Fundamental Research in Bangalore, India, and the Inter-University Center for Astronomy and Astrophysics in Pune, India. 

The discrepancy in the measurement of expansion

For more than a century, scientists have been documenting evidence of the universe's expansion. Despite all of these scientific observations, it has been impossible to determine the exact rate at which celestial objects are moving away from one another. 

The official release highlights that the pace of universe expansion has been susceptible to variations since its discovery due to measurement inconsistencies. 

According to the most recent data, the expansion is occurring at a rate ranging from 67.4 to 76.5 kilometers per second per megaparsec. The recession velocity is measured in kilometers per second to the distance (in megaparsecs).

This disparity is known as Hubble tension, and some experts say it is a "crisis in cosmology."

The team suggests that this possible method may be used to estimate the universe's expansion. 

“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.

Gravitational lensing of waves: A better way to measure the universe's expansion?
Massive objects such as galaxies, can bend gravitational waves from merging black holes, creating multiple copies of the same signal that reach Earth at different times.

The use of the gravitational method 

Measurements of the universe's expansion mostly depend on two parameters: velocity and distance.

Currently, two methods are used to estimate the distance between two objects. The first is based on the concept of "objects with known length." The second is "objects of known luminosity." 

And now gravitational wave is another new way to calculate the distance of cosmic objects. These waves are created by the violent collision of neutron stars or black holes.

In exceptional situations, lensing can cause several copies of the same gravitational wave signal to arrive on Earth at different times. The time it takes for each signal to reach the ground might be used to compute the universe's expansion rate.

“We understand very well just how sensitive gravitational wave detectors are, and there are no astrophysical sources of confusion, so we can properly account for what gets into our catalog of events,” Nerella said. “The new method has sources of error that are complementary to those of existing methods, which makes it a good discriminator.”

Scientists have yet to identify strongly lensed examples of these signals, but they are confident that the sensitivity of the next generation of ground-based detectors will allow them to do so. 

The first detection of lensed gravitational waves is expected in the next years. 

The study details have been published in the journal Physical Review Letters.

Study abstract:

Third-generation gravitational wave (GW) detectors are expected to detect millions of binary black hole (BBH) mergers during their operation period. A small fraction of them (∼1%) will be strongly lensed by intervening galaxies and clusters, producing multiple observable copies of the GW signals. The expected number of lensed events and the distribution of the time delay between lensed images depend on the cosmology. We develop a Bayesian analysis method for estimating cosmological parameters from the detected number of lensed events and their time delay distribution. The expected constraints are comparable to that obtained from other cosmological measurements, but probing a different redshift regime (z∼10) that is not explored by other probes.

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