For a scientist, perplexity is the most exhilarating state of mind, because unexpected phenomena are often the most direct path to discoveries. And a recent observation from one of the furthest corners of the universe -- an ancient and enormous black hole -- has set the astronomical world buzzing. It is hard to understand how a black hole became so big, so long ago.
Generally, black holes are the relic of massive and burned-out stars. They contain enough mass in a small enough volume to literally warp space and time, so much that not even light can escape them. One class of black holes originates from the death of stars; these contain as much mass as perhaps three to maybe thirty times that of our sun.
There is a second class of black holes that are much more massive and are found at the center of galaxies, like our own Milky Way. The recent scientific surprise involves a black hole from the second class, with a mass of about 800 million times that of our sun, that existed when our universe was a mere 690 million years old.
So why is that surprising? Black holes are like humans. They start out small and they grow over time. This particular black hole existed when the universe was only about 5% of its current age, and was located so far away that the light signaling its existence is only now arriving on Earth, after traveling for about thirteen billion years.
There is a lot of exciting and mind-blowing science that goes into understanding this most recent observation. The first point is that black holes are, well, black. They don't emit any light at all. Thus, what scientists observed was not the black hole itself, but rather a thing called a quasar, the first of which was discovered in 1962. Quasars, short for quasi-stellar object, are the brightest persistent things in the universe. They appear to be small and emit energy prodigiously.
The reason they seem small is because they are so incredibly far away. They were initially a mystery, and in the 1970s there were all sorts of types of speculation as to what they were, including that they might be a sort of "white hole" -- the other side of a black hole, where the mass and energy of a black hole was emitted into our universe.
We now know that quasars are actually supermassive black holes at the center of galaxies. These celestial monsters are millions or even billions of times the mass of our sun. But not all supermassive black holes are quasars. To be a quasar, the black hole must be feeding -- consuming the gas and stars around it at a breathtaking pace. As that matter spirals down into oblivion, it heats up to outrageous temperatures and emits light. It's that light that we see, and why we know that there is a large black hole at the center of this distant galaxy.
The recent observation is the most distant quasar ever observed. It is powered by an enormous black hole, and it existed when the universe was in its infancy. And that is the source of the mystery. Just how is it possible for such a massive black hole to exist when the cosmos was so young?
Astronomers have a pretty good handle on the broad strokes of how the universe has evolved. It started about 13.7 billion years ago in an event called the Big Bang, when all of the matter of the visible universe was compressed into a tiny volume. For reasons that are still being worked out, it "exploded," for the lack of a better word, starting out very hot and cooling as it went. In a scant three minutes, the protons, neutrons and electrons that make up ordinary matter existed, although the universe was too hot for atoms to form. At a time about 380,000 years after the Big Bang, the universe had cooled enough for atomic hydrogen and helium to form. At that point, the universe "went dark." It was relatively cool, transparent to light, and was nearly a uniform cloud of hydrogen and helium gas.
That "nearly" is important, as there were very small fluctuations in the density of that gas, and the denser portions started to attract other nearby gas, setting into motion the events that led to the universe we see today.
As the gas concentrated, it grew hotter, eventually turning into stars and galaxies. The very first galaxies formed when the universe was about 180 million years old, just about 500 million years before the recently observed quasar existed.
Five hundred million years may seem like a long time to us, but cosmically speaking, it's merely the blink of an eye. The fact that this black hole grew in such a short time challenges our models of how early galaxies formed. Stars would have to be born, die, and become black holes that then accumulated in close proximity in the center of the galaxy.
After that, there needed to be a period in which the black holes devoured one another in a sort of immense orgy of cosmic cannibalism. We know that black holes do indeed merge, as we've recently been able to observe the phenomena, but it is hard to understand how the process happened so quickly, and how such an incredibly massive black hole could form so early in the history of the universe.
So what explanations have astronomers offered to explain this observation? One is that a large black hole formed and then consumed matter around it at a dizzying pace. The problem with this explanation is that, as matter spirals into the black hole, it heats up and forms a kind of pressure that pushes back on other matter falling into the hole. The more the black hole consumes, the hotter it gets and the more it pushes back. Thus, the process should sort of self-regulate, and it is hard to understand how the hole could consume so much mass so quickly and be so big so soon after the Big Bang.
A second and exciting possibility is that heavy black holes were made much earlier in the history of the universe than expected. This is because it is very hard to understand how many stellar-massed black holes could combine together so early in the universe. As a possible solution, some astronomers have proposed that black holes with masses about a thousand times that of our sun formed a mere 66 million years after the Big Bang. This has exciting implications for studies of dark matter, another cosmic mystery. Dark matter is thought to be five times more prevalent than ordinary matter and is invisible. Perhaps dark matter is primordial black holes.
It's all too early to have completely absorbed the significance and implications of this new discovery, but the situation is exhilarating. Astronomers have a new and puzzling observation that could change how we understand a pivotal epoch in the history of our universe. It's truly an exciting time to be a scientist.
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