Has Webb broken cosmology?

Caroline Harper

- From theories of early galaxies to the expansion of the Universe, JWST has fundamentally challenged what we thought we knew, writes Caroline Harper

When the James Webb Space Telescope (JWST) launched on Christmas Day 2021, we knew its groundbreaking capabilities had the potential to rewrite the astronomy textbooks. And this incredible spacecraft has not disappointed.

The deployment of its giant segmented mirror and sun shield went without a hitch, its science instruments are operational and exceeding expectations, and the launch trajectory was so precise that there's fuel to maintain its orbit for many years to come.

Right from its first observations, the space telescope has given astronomers new puzzles to solve and new questions to pursue. In particular, it is challenging what we thought we knew about the early evolution of galaxies.

'Impossible' galaxies

JWST's intriguing deep-field observations of faint light from the early, distant Universe reveal stars and galaxies that seem to be much larger than expected.

The CEERS (Cosmic Evolution Early Release Science) survey, led by Prof Steven Finkelstein of the University of Texas at Austin, used JWST's NIRCam instrument to look back as far as the epoch of reionisation, just after the so-called dark ages of cosmic evolution, to study the structure of galaxies in the very early Universe.

It found more of them than predicted, and they appear bigger and brighter than expected. According to our best models of how the infant Universe developed, they aren't supposed to be there so early or look as they do.

Some of the first survey results have even indicated there are mature-looking disc galaxies reminiscent of our own Milky Way present as early as 10 billion years ago. We were expecting a more chaotic picture, with predominantly irregular galactic structures interacting violently. JWST's advanced resolution, coupled with its ability to observe very distant early objects in infrared, has made us think again about how soon galaxies began to form and mature.