Old before their time

摘要

The discovery of massive, evolved galaxies at much greater distances than expected ― and hence at earlier times in the history of the Universe ― is a challenge to our understanding of how galaxies form. Over the past two decades, astrophysicists have been spectacularly successful in explaining the early evolution of the Universe. Existing theories can account well for the time span from the Big Bang nearly 14 billion years ago until the Universe began to cool and form the first large structures. less than a million years later. But detailed explanations of how the original stew of elementary particles subsequently coalesced over time, to form the stars and galaxies seen in the present-day Universe, are still being refined. As they report on pages 181 and 184 of this issue, Glazebrook et al. and Cimatti et al. have discovered the most distant 'old' galaxies yet. But the existence of these objects at such an early epoch in the history of the Universe seems inconsistent with the favoured theory of how galaxies formed.