Housing thousands of galaxies and an absurd amount of stars, galaxy clusters are the most massive known objects in the universe. It was previously believed that these clusters formed when young starburst galaxies collided about 3 billion years after the Big Bang, but now, two teams of international researchers think that the pileup occurred much earlier. By peering into the universe’s ancient past, they saw the collisions beginning just 1.5 billion years after the Big Bang, drastically increasing the age of some of our oldest cosmic residents.

The research teams, led by Iván Oteo of the University of Edinburgh and Tim Miller of Yale University and Dalhousie University in Nova Scotia, used the Atacama Large Millimeter/submillimeter Array (ALMA) and the Atacama Pathfinder Experiment (APEX) telescopes to study the merging starburst galaxies. Both Oteo’s and Miller’s findings were published in the Astrophysical Journal.

Starburst galaxies produce stars at freakishly fast rates, popping out thousands each year, whereas the Milky Way produces roughly one per year. These galaxies were prominent after the Big Bang and are located a fair distance from Earth, with their light taking billions of years to reach us. By the time it finally does, we’re seeing these galaxies as they appeared billions of years ago.

Discovered while observing our 1.5-billion-year-old universe, the South Pole Telescope and the Herschel Space Observatory found two objects that appeared as indistinct smudges of light. Further observations by ALMA and APEX revealed them to be two extremely dense packs of galaxies, one made up of fourteen galaxies and the other made up of ten. They’re the most active regions of star formation ever witnessed in the infant universe — and the galaxies within them are on the brink of collision. These cosmic mash-ups, which were originally thought to have begun when the universe was 3 billion years old, would go on to create the cores of massive galaxy clusters.