It’s one of the biggest mysteries of the cosmos: What are these little red dots?
Massive galaxies? Dusty black holes? Black hole stars? Scientists have raised a lot of theories around what the crimson blobs in James Webb data might be.
When the James Webb Space Telescope launched on Christmas day in 2021, astronomers hoped it would reveal the earliest galaxies of our universe.
But amid those first glowing galaxies, Ivo Labbé of Swinburne University of Technology in Australia and his colleagues spotted something totally unexpected: 13 bright red blobs. When Labbé and team used the brightness to estimate the mass, and the color to estimate their distances, their calculations revealed a surprise: Six of them were nearly as massive as the Milky Way Galaxy, yet existed within the first 600 to 700 million years of the universe. Massive galaxies within the first billion years of the universe didn’t make sense for how astronomers think galaxies form and evolve, so the astronomers called them “universe breakers.”
But these titans were much too big compared to the galaxies researchers had expected, and so they kept investigating. The mystery the little red dots began to unfold. Despite being prolific in the first billion years of our universe’s life, they don’t seem to fit into any theoretical model or class of objects known to astronomers.
“It gets a bit puzzling,” says Princeton astronomer Bingjie Wang, who was on the discovery team with Labbé. “The story didn't add up.”
Now four years later, astronomers have found some 1,000 of these little red dots, including a couple dozen in the modern universe, and theories abound as to what they might be. The more astronomers learn about them, these little red dots seem to be revealing something fundamental about how the universe evolved and how objects grew within it.
(Here's a behind-the-scenes look at how the James Webb telescope was built.)
Why don’t these dots look like normal galaxies?
Labbé, Wang, and their colleagues discovered the little red dots in views of the sky captured June 21, 2022—some of the first data that Webb collected, when scientists were testing the instruments and showing off what the telescope could do. By the time the team published their results in a February 2023 issue of the journal Nature, other teams of astronomers had discovered more of these red blobs and captured them in greater detail.
“It was … a trickle of papers that that led to the realization that these things were really common,” says astronomer Dale Kocevski of Colby College in Maine, who has been observing and trying to understand these red blobs since 2022.
To figure out what the little red dots might be, if not simply really big galaxies, scientists turned to spectroscopy, a technique that spreads out the collected light by its wavelength, or color. This allows astronomers to see the intensity at each wavelength. Each color of light in the collected data, called a spectrum, reveals the chemical elements that make up the glowing material.
That light also travels across long distances, which keep growing because the universe is expanding. That expansion actually shifts colors toward the red end of the spectrum. But because astronomers understand the patterns of lines from each chemical element, they can still find those patterns even if they’re redder. By tracking those specific colors and how much they’ve shifted toward red, astronomers can determine how far away the object is. Scientists can also use those shifted lines to understand how gas at the light source is moving. That turned out to be crucial information to understanding the little red dots.
Hydrogen is the most prevalent material in the universe, but instead of a narrow marker for the element, light from the little red dots produced a blurred, or broadened, region in the data. That blurring is a result of incredibly fast movement, thousands of kilometers per second, says Kocevski, toward and away from the viewer. These blurred lines, he says, implies “something really massive, and gas orbiting it,” like a supermassive black hole with material swirling around it.
Once astronomers uncovered this broad hydrogen line, the main hypothesis shifted from massive galaxies to massive black holes that were actively pulling in matter at the center of their forming galaxies. The red color, astronomers thought, was due to a large amount of dust reddening the active black hole’s glow. But when they captured even more data, the evidence yet again pointed elsewhere and the active black holes began to look like something even more puzzling.
Black holes that pull in a lot of material host a surrounding accretion disk, which heats up and glows brightly in X-ray light. If the red dots were typical active black holes, this high-energy radiation should be visible. But only one of these objects shows up in X-ray light. Active black holes also flicker in brightness, due to hot spots, instabilities, and flares in the accreting material. “Little red dots basically don't change,” though, says Institute of Science and Technology Austria astronomer Jorryt Matthee. It became apparent, he adds, these were not normal black holes.
A black hole star is born
When researchers looked at a broader swath of colors in the spectra from these dots instead of the fine detail of a small section of data, they saw “peculiar shapes” in the light spectrum, says astronomer Anna de Graaff, of the Max Planck Institute for Astronomy in Heidelberg, Germany. This peculiar shape is a drop in intensity of light followed by a sudden jump.
Astronomers see this spectral shape in regions with very dense gas, like the gaseous atmospheres of a specific type of hot star. With this new bit of evidence, researchers altered their hypothesis again. They now think little red dots have a power source, likely a massive active black hole or perhaps a star ten thousand times the mass of our sun, that is enshrouded in a very dense atmosphere of gas. “That is a pretty crazy conclusion, but it is really what the data are telling [us],” says de Graaff. This idea of a black hole at the center of a dense cocoon of gas similar to a star’s atmospheric shroud, that is also actively drawing in matter, has been dubbed a black hole star or black hole envelope, and it’s been gaining momentum.
That’s a leading theory now, but with about a thousand little red dots discovered, astronomers are finding they don’t all look alike. “We call them as if they are one single class of objects,” says Xiaohui Fan, an observational cosmologist at the University of Arizona, “but there's no reason to believe they are.”
How little red dots differ from each other
Researchers keep discovering differences between the known little red dots, and those differences reveals possible connections to black holes. For example, one lone little red dot from the universe’s first two billion years spews X-rays.
Another sits behind a massive galaxy cluster, whose gravity warps space-time and causes the light from the little red dot to bend. Astronomers see multiple separate images, which means the light takes multiple separate paths, each that takes a different amount of time. The paths span 130 years, allowing scientists to study how the light from this little red dot gets brighter or dimmer over time. The astronomers found it does vary by about a factor of two, says Fan. And they’ve found a few more little red dots with changing brightnesses, which means they could be more like today’s active black holes than the first observations suggested.
Some of the differences seen in little red dots may come down to the angle at which we see them from here on Earth. University of Cambridge astronomer Roberto Maiolino suggests those differences can be explained if the surrounding gas is clumpy and not a shell. That means it can look different from different angles, depending on if your line of sight passes through a thick clump of gas or not.
That’s similar to normal black holes, those much closer to Earth. Astronomers have classified multiple types of active black holes in the modern universe, each with different characteristics based on how they’re oriented toward Earth and how much material they likely are pulling in.
(This supermassive black hole was formed when the universe was a toddler.)
The many similarities with other active black holes in today’s universe mean the little red dots may be their precursors that emerge before the behemoths begin producing more detectable X-rays, adds Kocevski. He is working to show the little red dots may actually be the same accreting black holes we see today, and astronomers are seeing some subset of today’s black holes at an earlier time in the universe’s evolution.
Whatever these are, says Matthee, they did not disappear, they just evolved into something else. “It's like little villages that got eaten by a city.”
A closer look at nearby red dots
The more images and spectra astronomers can capture of the little red dots, the more they can learn about them and how they fit into the existing cast of cosmic players.
Researchers have also discovered a couple dozen objects that look like little red dots in the modern universe, meaning closer than 2 billion light-years away. “Because they’re local, we can actually study their physical conditions around them much, much better, so that we can maybe get some clue of why they exist,” says Fan.
Four years after discovery, astronomers have written more than 700 papers relating to little red dots, and the hypotheses have changed over those years as more data came in, though the solution remains elusive.
Webb was built to find the first galaxies, to piece together how the earliest cosmic structures formed. That was expected, but “secretly you hope to find a source you’ve never seen before,” says de Graaff. “I think the little red dots really deliver that.”
