Scientific discoveries never cease. Just when you think we’ve found the oldest fossil, animal, or planet, there’s another research team announcing that they’ve found one even older. Even still, the discovery of the oldest known black hole is pretty impressive: it formed just millions, not billions, of years after the Big Bang.
And because we know it’s there, we can learn about the conditions of the universe when it formed. About 13.8 billion years ago, our universe existed as a singularity: a point of infinite gravity and density where the laws of physics break down. (What happened before that is up for debate). Then, in trillionth of a second, all hell broke loose. The singularity exploded, doubling in size many times over at a rate faster than the speed of light in an event we know as the Big Bang.
That early universe looked very different than it does today. It was a dense plasma soup that was positively buzzing with subatomic particles, including photons (light) that scattered off of free electrons. At first, the universe was too hot for atoms to form, but it eventually cooled enough to form hydrogen (and eventually helium) atoms that were ionized, or missing an electron. Those atoms attracted electrons to become neutrally charged, allowing light to travel freely for the first time (and creating what we know as the cosmic microwave background). But that freedom was short lived, because the neutral atoms eventually formed a “cosmic fog” that absorbed the light, marking the start of what scientists call the Dark Ages of the early universe.
Even once the earliest stars began to form, this fog would have snuffed out their light and kept the universe pitch black. Eventually, around 400 million years after the Big Bang, there were enough young stars and quasars — the bright jet of light blasting out of a black hole — to create enough ultraviolet light to strip those atoms of their electrons once again in an event called “reionization.” That gradually burned off the fog, returning the universe to the light-filled state it had been in before the Dark Ages. But this time, the light wasn’t just the Big Bang’s leftovers. It was in constant production, thanks to a young generation of stars. By the end of reionization, 1 billion years after the Big Bang, the fog was completely gone and the universe had gone from opaque to transparent. Let there be light … again!
That’s the gist of what astronomers think happened in the early universe. But that story is full of holes. We still don’t know when the first stars formed, or which light sources were responsible for reionization and why. But thanks to the laws of physics, it’s possible to look back in time. You see objects in the sky not as they are, but in the state they were in when the light began traveling to you. It takes the light from the sun eight minutes to reach you, so the sun you see is actually eight minutes in the past. Proxima Centauri, our closest star, is about four light-years away, so you see it four years in the past. Our universe is a little more than 13 billion years old, so if by some miracle we could see something 13 billion light-years away, we could glimpse the beginning of the universe itself.
That’s why the discovery of this ancient black hole is so groundbreaking. Because of its distance, we know that it formed 690 million years after the Big Bang: right in the middle of reionization. In fact, the gas around this black hole and its telltale quasar is half neutral, half ionized, just as you’d expect from that era. The black hole is much bigger than you’d expect, however, at 780 million times the mass of our sun. It didn’t have enough time to grow to that size, and scientists are puzzling over how that was possible. One theory? It just started bigger, possibly because collapsing clouds in the early universe gave birth to supersized black holes.
New technology is making discoveries about the universe come at breakneck speed. The researchers, led by astronomer Eduardo Bañados, hope to discover more black holes and quasars like this one, and hopefully solve the new puzzles this discovery has posed.