Webb and the galaxies that seem to have formed too early

What was found

Light takes time to travel, so looking far away means looking into the past. Webb was built to see infrared light, which is what the earliest galaxies' light has been stretched into by the expansion of space.

Within its first year Webb spotted galaxies whose light left them when the universe was under 400 million years old, about three percent of its present age. Several of them looked surprisingly bright and surprisingly well formed. Some appeared to contain more stars than the standard model of galaxy growth expected to exist that early.

Why that is surprising

In the standard picture, galaxies grow slowly. Gas falls into clumps of dark matter, cools, and forms stars over hundreds of millions of years. The amount of gas available that early should limit how big a galaxy can get. Seeing objects that look too big, too soon, suggests something in that story is off.

The candidate explanations

Astronomers have several ordinary explanations, and the truth is probably a mix.

• Stars formed more efficiently early on. With fewer heavy elements around, early gas may have turned into stars faster than today's gas does.
• The light is not all from stars. Young black holes swallowing gas can shine brightly and be mistaken for starlight, making a galaxy look more massive than it is.
• Distances were overestimated. Early distances came from colours alone. When spectra were taken, some galaxies turned out to be closer than first thought.
• Star mass was over-counted. Very bright, short-lived stars can dominate the light while contributing little to the total mass.

What has not happened

Nothing so far breaks the underlying model of the universe: a hot beginning, expansion, dark matter, dark energy. The pressure is on the sub-model of how the first galaxies grew. That is the layer scientists always expected to revise once they could see it.

How to read the next headline

When a story says a galaxy is "impossible", check whether the distance came from a spectrum or from colours alone. Spectrum-confirmed results are the ones that count. Then check whether the mass estimate has been revised. Most of the "impossible" galaxies of 2022 and 2023 have since become merely interesting.