Surprisingly Evolved Galaxies in the Early Universe

This astrophysics preprint describes A Population of Massive and Evolved Galaxies at z>=5, when the universe was less than a billion years old, which "... seems surpising at first sight". z is the redshift which according to the standard Big Bang models of cosmology is related to the age of the galaxy.

The galaxies found in this study are remarkable in that they contain a large stellar mass, have small physical sizes and that their main epoch of star formation occured at z >= 10. Galaxies with similar properties have, however, also been found by others.

Here's another preprint Morphologies of Two Massive Old Galaxies at z ~ 2.5 which says

Considerable observational evidence has built up over
the past few years that a substantial fraction of the mas-
sive galaxies around us today were already massive at
very early epochs.