Ask a Scientist, Science & Technology

Peering into the universe with gravitational lensing

Radio waves coming from galaxies millions or billions of light-years away—an immense distance compared to only eight light-minutes between the Earth and the Sun—gradually fade as they lose energy. Many become essentially invisible even to today’s powerful telescopes by the time they reach our little, blue planet. 

So it’s not surprising that the recent news from Arnab Chakraborty, a postdoctoral fellow in the Department of Physics at McGill, was met with great enthusiasm and interest when he detected a radio signal from a distant galaxy. Chakraborty, who works with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) telescope, picked up the longest-range radio signal to date—from galaxy SDSSJ0826+5630—thanks to a phenomenon called gravitational lensing. 

As the name suggests—physicists are not quite as inventive with terminology as biologists—gravitational force can and does act as a lens. It collects and concentrates waves into a single radiation beam, which is a lot like a converging optical lens commonly prescribed to shortsighted patients.

“There is [another] source between the galaxy and us, the observer, which acts as a lens,” Chakraborty said in an interview with The McGill Tribune. “If there is a large amount of mass, like a black hole, it will bend the space-time around itself, so when light is passing through [a region near the massive object], it will be bent.” 

The bending effect magnifies signals—kind of like burning paper with a loupe on a sunny day—to make detection significantly easier. 

Objects that bend light may sound like science fiction, but Einstein’s theory of relativity provides a consistent explanation for this effect. Imagine stretching a tablecloth from all its edges and then putting an object in the middle. The matter around the object—the fabric, in this case—becomes curved. The same curvature is applied to the space around extremely massive bodies, like planets and stars. Electromagnetic waves such as light change their trajectory because of this “lensing” though space distortion. 

What were the chances of finding something that huge between our Milky Way and some distant galaxy in the observable universe’s outer edge? As it turns out, it’s largely a matter of probability.

“We are lucky in that sense, it is a natural phenomenon, [otherwise] it would not be possible to detect a galaxy so far away from us,” Chakraborty explained. 

At the same time, the odds of a “gravitational lens” being out there increase with the distance. 

“If we want to see a far galaxy, it can always happen that in the path, there is another galaxy or a cluster of galaxies,” Chakraborty said. “Presence of a huge mass will bend light and magnify it.”

The probability of a black hole being positioned between the Earth and the Sun is ridiculously low compared to Earth and, for example, the Andromeda Galaxy, which is about two million light-years away from us. 

Signals coming from very distant objects are “running late,” which means that they may reach the Earth within minutes, as is the case with our Sun, or within millions of years. Signals are not transmitted instantaneously; their speed is limited by the speed of light, causing a delay. In some cases, the original source might well be dead in our ‘present’ on Earth. 

The progress made by Chakraborty and his team is only the first step toward building a complete picture of how the universe works. As more signals are detected, they can be compiled to establish an image of what the universe looked like in the past. 

“When we have more observations and more detections, we can do studies to understand the evolution of galaxies over cosmic times,” Chakraborty said. 

Detections can then be compiled into a “lensing catalogue” so that scientists can gain insight into star formation. 

“Currently, we do not have that information since it is just one detection. I think in future we will [obviously] push there,” Chakraborty added. 

If more sensitive telescopes are developed in the future, astronomers may be able to detect waves originating billions of light-years away by way of a more advanced technique called gravitational microlensing. But until then, traditional gravitational lensing is perhaps the exclusive route to researching distant galaxies. 

Share this:

Leave a Comment

Your email address will not be published.


Read the latest issue