Astronomers have found an alignment of galaxies along our line of sight that’s so rare, it ought to be impossible. Is it just luck — or does it tell us something more?
The Hubble Space Telescope image shows it clearly: a giant blue arc nestled among a massive cluster of galaxies. Light from a distant, large, spiral-type background galaxy was warped into an arc as it passed the massive cluster en route to Earth. Einstein predicted such gravitational light-bending in 1916, and with the advent of Hubble, warped images of extremely far galaxies have become a relatively common sight. But there’s a problem with this one: it shouldn’t exist.
“When I first saw it, I kept staring at it, thinking it would go away,” says lead scientist Anthony Gonzalez (University of Florida). “According to a statistical analysis, arcs should be extremely rare at that distance.”
The problem is not that the background galaxy exists. Though its distance is not well known, we’re probably seeing it from when the universe was between 0.7 and 3.7 billion years old. The first galaxies formed when the universe was less than about 1 billion years old. So the galaxy is young, but not impossibly young.
Nor is the problem the existence of such a massive galaxy cluster. It’s a big ’un though. Weighing as much as 500 trillion Suns, it’s five to 10 times more massive than other clusters its age. That puts it on track to become one of the most massive clusters in the present-day universe, with as much mass as the Coma or El Gordo galaxy clusters.
“The galaxy-cluster community has been calling these [massive clusters] pink elephants, and this guy’s the pinkest,” notes Daniel Stern (JPL), a co-author of the study.
Still, the existence of such a massive cluster in a universe only 4 billion years old might be rare, but it’s not impossible.
The almost-impossibility lies in the chance alignment of such a large background galaxy and such a large foreground cluster. The odds of there being enough of both are small. So small, in fact, that Gonzalez and his co-authors write in their paper, “This arc simply should not exist” (their emphasis).
One possible explanation requires a change to the earliest moments of the Big Bang. The current model says that the newborn universe went through a super-expansion, or “inflation,” that lasted only from 10-36 to 10-33 seconds. During that moment, inflation enlarged microscopic quantum fluctuations enough that they became the gravitational seeds for the formation of galaxies. The physics all works out beautifully.
The question is, what exactly did these seeds look like? Some versions of inflation produce seeds that tend to be on the large side. Larger seeds would make large, star-forming galaxies and massive galaxy clusters more common in the early universe. If that were true, the chance alignment of a large galaxy behind a massive cluster becomes more likely.
Exotic physics in the early universe make astronomers uneasy. But other explanations don’t work well. For example, Gonzalez points out that young galaxy clusters may have been more concentrated in their centers. That would enable clusters to distort light from more background galaxies, making arcs like this one more common. But they still wouldn’t be common enough — the odds of finding a giant arc at such a great distance would still be near zero.
“I’m not yet convinced by any of these explanations,” says Gonzalez. “After all, we have found only one example.” But it should spur the search for more.