Black holes are finally winning some respect. After long regarding them as agents of destruction or dismissing them as mere by-products of galaxies and stars, scientists are recalibrating their thinking. Now it seems that black holes debuted in a constructive role and appeared unexpectedly soon after the Big Bang. “Several years ago, nobody imagined that there were such monsters in the early universe,” says Penn State astrophysicist Yuexing Li. “Now we see that black holes were essential in creating the universe’s modern structure.”
Black holes, tortured regions of space where the pull of gravity is so intense that not even light can escape, did not always have such a high profile. They were once thought to be very rare; in fact, Albert Einstein did not believe they existed at all. Over the past several decades, though, astronomers have realized that black holes are not so unusual after all: Supermassive ones, millions or billions of times as hefty as the sun, seem to reside at the center of most, if not all, galaxies. Still, many people were shocked in 2003 when a detailed sky survey found that giant black holes were already common nearly 13 billion years ago, when the universe was less than a billion years old. Since then, researchers have been trying to figure out where these primordial holes came from and how they influenced the cosmic events that followed.
In August, researchers at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University ran a supercomputer simulation of the early universe and provided a tantalizing glimpse into the lives of the first black holes. The story began 200 million years after the Big Bang, when the universe’s first stars formed. These beasts, about 100 times the mass of the sun, were so large and energetic that they burned all their hydrogen fuel in just a few million years. With no more energy from hydrogen fusion to counteract the enormous inward pull of their gravity, the stars collapsed until all of their mass was compressed into a point of infinite density.
The first-generation black holes were puny compared with the monsters we see at the centers of galaxies today. They grew only slowly at first—adding just 1 percent to their bulk in the next 200 million years—because the hyperactive stars that spawned them had blasted away most of the nearby gas that they could have devoured. Nevertheless, those modest-size black holes left a big mark by performing a form of stellar birth control: Radiation from the trickle of material falling into the holes heated surrounding clouds of gas to about 5,000 degrees Fahrenheit, so hot that the gas could no longer easily coalesce. “You couldn’t really form stars in that stuff,” says Marcelo Alvarez, lead author of the Kavli study.
Even as Alvarez’s computer model offered a glimpse into the universe’s infancy, it sowed confusion about what happened next. In 2007 scientists spotted a billion-solar-mass black hole that existed some 840 million years after the Big Bang, the earliest and most distant one ever observed. (Black holes themselves are invisible, but astronomers detect them by looking for the brilliantly hot gas that swirls around them before getting sucked in.) This past September another research team announced it had found a large, star-forming galaxy surrounding that black hole. These discoveries were puzzling, to say the least. About 400 million years after the Big Bang, the universe still consisted of scattered stars and small, starving black holes. Less than 500 million years later, it was full of monster black holes embedded in vast galaxies. How did things change so rapidly?
Penn State’s Li is trying to find out. While Alvarez’s simulations focus mostly on individual stars and black holes, Li studies the interaction of those objects and their influence on large-scale structures in the early universe. Her work shows that the first black holes were enveloped by halos of dense, invisible matter tens of thousands of times more massive. Together, these constituted protogalaxies, building blocks of today’s galaxies. During a period of frequent, violent collisions among the protogalaxies, their resident black holes experienced rapid growth spurts by merging with one another and gobbling up new supplies of gas and dust. A 100-solar-mass black hole ballooned into a billion-mass beast within 800 million years, and in especially dense regions that growth could have occurred even more quickly. During this dynamic period, Li’s model shows, black holes suddenly became a lot more star-friendly. Merging protogalaxies sent out shockwaves that compressed dense clumps of gas, helping trigger widespread star birth even in regions previously dominated by black hole radiation. In a remarkably short period of time, black holes shifted from lightweight bullies to supermassive centerpieces of star-breeding galaxies.
Although this simulation offers a comprehensive account of this formative epoch, Li concedes that her models are still just models; they are no match for direct observation. So while she and other theorists refine their calculations, other astronomers are using powerful telescopes to peer ever further back in time, looking for objects that are currently known only from computer simulations. “There are aggressive campaigns to search for the first supermassive black holes,” Li says. “We still may not have found the very first ones.” She says it would not surprise her if the earliest of these giant black holes appeared as little as 500 million years after the birth of the universe.
The recently refurbished Hubble Space Telescope will aid this search. This past April, one of Li’s Penn State colleagues discovered the burst of energy from a star that exploded, probably in the process of collapsing to form a black hole, when the cosmos was just 630 million years old.Hubble’s successor, the James Webb Space Telescope, will delve even deeper following its 2014 launch.
Soon, astronomers may be able to directly observe the improbable era when black holes were among the most important objects in the universe, helping to bring order to the Big Bang’s formlessness. “In theoretical and observational astronomy,” Li says, “this is the cosmic frontier.”
