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Rebekah Hounsell: Tracking Cosmic Light to Untangle the Universe’s Darkest Mysteries
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By NASA
Attendees line up to enter the theater for a screening of the new NASA+ documentary “Cosmic Dawn: The Untold Story of the James Webb Space Telescope,” Wednesday, June 11, 2025, at the Greenbelt Cinema in Greenbelt, Maryland. Featuring never-before-seen footage, Cosmic Dawn offers an unprecedented glimpse into the assembly, testing, and launch of NASA’s James Webb Space Telescope.NASA/Joel Kowsky Attendees line up to enter the theater for a screening of the new NASA+ documentary “Cosmic Dawn: The Untold Story of the James Webb Space Telescope,” Wednesday, June 11, 2025, at the Greenbelt Cinema in Greenbelt, Maryland. Following the screening, Jacob Pinter, host of NASA’s Curious Universe podcast, led a discussion with Sophia Roberts, a NASA video producer who documented the Webb project, and Paul Geithner, former deputy project manager for Webb.
Featuring never-before-seen footage, Cosmic Dawn offers an unprecedented glimpse into the assembly, testing, and launch of NASA’s James Webb Space Telescope.
Watch the documentary.
Image credit: NASA/Joel Kowsky
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By NASA
A funky effect Einstein predicted, known as gravitational lensing — when a foreground galaxy magnifies more distant galaxies behind it — will soon become common when NASA’s Nancy Grace Roman Space Telescope begins science operations in 2027 and produces vast surveys of the cosmos.
This image shows a simulated observation from NASA’s Nancy Grace Roman Space Telescope with an overlay of its Wide Field Instrument’s field of view. More than 20 gravitational lenses, with examples shown at left and right, are expected to pop out in every one of Roman’s vast observations. A journal paper led by Bryce Wedig, a graduate student at Washington University in St. Louis, Missouri, estimates that of those Roman detects, about 500 from the telescope’s High-Latitude Wide-Area Survey will be suitable for dark matter studies. By examining such a large population of gravitational lenses, the researchers hope to learn a lot more about the mysterious nature of dark matter.Credit: NASA, Bryce Wedig (Washington University), Tansu Daylan (Washington University), Joseph DePasquale (STScI) A particular subset of gravitational lenses, known as strong lenses, is the focus of a new paper published in the Astrophysical Journal led by Bryce Wedig, a graduate student at Washington University in St. Louis. The research team has calculated that over 160,000 gravitational lenses, including hundreds suitable for this study, are expected to pop up in Roman’s vast images. Each Roman image will be 200 times larger than infrared snapshots from NASA’s Hubble Space Telescope, and its upcoming “wealth” of lenses will vastly outpace the hundreds studied by Hubble to date.
Roman will conduct three core surveys, providing expansive views of the universe. This science team’s work is based on a previous version of Roman’s now fully defined High-Latitude Wide-Area Survey. The researchers are working on a follow-up paper that will align with the final survey’s specifications to fully support the research community.
“The current sample size of these objects from other telescopes is fairly small because we’re relying on two galaxies to be lined up nearly perfectly along our line of sight,” Wedig said. “Other telescopes are either limited to a smaller field of view or less precise observations, making gravitational lenses harder to detect.”
Gravitational lenses are made up of at least two cosmic objects. In some cases, a single foreground galaxy has enough mass to act like a lens, magnifying a galaxy that is almost perfectly behind it. Light from the background galaxy curves around the foreground galaxy along more than one path, appearing in observations as warped arcs and crescents. Of the 160,000 lensed galaxies Roman may identify, the team expects to narrow that down to about 500 that are suitable for studying the structure of dark matter at scales smaller than those galaxies.
“Roman will not only significantly increase our sample size — its sharp, high-resolution images will also allow us to discover gravitational lenses that appear smaller on the sky,” said Tansu Daylan, the principal investigator of the science team conducting this research program. Daylan is an assistant professor and a faculty fellow at the McDonnell Center for the Space Sciences at Washington University in St. Louis. “Ultimately, both the alignment and the brightness of the background galaxies need to meet a certain threshold so we can characterize the dark matter within the foreground galaxies.”
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This video shows how a background galaxy’s light is lensed or magnified by a massive foreground galaxy, seen at center, before reaching NASA’s Roman Space Telescope. Light from the background galaxy is distorted, curving around the foreground galaxy and appearing more than once as warped arcs and crescents. Researchers studying these objects, known as gravitational lenses, can better characterize the mass of the foreground galaxy, which offers clues about the particle nature of dark matter.Credit: NASA, Joseph Olmsted (STScI) What Is Dark Matter?
Not all mass in galaxies is made up of objects we can see, like star clusters. A significant fraction of a galaxy’s mass is made up of dark matter, so called because it doesn’t emit, reflect, or absorb light. Dark matter does, however, possess mass, and like anything else with mass, it can cause gravitational lensing.
When the gravity of a foreground galaxy bends the path of a background galaxy’s light, its light is routed onto multiple paths. “This effect produces multiple images of the background galaxy that are magnified and distorted differently,” Daylan said. These “duplicates” are a huge advantage for researchers — they allow multiple measurements of the lensing galaxy’s mass distribution, ensuring that the resulting measurement is far more precise.
Roman’s 300-megapixel camera, known as its Wide Field Instrument, will allow researchers to accurately determine the bending of the background galaxies’ light by as little as 50 milliarcseconds, which is like measuring the diameter of a human hair from the distance of more than two and a half American football fields or soccer pitches.
The amount of gravitational lensing that the background light experiences depends on the intervening mass. Less massive clumps of dark matter cause smaller distortions. As a result, if researchers are able to measure tinier amounts of bending, they can detect and characterize smaller, less massive dark matter structures — the types of structures that gradually merged over time to build up the galaxies we see today.
With Roman, the team will accumulate overwhelming statistics about the size and structures of early galaxies. “Finding gravitational lenses and being able to detect clumps of dark matter in them is a game of tiny odds. With Roman, we can cast a wide net and expect to get lucky often,” Wedig said. “We won’t see dark matter in the images — it’s invisible — but we can measure its effects.”
“Ultimately, the question we’re trying to address is: What particle or particles constitute dark matter?” Daylan added. “While some properties of dark matter are known, we essentially have no idea what makes up dark matter. Roman will help us to distinguish how dark matter is distributed on small scales and, hence, its particle nature.”
Preparations Continue
Before Roman launches, the team will also search for more candidates in observations from ESA’s (the European Space Agency’s) Euclid mission and the upcoming ground-based Vera C. Rubin Observatory in Chile, which will begin its full-scale operations in a few weeks. Once Roman’s infrared images are in hand, the researchers will combine them with complementary visible light images from Euclid, Rubin, and Hubble to maximize what’s known about these galaxies.
“We will push the limits of what we can observe, and use every gravitational lens we detect with Roman to pin down the particle nature of dark matter,” Daylan said.
The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation by NASA’s Jet Propulsion Laboratory in Southern California; Caltech/IPAC in Pasadena, California; the Space Telescope Science Institute in Baltimore; and a science team comprising scientists from various research institutions. The primary industrial partners are BAE Systems, Inc. in Boulder, Colorado; L3Harris Technologies in Melbourne, Florida; and Teledyne Scientific & Imaging in Thousand Oaks, California.
By Claire Blome
Space Telescope Science Institute, Baltimore, Md.
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Last Updated Jun 12, 2025 EditorAshley BalzerContactAshley Balzerashley.m.balzer@nasa.govLocationNASA Goddard Space Flight Center Related Terms
Nancy Grace Roman Space Telescope Astrophysics Dark Matter Galaxies Galaxies, Stars, & Black Holes Galaxies, Stars, & Black Holes Research The Universe Explore More
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By NASA
What does it take to gaze through time to our universe’s very first stars and galaxies?
NASA answers this question in its new documentary, “Cosmic Dawn: The Untold Story of the James Webb Space Telescope.” The agency’s original documentary, which chronicles the story of the most powerful telescope ever deployed in space, was released Wednesday, June 11.
Cosmic Dawn offers an unprecedented glimpse into the delicate assembly, rigorous testing, and triumphant launch of NASA’s James Webb Space Telescope. The documentary showcases the complexity involved in creating a telescope capable of peering billions of years into the past.
Cosmic Dawn is now available for streaming on NASA’s YouTube, NASA+, and select local theaters. The trailer is available on NASA+ and YouTube.
Relive the pitfalls and the triumphs of the world’s most powerful space telescope—from developing the idea of an impossible machine to watching with bated breath as it unfolded, hurtling through space a million miles away from Earth. Watch the Documentary on YouTube The film features never-before-seen footage captured by the Webb film crew, offering intimate access to the challenges and triumphs faced by the team at NASA’s Goddard Space Flight Center in Greenbelt, Maryland — the birthplace of Webb.
“At NASA, we’re thrilled to share the untold story of our James Webb Space Telescope in our new film ‘Cosmic Dawn,’ celebrating not just the discoveries, but the extraordinary people who made it all happen, for the benefit of humanity,” said Rebecca Sirmons, head of NASA+ at the agency’s headquarters in Washington.
From its vantage point more than a million miles from Earth and a massive sunshield to block the light of our star, Webb’s First Deep Field the deepest and sharpest infrared images of the universe that the world had seen.
Webb’s images have dazzled people around the globe, capturing the very faint light of the first stars and galaxies that formed more than 13.5 billion years ago. These are baby pictures from an ancient past when the first objects were turning on and emitting light after the Big Bang. Webb has also given us new insights into black holes, planets both inside and outside of our own solar system, and many other cosmic phenomena.
Webb was a mission that was going to be spectacular whether that was good or bad — if it failed or was successful. It was always going to make history
Sophia roberts
NASA Video Producer
NASA’s biggest and most powerful space telescope was also its most technically complicated to build. It was harder still to deploy, with more than 300 critical components that had to deploy perfectly. The risks were high in this complicated dance of engineering, but the rewards were so much higher.
“Webb was a mission that was going to be spectacular whether that was good or bad — if it failed or was successful,” said video producer Sophia Roberts, who chronicled the five years preceding Webb’s launch. “It was always going to make history.”
NASA scientists like Nobel Laureate Dr. John Mather conceived Webb to look farther and deeper into origins of our universe using cutting edge infrared technology and massive mirrors to collect incredibly rich information about our universe, from the light of the first galaxies to detailed images of planets in our own solar system.
To achieve this goal, NASA and its partners faced unprecedented hurdles.
Webb’s development introduced questions that no one had asked before. How do you fit a telescope with the footprint of a tennis court into a rocket? How do you clean 18 sensitive mirrors when a single scratch could render them inoperable? How do you maintain critical testing while hurricane stormwater pours through ceilings?
A technician inspects the James Webb Space Telescope primary mirrors at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.NASA/Sophia Roberts Cosmic Dawn captures 25 years of formidable design constraints, high-stake assessments, devastating natural disasters, a global pandemic and determined individuals who would let none of that get in the way of getting this monumental observatory to its rightful place in the cosmos.
“There was nothing easy about Webb at all,” said Webb project manager Bill Ochs. “I don’t care what aspect of the mission you looked at.”
Viewers will experience a one-of-a-kind journey as NASA and its partners tackle these dilemmas — and more — through ingenuity, teamwork, and unbreakable determination.
“The inspiration of trying to discover something — to build something that’s never been built before, to discover something that’s never been known before — it keeps us going,” Mather said. “We are pleased and privileged in our position here at NASA to be able to carry out this [purpose] on behalf of the country and the world.”
Bound by NASA’s 66-year commitment to document and share its work with the public, Cosmic Dawn details every step toward Webb’s launch and science results.
Learn more at nasa.gov/cosmicdawn By Laine Havens,
NASA’s Goddard Space Flight Center, Greenbelt, Md.
Media Contact:
Katie Konans,
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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Last Updated Jun 11, 2025 Related Terms
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By NASA
Explore Webb Webb News Latest News Latest Images Webb’s Blog Awards X (offsite – login reqd) Instagram (offsite – login reqd) Facebook (offsite- login reqd) Youtube (offsite) Overview About Who is James Webb? Fact Sheet Impacts+Benefits FAQ Science Overview and Goals Early Universe Galaxies Over Time Star Lifecycle Other Worlds Observatory Overview Launch Deployment Orbit Mirrors Sunshield Instrument: NIRCam Instrument: MIRI Instrument: NIRSpec Instrument: FGS/NIRISS Optical Telescope Element Backplane Spacecraft Bus Instrument Module Multimedia About Webb Images Images Videos What is Webb Observing? 3d Webb in 3d Solar System Podcasts Webb Image Sonifications Webb’s First Images Team International Team People Of Webb More For the Media For Scientists For Educators For Fun/Learning 5 Min Read NASA’s Webb ‘UNCOVERs’ Galaxy Population Driving Cosmic Renovation
White diamonds show the locations of 20 of the 83 young, low-mass, starburst galaxies found in infrared images of the giant galaxy cluster Abell 2744. Full image and description shown below. Credits:
NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025 Astronomers using data from NASA’s James Webb Space Telescope have identified dozens of small galaxies that played a starring role in a cosmic makeover that transformed the early universe into the one we know today.
“When it comes to producing ultraviolet light, these small galaxies punch well above their weight,” said Isak Wold, an assistant research scientist at Catholic University of America in Washington and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “Our analysis of these tiny but mighty galaxies is 10 times more sensitive than previous studies, and shows they existed in sufficient numbers and packed enough ultraviolet power to drive this cosmic renovation.”
Wold discussed his findings Wednesday at the 246th meeting of the American Astronomical Society in Anchorage, Alaska. The study took advantage of existing imaging collected by Webb’s NIRCam (Near-Infrared Camera) instrument, as well as new observations made with its NIRSpec (Near-Infrared Spectrograph) instrument.
Image A: Webb search finds dozens of tiny, young star-forming galaxies
Symbols mark the locations of young, low-mass galaxies bursting with new stars when the universe was about 800 million years old. Using a filter sensitive to such galaxies, NASA’s James Webb Space Telescope imaged them with the help of a natural gravitational lens created by the massive galaxy cluster Abell 2744. In all, 83 young galaxies were found, but only the 20 shown here (white diamonds) were selected for deeper study. The inset zooms into one of the galaxies.
Download high-resolution images from NASA’s Scientific Visualization Studio NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025 The tiny galaxies were discovered by Wold and his Goddard colleagues, Sangeeta Malhotra and James Rhoads, by sifting through Webb images captured as part of the UNCOVER (Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization) observing program, led by Rachel Bezanson at the University of Pittsburgh in Pennsylvania.
The project mapped a giant galaxy cluster known as Abell 2744, nicknamed Pandora’s cluster, located about 4 billion light-years away in the southern constellation Sculptor. The cluster’s mass forms a gravitational lens that magnifies distant sources, adding to Webb’s already considerable reach.
Image B: Galaxy cluster helps reveal young, low-mass galaxies bursting with stars
White diamonds show the locations of 20 of the 83 young, low-mass, starburst galaxies found in infrared images of the giant galaxy cluster Abell 2744. This composite incorporates images taken through three NIRCam filters (F200W as blue, F410M as green, and F444W as red). The F410M filter is highly sensitive to light emitted by doubly ionized oxygen — oxygen atoms that have been stripped of two electrons — at a time when reionization was well underway. Emitted as green light, the glow was stretched into the infrared as it traversed the expanding universe over billions of years. The cluster’s mass acts as a natural magnifying glass, allowing astronomers to see these tiny galaxies as they were when the universe was about 800 million years old. NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025 For much of its first billion years, the universe was immersed in a fog of neutral hydrogen gas. Today, this gas is ionized — stripped of its electrons. Astronomers, who refer to this transformation as reionization, have long wondered which types of objects were most responsible: big galaxies, small galaxies, or supermassive black holes in active galaxies. As one of its main goals, NASA’s Webb was specifically designed to address key questions about this major transition in the history of the universe.
Recent studies have shown that small galaxies undergoing vigorous star formation could have played an outsized role. Such galaxies are rare today, making up only about 1% of those around us. But they were abundant when the universe was about 800 million years old, an epoch astronomers refer to as redshift 7, when reionization was well underway.
The team searched for small galaxies of the right cosmic age that showed signs of extreme star formation, called starbursts, in NIRCam images of the cluster.
“Low-mass galaxies gather less neutral hydrogen gas around them, which makes it easier for ionizing ultraviolet light to escape,” Rhoads said. “Likewise, starburst episodes not only produce plentiful ultraviolet light — they also carve channels into a galaxy’s interstellar matter that helps this light break out.”
Image C: A deeper look into small, young, star-forming galaxies during reionization
At left is an enlarged infrared view of galaxy cluster Abell 2744 with three young, star-forming galaxies highlighted by green diamonds. The center column shows close-ups of each galaxy, along with their designations, the amount of magnification provided by the cluster’s gravitational lens, their redshifts (shown as z — all correspond to a cosmic age of about 790 million years), and their estimated mass of stars. At right, measurements from NASA’s James Webb Space Telescope’s NIRSpec instrument confirm that the galaxies produce strong emission in the light of doubly ionized oxygen (green bars), indicating vigorous star formation is taking place. NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025 The astronomers looked for strong sources of a specific wavelength of light that signifies the presence of high-energy processes: a green line emitted by oxygen atoms that have lost two electrons. Originally emitted as visible light in the early cosmos, the green glow from doubly ionized oxygen was stretched into the infrared as it traversed the expanding universe and eventually reached Webb’s instruments.
This technique revealed 83 small starburst galaxies as they appear when the universe was 800 million years old, or about 6% of its current age of 13.8 billion years. The team selected 20 of these for deeper inspection using NIRSpec.
“These galaxies are so small that, to build the equivalent stellar mass of our own Milky Way galaxy, you’d need from 2,000 to 200,000 of them,” Malhotra said. “But we are able to detect them because of our novel sample selection technique combined with gravitational lensing.”
Image D: Tiny but mighty galaxy helped clear cosmic fog
One of the most interesting galaxies of the study, dubbed 41028 (the green oval at center), has an estimated stellar mass of just 2 million Suns — comparable to the masses of the largest star clusters in our own Milky Way galaxy. NASA/ESA/CSA/Bezanson et al. 2024 and Wold et al. 2025 Similar types of galaxies in the present-day universe, such as green peas, release about 25% of their ionizing ultraviolet light into surrounding space. If the low-mass starburst galaxies explored by Wold and his team release a similar amount, they can account for all of the ultraviolet light needed to convert the universe’s neutral hydrogen to its ionized form.
The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).
To learn more about Webb, visit:
https://science.nasa.gov/webb
By Francis Reddy
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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Laura Betz – laura.e.betz@nasa.gov
NASA’s Goddard Space Flight Center, Greenbelt, Md.
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