This post is the third in a series that accompanies the upcoming publication of my book ‘Gravity’s Engines: How Bubble-Blowing Black Holes Rule Galaxies, Stars, and Life in the Cosmos’ (Scientific American/FSG).

In space it's a good thing that you can't hear black holes scream.

Although some of the most incontrovertible evidence for the existence of supermassive black holes comes from observing their gravitational effect on neighboring stars, there are other even more spectacular clues. The clearest of these were unwittingly seen in extraterrestrial radio signals as far back as the late 1930's, and then observed with increasing fidelity as better and better technology allowed astronomers to map out the cosmos.

Among the brightest of these astronomical radio wave sources are some extraordinary and very distant structures. Many of these puzzling forms not only span hundreds of thousands of light years but are strikingly asymmetrical, while others are 'doubled', like the two ends of a vast dumbbell. The image here is a modern radio map of one of the most famous such objects, known as Cygnus A.

Cygnus A mapped in microwaves (NRAO/VLA)








This beast is some 600 million light years from the Milky Way, and end-to-end it spans about 500,000 light years. The radiation we're seeing comes from electrons that are moving at close to the speed of light and cooling themselves down by shedding microwave photons (among other frequencies) as they spiral around intergalactic magnetic fields. These fast-moving particles are transported in the thin beams, or 'jets', that appear to emerge from a point of brightness at the very center, before they splash into the great lobes of light to either side. But what powers such an enormous system of subatomic acceleration?

First let's put it in perspective. Here is the galaxy at the very center of Cygnus A as seen by the Hubble Space Telescope.

The galaxy of Cygnus A in visible light (Hubble Space Telescope/NASA/ESA)

It's a bit of a messy object. Lots of stars, lots of dust and gas. It carries the markings of interstellar turmoil, and hints at the presence of other intense sources of radiation.


But things get really interesting when we place this image to approximate scale on top of the radio map of Cygnus A.

The galaxy of Cygnus A to approximate scale superimposed on the radio map









See that? The visible galaxy is tiny compared to the rest of Cygnus A.

This is a terrific clue to the incredible amount of energy being pumped out to intergalactic space - from a very small point of origin. Back in the 1960's and 70's it was the sheer scale of the energy budget of objects like Cygnus A that challenged astronomers and physicists to come up with a sufficiently efficient power source. Nothing chemical or nuclear seemed to come close to meeting the requirements, and there was the striking morphology to explain as well - all that energy seemingly channeled into narrow beams of highly relativistic (near light speed) particles.

Representation of a gravity well, 3 dimensions collapsed to 2 (Credit: AllenMcC)

Although it was a contentious issue, it eventually became clear that there was in fact a natural power source of the required efficiency; gravity. As matter falls into a gravity well it is accelerated, and the further down the gravity well that matter can fall, the greater the velocity it can reach. The ultimate in gravity wells are those produced by black holes, which by definition are so compact and so 'steep' that light itself is unable to climb away again (at least not without giving up all its energy). Matter that becomes ensnared by a hole can accelerate to close to the speed of light, but before it disappears across the event horizon the friction and turbulence of its spiraling path can generate a phenomenal amount of energy that escapes and floods out. Add some other factors, like black hole spin and black hole electrical charge, and a supermassive hole can build you a machine that's as much as fifty times more efficient than nuclear fusion at converting matter into energy - and it can also squirt that energy out in narrow particle beams.

So just how loud can an energy spewing supermassive black hole be? It depends a lot on the circumstances; the size of the hole, its rate of spin, and how much matter is feeding into it, but there are some examples that help give us an idea of what to expect.

An image in X-ray light of the Perseus galaxy cluster (NASA/CXC/IoA/A. Fabian et al.)

This is an image of X-ray light being emitted by million degree gas that sits inside a galaxy cluster called Perseus. Within the gas, at the center, is a galaxy called Perseus A that contains a supermassive black hole. Every few hundred thousand years that black holes goes on a feeding frenzy and ejects particles outwards. Except that deep inside the galaxy cluster those particles bump right into the material around them, and like a giant blowing air through a straw, the black hole inflates enormous bubbles inside the hot gas. These are the darker looking patches in the image.

Incredibly, the inflation of these bubbles is akin to firing up a colossal sub-woofer, setting sound waves in motion that flood outwards as ripples that are over a hundred thousand light years in width (you can see them in this picture). That's a note played at a frequency a million, billion times lower than anything the human ear can detect. And the output is a whopping ten-to-the-power-of-thirty-seven watts, or about ten billion times the energy of our Sun. It is indeed fortunate that this primal scream is eventually lost to the vacuum of space.

Just as your annoying neighbor may keep you awake at night, what can a bellowing black hole like this do to the universe around it? It turns out that it may play an unexpectedly important role in creating the appearance of our cosmic surroundings. be continued.