What is gravitational lensing and how can the James Webb Telescope use it?
If you’ve seen the first images from the James Webb Space Telescope (JWST) this week (and let’s face it, how could you not?), you might have heard the term “gravitational lensing” being thrown around. But what does it mean exactly? And how can it help this new telescope make discoveries?
The very first image from James Webb showed a glittering starscape of the galaxy cluster SMACS 0723. There’s a lot going on in the image, but stare long enough and you might have noticed that some of the galaxies appear to be smudged, with their light stretched out. That’s not an artefact of the telescope however – it’s a distortion of reality itself, in the region of space being photographed.
Gravity is inextricably linked to mass, so the more mass an object has, the stronger its gravitational influence. That force is what sticks us to the Earth, and what keeps the planet circling the Sun. But the way it works is weirder than many people might realize – gravity deforms the very fabric of spacetime.
“Say there is a collection of massive galaxies close to each other; i.e. a galaxy cluster,” Dr. Themiya Nanayakkara, Chief Scientist of the James Webb Australian Data Centre, told New Atlas. “What will happen is because the collective mass here is very big, it will create a bend in space around that – a similar effect can be observed on a bed mattress when you put a heavy ball on it. So when light from a background galaxy passes through this area, the path it has to travel gets curved. This results in elongated images of background galaxies.”
This phenomenon is known as gravitational lensing, and its effects are clearly visible in the new images from Webb.
“All matter creates these distortions in space, including you and I, but the effects of gravitational lensing are only noticeable when a truly astronomical amount of matter is present to act as a lens like, for example, the combined cluster of galaxies seen in the new JWST images,” Robin Cook, an astronomer at ICRAR and ISC, told us. “In these images, we are seeing the light from very distant galaxies having their light bent around the foreground cluster creating impressive and mind-bending arcs. Because galaxy clusters aren’t perfect spheres, this phenomenon can often create strange features like mirror images of the same galaxy.”
As the name suggests, gravitational lensing can magnify very distant objects, including some that are too far away to be seen otherwise. That gives astronomers an incredible peek into the farthest reaches of space and time.
“The key feature here is that the gravitational lenses don't just distort the light, but they also magnify it, like a telescope,” said Professor Ray Norris of CSIRO, Australia's national science agency. “So if you view the distant universe with the JWST through a gravitational lens, it's as if you are fitting an extra lens to the JWST which lets you see even more distant galaxies than you would without the gravitational lens.”
And the James Webb Space Telescope is particularly well-suited to taking advantage of this phenomenon.
“The factors that make JWST especially adept at utilizing this phenomenon are the same ones that make it good at so many things: a big mirror working at infrared wavelengths,” explained Professor Matthew Colless, Director of the Research School of Astronomy & Astrophysics at Australian National University. “The big mirror helps in two ways, as it not only collects more light (so fainter things can be seen) but also produces sharper images (so smaller things can be seen). Working at infrared wavelengths is also helpful, because the redshift effect means that very distant objects in the universe have their light shifted along the spectrum from the visible to the infrared. Since the targets astronomers are looking at with the help of gravitational lenses are faint, small, and very distant JWST wins substantially over smaller telescopes working at visible wavelengths, like Hubble.”
There’s a lot that we can learn from the unprecedented power of the James Webb, combined with gravitational lensing, which can let astronomers study the universe as it was in the very early years after the Big Bang.
“JWST is already imaging some of the first generations of galaxies that have formed in the universe,” said Professor Cathryn Trott, International Centre for Radio Astronomy Research, Curtin University. “It also has the ability to take their spectra, whereby the light from the galaxy is split into its constituent wavelengths, allowing astronomers to identify individual elements. Already the spectra from the lensed galaxies in the SMACS 0723 field is showing us that galaxies that formed within the first few hundred million years of the universe have oxygen, an element that is only produced within stars. These observations, combined with the observation of primordial hydrogen gas using low-frequency radio telescopes, such as the Murchison Widefield Array, tell us when the first generations of stars formed in the universe, and how galaxies evolved from the small and clumpy ones observed with JWST, to those around us today.”
“And we might also get a precise new measurement of the expansion rate and age of the universe by measuring the differences in light travel time around gravitational lenses for transient events like exploding stars or flickering quasars,” Colless added. “So we expect the combination of JWST and gravitational lensing to be an extraordinarily powerful way of exploring the distant universe.”
The stunning first images are just a taste of what’s to come, and we can’t wait to see what discoveries the James Webb Space Telescope will make over the coming years.
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