First galaxies assemble

Within a few hundred million years of the Big Bang, stars, gas and dark matter gathered into the first galaxies. They were small and irregular compared with the Milky Way, but they formed stars rapidly. Astronomers can now see some of them directly: because light takes time to travel, looking very far away means looking back in time. In 2024 the James Webb Space Telescope confirmed a galaxy called JADES-GS-z14-0, seen as it was about 300 million years after the Big Bang, and others nearly as early.

Galaxies formed because the early universe was not perfectly smooth. Slight differences in density, visible in the cosmic background radiation, grew as gravity pulled matter toward denser regions. Dark matter, which does not emit light but outweighs ordinary matter about five to one, gathered first into clumps called halos. Hydrogen and helium gas fell into these halos, cooled and formed stars, including the first stars. Small halos and their galaxies then merged again and again, building larger galaxies over billions of years, a process called hierarchical growth.

Seeing these galaxies required a special telescope. As the universe expands, light traveling across it is stretched to longer wavelengths, an effect called redshift, so visible and ultraviolet light from the first galaxies arrives as infrared. The Hubble Space Telescope could see back to about 400 million years after the Big Bang. Webb, launched in December 2021 with a 6.5-meter mirror and designed to observe infrared, pushed further. It found more early galaxies, and brighter ones, than many models predicted, leading astronomers to debate whether stars formed faster and more efficiently than expected.