Open questionCosmology · Universe

Early galaxies

JWST keeps finding galaxies that look more grown up than expected in the first billion years. Either galaxies form faster than we thought, or something in the timeline is off.

Open question · the first billion years

Tension
Galaxies at redshift 10 to 14 look too massive too early
Telescope
JWST, observing in the infrared
Implication
Star formation may be more efficient, or early dust differs
Alternatives
Revised mass estimates, a different stellar mix
Status
Active, with models being re-tuned

Galaxies in the first billion years

NASA, ESA, CSA, STScI
Webb's First Deep Field with thousands of distant galaxies
Webb's First Deep Field, with thousands of galaxies, some seen as they were within the first billion years. Image: NASA, ESA, CSA, STScI.

Too mature, too soon

JWST's deep fields reveal bright galaxies within a few hundred million years of the Big Bang. Some hold more stars than simple models allow that early. If the measurements stand, galaxy formation starts faster than the standard picture expects.

Because their light is stretched into the infrared, JWST is the first telescope able to see this era clearly.

Reading the evidence carefully

Mass estimates depend on assumptions about stars, dust and how much light each type produces. Follow-up spectra show that a few candidates are less massive than first thought. The broader trend, however, has not gone away, and several of the early galaxies hold up under closer inspection.

What it changes

If early galaxies really are more massive, star formation must be more efficient, feedback from young stars weaker, or the first stars built differently. That would ripple through models of reionization, galaxy growth and the timeline of the early universe.

Key publications

YearPaperVenueLink
2023Labbé et al., A population of red candidate massive galaxies ~600 Myr after the Big BangNature 616, 266DOI
2023Finkelstein et al., CEERS Key Paper. I. An Early Look into the First 500 Myr of Galaxy Formation with JWSTApJL 946, L13DOI
2023Arrabal Haro et al., Confirmation and refutation of very luminous galaxies in the early UniverseNature 622, 707DOI

Selected papers for further reading. DOI links open the publisher's record; preprints are linked on arXiv.

Worth knowing

  • Some of these galaxies formed when the universe was under 5% of its current age.

  • JWST sees them in infrared because their light is redshifted.

  • The record holders keep moving as surveys go deeper.

Recommended videos

  • We Found Galaxies Too Old for the Universe PBS Space Time
  • Incredible Discoveries Of The James Webb Telescope BBC Earth Science

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