“The problem is, you’re trying to find these really faint things, but you’re looking behind these really bright things. The brightest galaxies in the universe are in clusters, and those cluster galaxies are blocking the background galaxies we’re trying to observe.” -Rachael Livermore
One of the biggest puzzles in science is exactly how the Universe became transparent to visible light. Neutral atoms — cosmic dust — blocks visible light, and yet before there were stars, that’s all we had. According to theory, it should be large numbers of small, faint, ultra-distant galaxies that made it transparent, but they’ve never been seen.
However, thanks to the combined power of the Hubble Space Telescope, gravitational lensing and a new foreground light-removal technique, galaxies 100 times fainter thank the ones visible in the Hubble eXtreme Deep Field — the longest-exposure image ever — have now been revealed. These galaxies, seen in two Frontier Fields’ clusters so far, are the ‘missing link’ needed to explain reionization.