Carl Sagan once said that to make an apple pie from scratch you must first invent the universe. He was right. And in inventing the universe you will need to build all the objects and structures found in it.
These are the planets, stars, white dwarfs, neutron stars, black holes, gas, dust, galaxies, galaxy clusters and superclusters. Eventually, when this cosmic mix has cooked long enough, the molecular arrangements will emerge to produce that pie. But how does the universe build all this stuff? This is one of the biggest unsolved puzzles in science.
We do know something about how cosmic objects are constructed: The glue used to build them is gravity. The number and variety of objects we see in the sky are in part determined by the effect of gravity on the tiny bumps and kinks of matter that the universe started out with almost 14 billion years ago.
The second piece of the problem is a little trickier: It’s all about resistance to building, or even destruction. Matter in the universe, rather perversely, creates many obstacles to its own assembly, the most fundamental of which comes from pressure, which is in turn related to temperature. The forces of gas pressure resist compression, and the hotter the gas gets, the greater its pressure.
In 1902, a young physicist at the University of Cambridge named James Jeans figured out that if you measured the temperature and density in a nebula, you could calculate the size of a region that would be hovering in balance, poised to collapse. A smaller region would have insufficient gravity to overcome its gas pressure. A bigger region would have insufficient gas pressure to resist gravity’s embrace. This critical point is known as the Jeans Mass.
In other words, any nebula bigger than its Jeans Mass is almost inevitably in the process of collapsing and condensing to make stars. Similarly, any cloud of gas that is actively cooling down by emitting radiation stands a good chance of cooling enough that it will begin to collapse under its own gravity.
We’d have to wait around for hundreds of thousands of years to notice a nebula collapsing to make stars. Because the human time scale is so short, Jeans found a way for us to deduce actions of matter that are happening at a snail’s pace from our terrestrial perspective. To make objects, gravity must always overwhelm pressure.
In the outer realms of a galaxy cluster the gas cools at a very slow rate. In the center, though, the process can cool down hundreds of times the mass of the sun in gas every year. That may not sound like much, but a typical cluster has been around for billions of years, so that adds up to an awful lot of material turning into a thick, cold nebula and condensing into stars.
It was in the 1970s, with the advent of Earth-orbiting X-ray telescopes, that scientists became aware of this characteristic of galaxy clusters. At the center of some clusters, X-ray emissions indicated the gas was cooling down in less than 10 million years, the blink of a cosmic eye -- a runaway process that soon earned the name “cooling flow.”
It seemed logical that all that cooling gas should end up condensing within the big elliptical galaxy that sits in the center of most large galaxy clusters. And as a result, much new star formation should be occurring there. But there wasn’t much evidence to support this. The giant galaxies at the centers of clusters simply didn’t contain a huge excess of young stars.
In 1994, the English astronomer Andrew Fabian tried to explain what might be going on. It was conceivable, he suggested, that the cooling gas was turning into cold molecules of simple compounds like carbon monoxide, or magnetic fields might be channeling and constraining the gas, and perhaps hot and cold gases were coexisting in structures that fooled our telescopes.
As with so many puzzles in science, new observations would clarify things. In 1999, within five months of each other, two mammoth telescopes were launched into Earth’s orbit -- NASA’s Chandra X-ray Observatory and the European Space Agency’s Newton Observatory. Both were designed to collect more X-ray light from astrophysical objects than ever before.
And they showed the gas, cooling. Down and down it went. And then ... nothing. You can imagine the scientists’ consternation. In cluster after cluster, the gas cooled as expected, then just as it reached a temperature of a little more than 10 million degrees, it stopped. Not only did it stop, it seemed to vanish, with just a trickle carrying on down to lower and lower temperatures. It felt like watching a great ocean liner gracefully sailing off to the horizon, and then suddenly turning into a dinghy before dropping out of sight.
Perhaps an unknown mechanism was heating the gas and getting it quickly back into the general mix before astronomers noticed it. Or perhaps the just-cooled gas was mixing with either much hotter or much cooler gas, or was obscured by a hitherto unseen blanket of cool material that blocked the X-rays from our view.
There was another possibility. Maybe energy from a central supermassive black hole was halting the cooling. But how? Scientists needed something to indicate exactly where to look. In the end there were several such signposts, but one in particular was so big and clear that in retrospect it’s almost embarrassing it took so long to connect the dots.
The answer came in a study of Perseus, a huge cluster of hundreds of galaxies about 250 million light-years from our solar system. Perseus is one of the largest clusters in our cosmic neighborhood; if we could see it with our eyes, it would cover a patch of sky four times broader than the full moon. It stretches across 12 million light-years and contains a thousand times the mass of the Milky Way. As in all such vast gravitational wells, the bulk of the normal matter in Perseus consists of gas that is trying to cool off. And just as in so many clusters, within the center is the unmistakable signature of particles that have been spewed from around a supermassive black hole, glowing with radio emission.
A puzzle about Perseus had emerged in the early 1980s with its first detailed X-ray images. There in one corner was a strange dark zone, what looked on the images like a dirty thumbprint, blotting out perhaps a hundred thousand light-years of X-ray glow.
Fabian and his colleagues spent nearly two years gathering almost 280 hours worth of photons to generate the most precise image of Perseus possible. And what an image it was! Perseus looked like a smooth pond after a giant pebble has been thrown in. There were bubble-like gaps, and there were ripples -- actual waves through intergalactic space -- moving outward from the supermassive black hole at the core.
Fabian’s team looked at the data every way, holding it up like a faceted diamond to see the shifting colors. The mysterious gaps high in the cluster’s outer regions were rising bubbles. And down toward the core were new bubbles, with ripples between them.
Apparently, the buoyantly rising bubbles were lifting and pushing the cooling gas aside, preventing it from funneling down to the core. The bubbles set in motion sound waves that dispersed energy throughout Perseus, keeping it at a perfect simmer.
A simple physics experiment brings the principle to life: Place a music loudspeaker on its back, so that it forms a shallow cup. Then add a sprinkling of rice grains. With the music off, they roll down to the middle of the speaker cup, but when you turn on the music and turn up the volume -- perhaps a bit of Bach or heavy metal -- the bass notes (longer sound waves) will vibrate the speaker and push at the grains. Given the right pitch and volume, they will bounce back out up the sides of the cup, just like the gas in Perseus.
Every few million years a supermassive black hole in Perseus is being fed matter. When this happens, an outburst of energy from its jets and radiation inflates bubbles of high-pressure, fast-moving particles into the comparatively cool and dense gas of the cluster. As these bubbles inflate, they release a wave of energy into the gas. The black hole is driving the ultimate subwoofer. The note it’s playing? Fifty-seven octaves below B flat above middle C, in case you were curious. That’s approximately 300,000 trillion times lower in frequency than the human voice. And the output is a planet-disintegrating 10^37 watts. Supermassive black holes make a very powerful sound system.
By now, evidence of similar bubbles has been found in 70 percent of all known galaxy clusters. There are also ethereal fingerprints of gas trickles that manage to cool all the way down before being heated and rearranged, and in among these are large, young, blue stars. The only viable explanation is that new stellar systems are forming here. The number is limited, because black holes are controlling the production line.
(Caleb Scharf, an astrophysicist, is the director of Columbia University’s Astrobiology Center and author of the blog Life, Unbounded. This is the second of five excerpts from his new book, “Gravity’s Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars and Life in the Cosmos,” which will be published on Aug. 14 by Farrar, Straus and Giroux. The opinions expressed are his own. Read part 1, part 3, part 4, part 5.)
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