Astronomers don’t know how or when these alignments form. To probe this mystery, West used Hubble observations to peer back 10 billion years, to when the universe was one-third of its current age. His team studied the light from 65 giant galaxies. They found that the brightest galaxies in the hearts of galaxy clusters were aligned with their surroundings. This is the farthest back that researchers have observed this phenomenon.
The researchers suggest several explanations for this. West’s preferred theory involves the cosmic web, the Swiss cheese-like structure of the observable universe. Galaxies are concentrated in thread-like filaments that weave around large voids of mostly empty space. Big, bright galaxies sit inside the center of a galaxy cluster, which is inside a cluster of galaxy clusters, inside one of these filaments. When big, bright galaxies align with their surroundings, they may be taking cues from this vast network. To extend the web metaphor, imagine that a galaxy is like a spider, resting in the middle of a cluster.
“The spider is there at the center waiting for its next meal, but in this case, the next meal is a small galaxy, not an insect,” West said. “But the web isn’t circular. The web is elongated in the same way it’s tracing this cosmic web, this network of filaments. And so the spider turns in different directions to eat, depending on which way the web is oriented.”
Gravity could also be the culprit, as it often is. Over time, gravity may have pulled the big galaxies into alignment with its neighbors. Or the alignments could have been set when the galaxies first formed. It could be a mix of all three.
The biggest galaxies of the universe have undergone a turbulent evolution. Elongated galaxies get their shape from violent collisions that allow them to swallow up more matter. Computer simulations have shown that “if you take two galaxies and you just collide them head on, the net result is a bigger galaxy that’s elongated in the direction that the collision occurred,” West said. “They’re the result of probably multiple mergers or cannibalism of smaller galaxies over billions of years, and that imprints this elongation in them and these preferred orientations.”
What about our own galaxy? The Milky Way doesn’t seem to be a preferred orientation, West said. Unlike the massive, spider web-like galaxies of the early universe, the Milky Way has had a calm upbringing. The clusters housing massive galaxies are like the bustling metropolises of the cosmos, while the Milky Way's region is a small village, West said.
“The Milky Way, like all galaxies, has probably been influenced in one way or another by its environment. But in these massive galaxy clusters, where we find these giant galaxies, they’re really intense environments,” West said. “There’s a lot of galaxies, there’s a lot more merging, a lot more cannibalism.” All that activity has likely had a dramatic effect on their development, and in turn, their place in their slice of universe.