First-Ever ‘Einstein Zig-Zag’ Found in Space Could Help Solve the Universe’s Expansion Mystery

A light source in space originally believed to be a galaxy bending light from a distant active galactic core is actually an extremely rare and first-of-its-kind gravitational lens, according to a team of researchers that studied the system.

The system is called J1721+8842, and it was first discovered in 2017. At the time, the system was thought to be a galaxy bending the light of a faraway quasar—an energetic galactic nucleus. But after two years of observation with the Nordic Optical Telescope—as well as data from the James Webb Space Telescope—the recent team posits that the object is actually a compound lens, made up of two aligned galaxies. Furthermore, the team posits that light traveled through the lens in a zig-zag pattern. The team’s research is currently hosted on the preprint server arXiv, and it suggests that the rare structure could help answer some fundamental questions about the cosmos.

“In this letter, we present evidence from the light curves obtained at the Nordic Optical Telescope (NOT), new redshift measurements from the James Webb Space Telescope (JWST) Near InfraRed Spectrograph (NIRSpec), and updated lens models, which unambiguously confirm the scenario where one single source is lensed in J1721+8842,” the team wrote.

Gravitational lenses are objects with significant enough gravitational fields that they bend light emanating from other sources in the universe. Gravitational lensing was proposed by Einstein as early as 1912.

Gravitational lenses are of use to astronomers because they magnify distant light that would otherwise be too faint to see. In other words, gravitational lenses are portals into very distant and ancient parts of the universe; in 2022, astronomers leveraged a gravitational lens to spot Earendel, the oldest known star.

A graphic showing the source and double lens of the system. Graphic: Dux et al. 2024

Sometimes, gravitational lenses form rings of light in the sky, which are called Einstein rings. Last year, one team posited that some Einstein rings boosted the case for axions in physicists’ race to determine the phenomena responsible for dark matter, the 27% of the universe that we know to be there but cannot directly observe.

Earlier this year, a team from Lawrence Berkeley National Laboratory identified an exquisite gravitational lens composed of a configuration of galaxies that they said was the equivalent of “eight needles precisely lined up” in a haystack. Within that lens was an Einstein Cross, which indicated the symmetrical distribution of mass (including dark matter) across the lens.

Unlike previous gravitational lenses, though, J1721+8842’s structure indicates that light in the lens zigzagged through the two galaxies; hence, the first-ever “Einstein zig-zag lens.”

“Full lens models, time-delay measurements and cosmology constraints derived from this system will be published in follow-up papers as part of the TDCOSMO collaboration,” the researchers added. This means the double lens system can help astrophysicists’ understanding of the Hubble constant, the number that describes the rate of the universe’s expansion. The constant is different depending on how you calculate it, a problem known as the Hubble tension.

Being able to probe the compound lens for a new measurement of the Hubble constant will help astronomers understand whether the figure matches up with the cosmological model or not. The team noted that the lens “can also constrain ratio of distances between the observer, the lens and the two sources, allowing precise measure of the expansion history of the Universe.”

State-of-the-art telescopes are a marvel of modernity, and can help answer some of the most essential questions of human in existence—where did everything come from, and where are we going, for starters. But gravitational lenses make the telescopes’ work easier, by letting the laws of gravity magnify some of the more distant reaches of our universe. Besides the insights they can provide, the lenses deserve recognition in their own right. J1721+8842 is an Einstein zig-zag in space—I mean, that just sounds cool as hell.