Gravitational Waves Reveal Universe’s Expansion

Gravitational wave cosmology is opening a new window into the universe’s past.

AsianScientist (Jul. 20, 2023) –Since their first detection in 2015, gravitational waves have been used to study some of the most exciting phenomena in the universe such as colliding black holes or neutron stars. As space researchers detect more gravitational waves, they continue to open up new windows through which astronomers probe the fundamental mysteries of the universe.

Gravitational waves are ripples in spacetime that propagate at the speed of light. These waves are created by massive accelerating objects, such as black holes orbiting each other. As a gravitational wave spreads out, it stretches and contracts space at an insanely fast rate. Their propagation carries hints about how galaxies are moving apart from each other as the universe expands.

In a paper published in Physical Review Letters, a team of Indian scientists outlined a way to use future gravitational wave data to study the expansion of the universe.

Asian Scientist Magazine spoke to Parameswaran Ajith, an astrophysicist at the International Centre for Theoretical Sciences based in Bangalore, India and one of the co-authors of the study.

“Since the universe is expanding, there is a definite relationship between the distance to the galaxies and the velocity at which distant galaxies are moving away from us” Ajith explained.

Light or gravitational waves traveling from a distant object, such as a quasar or the bright center of an active galaxy, bends around a heavy object, such as another galaxy. In a phenomenon known as strong lensing, the galaxy acts like a lens and produces multiple images of the same quasar in the night sky (or in a telescope). While this lensing of gravitational waves has not been detected yet, physicists expect to detect it with more gravitational wave data.

When waves are bent by a heavy object, they take different paths, introducing a time delay between the observations on the detectors. What does that say about how fast the universe is expanding?

“It turns out that the universe’s expansion influences the time delay between the multiple copies of the lensed gravitational waves, said Ajith.

The researchers developed a model of how galaxies are distributed in the universe. The likelihood of time delays in gravitational wave detection depends on the Hubble constant, a term that underlies the rate at which the universe is expanding, and the density of matter.

In the future, astronomers will have access to far more gravitational wave detection data, due to the fact that they’d have been looking for them longer and with better detectors. Armed with that data, this model will allow astronomers to play around with the parameters to figure out more accurate values of the constants.

Currently, astronomers study the lensing of light and radio waves to determine the Hubble’s constant. However, dust and gasses weaken their propagation, limiting their utility. There is no consensus on what the exact value of the Hubble constant is.

Gravitational waves interact weakly with matter, allowing them to travel longer unimpeded. In other words, they allow astronomers to look deeper in the past, and help them get closer to determining the exact value of the Hubble constant. But there’s more to gravitational wave cosmology.

Studying lensing of gravitational waves could also shine some light on other big questions of the universe, such as the nature of dark matter. Ajith added that “one might be able to understand the properties of dark matter particles from the time delay distribution of gravitationally lensed gravitational waves that we will observe in the future.”

Source: International Centre for Theoretical Sciences, TIFR ; Image: Shutterstock

The paper can be found at: Cosmography Using Strongly Lensed Gravitational Waves from Binary Black Holes

Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.


Sachin Rawat is a freelance science writer & journalist based in Bangalore, India.

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