James Webb Telescope Rewrites the Story of the Early Universe
New observations from the James Webb Space Telescope are challenging existing theories about the early universe, from unexpectedly massive black holes to galaxies that are too bright. Astronomers are developing new models to explain these phenomena.
The James Webb Space Telescope (JWST) has become a window into the universe's infancy, revealing objects that defy prior expectations. Among the most puzzling are hundreds of "little red dots" — compact, reddish sources that appear in images of the cosmos around 650 million years after the Big Bang. These objects were unknown before the telescope began its work in 2022.
Astrophysicist Charlotte Mason of the Cosmic Dawn Center in Copenhagen studies these dots and often sketches diagrams to make sense of them. One idea is that each dot is a black hole hidden inside a thick cloud of gas—possibly a novel class of object called a "black hole star," where the gas glows like a stellar atmosphere. But when Mason and her team analyzed the light from one such dot, they found no characteristic changes that would occur if the light passed through dense gas. Mason now suspects the gas is clumpy rather than uniform.
JWST has also spotted ancient black holes that seem far too massive for their age. Jenny Greene, an astrophysicist at Princeton, notes that billion-solar-mass black holes existed just a few hundred million years after the Big Bang, requiring extraordinary growth rates. Theoretical limits like the Eddington cap—which normally halts feeding—may be circumvented by so-called super-Eddington accretion, where the infalling gas overpowers radiation pressure. Another option is that seeds formed from collapsing gas clouds directly, but simulations produce too few such seeds.
In 2024, JWST observed one black hole, seen from when the universe was about 1.5 billion years old, devouring material at roughly 40 times the Eddington limit. Another little red dot, about 750 million years after the Big Bang, turned out to be a "naked" black hole of around 50 million solar masses with no visible stars—hinting that it may have been born large through direct collapse.
The early galaxies themselves are another surprise. They are often too bright and too numerous. While some scientists initially wondered if the standard cosmological model was flawed, theorists now offer several explanations: more efficient star formation, periodic bursts, or oversized stars. Simulations have improved greatly since JWST's launch, helping researchers like Hakim Atek of the Sorbonne match simulated galaxies to observed ones.
Data from JWST's Mid-Infrared Instrument shows that ancient galaxies have unexpectedly varied properties, which may point to episodic star formation. Some galaxies seem to have cleared away their gas, while others are rich in it. An overabundance of nitrogen suggests many massive stars existed early on. As the telescope continues to gather data, astronomers are gradually piecing together a more complete picture of cosmic dawn.
