From its vantage point 1 million miles away from Earth, NASA’s Webb telescope just started to unravel new insights about the universe. So, what is most exciting about the latest data caught in the “spider web” of the telescope’s 18 hexagonal segments of its primary mirror?
The new Webb data shows evidence for water vapor, hazes and some previously unseen clouds, on the gas-giant planet WASP-96b. The planet’s mass is half of Jupiter’s mass and transits in front of its star every 3.4 days, allowing a small fraction of the star’s light to pass through its atmosphere and reveal its composition to Webb’s instruments. This planet is not expected to host life-as-we-know-it because it does not possess a thin atmosphere on top of a rocky surface, like the conditions on Earth.
But there was also a “deep image” of the cosmos that was released in a dedicated White House event, hosted by President Biden and Vice President Harris. The image shows numerous red arcs stretched around a cluster of galaxies, named SMACS 0723, located about 5 billion light years away. NASA Administrator Bill Nelson noted, “We’re looking back more than 13 billion years,” an unusual statement to be heard in the White House, which makes plans on a timescale of four years.
These amazingly sharp arcs were observed thanks to the unprecedented angular resolution of Webb’s optics. They feature ancient small galaxies from early cosmic times which happened to lie behind the cluster, so that their images were deformed by the effect of gravitational lensing. Clusters of galaxies, like SMACS 0723, contain a concentration of about a thousand Milky-Way-like galaxies, buzzing around at 5 percent the speed of light or a thousand miles per second. Most of the cluster mass is made of dark matter, an invisible substance that fills the dark gaps in Webb’s image. The luminous cores of galaxies are like fish swimming in a container filled with transparent water, bound together by gravity, which serves as the “aquarium” walls.
Ever since Fritz Zwicky observed clusters of galaxies in 1933, we’ve known that most of the matter in them is invisible. While Zwicky inferred that dark matter must exist in order to bind the fast-moving galaxies, the same gravitational potential well can be probed directly through its lensing effect on background galaxies.
The Webb telescope achieves unprecedented sensitivity to the faint galaxies that produced the first light during the dark ages of the universe, hundreds of millions of years after the Big Bang. Its unprecedented ability to peer back in time stems from its observing site far away from the glowing terrestrial atmosphere, the area of its “light bucket” being 7.3 times larger than that of the Hubble Space Telescope, and its high sensitivity to the infrared band into which starlight from early cosmic times is redshifted.
In its released “deep image,” the $10 billion Webb telescope, is aided by the natural gravitational lens of SMACS 0723, graciously provided to us for free. The cluster lens magnifies distant sources behind it by bending their light. The combination of the Webb telescope and the cluster’s magnifying power allows us to peer deeper into the universe than ever before.
In a 1936 paper, “Lens-Like Action of a Star by the Deviation of Light in the Gravitational Field,” Albert Einstein predicted that a background star could be gravitationally lensed into a ring if it is located precisely behind a foreground star. This “Einstein ring” is an outcome of the cylindrical symmetry around the lens. A cluster of galaxies is not perfectly symmetric and so sources behind its center are lensed into a partial ring, or an arc — as evident from Webb’s image.
In 1992, I entered the neighboring office of Andy Gould, a postdoctoral fellow at the Institute for Advanced Study at Princeton, where Einstein wrote his lensing paper. Gould worked extensively on gravitational lensing by compact objects, considering the possibility that the dark matter is made of them. I asked Gould whether he ever considered the contribution of a planet to the lensing effect by a star. Gould responded promptly, “planets have a negligible mass relative to their host star and so their impact on the combined lensing effect would be negligible.” I accepted the verdict of the local lensing expert and retreated quietly. But 10 minutes later, Gould showed up in my office and said, “I was wrong … the Einstein ring radius of planets scales as the square root of their mass and so their effect is measurable and could serve as a new method for discovering planets around distant stars. Let’s write a paper about that.” So, we did.
Today, gravitational lensing is the main method by which planets are discovered around distant stars where the transit method is less practical because the stars are too faint.
This anecdote from 30 years ago weaves together the themes of the two Webb images that were just unraveled.
A decade ago, I wrote “How Did the First Stars and Galaxies Form?” and co-wrote “The First Galaxies in the Universe“. Both books described theoretical expectations for what the Webb telescope might find in the context of the scientific story of genesis: “Let there be light.” Last year, I co-authored another, “Life in the Cosmos.” There is no doubt that I would be glad if the forecasts in these textbooks will be confirmed by future Webb data. But even better, I would be thrilled if Webb’s data will surprise us with new discoveries that were never anticipated.
Avi Loeb is head of Harvard’s Galileo Project, a systematic scientific search for evidence of extraterrestrial technological artifacts. Loeb is the founding director of Harvard’s Black Hole Initiative, the director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and he chairs the advisory board for the Breakthrough Starshot project. He is the author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth.”