October 11, 2003

NASA’s Chandra X-ray Observatory, after its four years of X-ray imagining, recently monitored a very deep sound coming from a massive black hole 250 million light years from Earth. The black hole sits at the hub of a giant galaxy called NGC 1275, in the Perseus cluster of galaxies. The galactic cluster of Perseus is situated 230 millions light years away from Earth and contains several small member-galaxies including the NGC 1275. The Chandra X-ray Observatory has detected ripples or waves in the gas isolated throughout the Perseus cluster.

Astronomers have detected that this note is the deepest sound ever detected in the universe. One might wonder, with a tone that deep and the energy being released from this particular black hole, if everything in the vicinity would be trembling so greatly that it would fall apart? But then again it’s a good thing that the galaxy is 250 million light years away.

It’s a rather interesting fact that we go through tons of information everyday regarding black holes and galaxies and still there’s very little we know about what black holes really are. Einstein’s general theory of relativity describes gravity as a curve of space-time caused by the presence of matter. On the other hand, according to Newton’s theory very massive or dense objects generate much stronger gravity but the smallest objects imaginable are predicted by General Relativity to have such strong gravity that nothing, not even light, can escape their grip. Scientists today call such objects black holes.

In modern science, a black hole is a region of space-time from which nothing can escape even light. To see why this happens, imagine throwing a tennis ball into the air. The harder you throw the tennis ball, the faster it will be travelling when it leaves your hand and the higher the ball will go before turning back. If you throw it hard enough it will never return, the gravitational attraction will not be able to pull it back down.

So as a body is compressed into a smaller and smaller volume, the gravitational attraction increases, and hence the escape velocity or speed also increases. Things have to be thrown harder and harder to escape. Eventually a point is reached when even light, which travels at 186 thousand miles a second, does not travel fast enough to escape. At this point, nothing can get out as nothing can travel faster than light. This is a black hole.

You must be aware that it’s impossible to see a black hole directly because no light can escape from them but there is good reason to think they exist. Scientific theories tell us that when a large star has burnt all its fuel it explodes into a supernova. The stuff that is left collapses down to an extremely solid object known as a neutron star. We know that these objects exist because several have been found using radio telescopes.

If the neutron star is too large, the gravitational forces overpower the pressure gradients and collapse. The neutron star continues to shrink until it finally becomes a black hole. This mass limit is only a couple of solar masses, that is about twice the mass of our sun, and so we should expect at least a few neutron stars to have this mass.

Now the question is do black holes actually exist? Most physicists believe they do, basing their views on a growing body of observations. In fact, present theories of how the cosmos began, partially rest on Einstein’s work and predict the existence of the black holes. Yet Einstein himself strongly denied their reality, to him black holes were a mere mathematical curiosity. He died in 1955, before the term “black hole” was thought up or understood and observational evidence for black holes began to accumulate.

Coming back to the point, the detection made by astronomers using Nasa’s Chandra X-ray Observatory is the first of its kind. They have found out that this is the deepest note ever generated in the cosmos; a B-flat flying through space like a ripple or a wave on an invisible pond.

In musical terms the pitch of the sound is the note B flat. A “B-flat” is among the various notes in a piano. The note is 57 octaves lower than middle C and a million, billion times deeper than the limit of human hearing and thus it is said that no human will actually hear the note. A typical piano has about seven octaves. NASA astronomers using the Chandra X-ray Observatory monitored 53 hours of noise coming from the centre of the Perseus galaxy.

The note also strikes an important chord with astronomers, who say it just might help them understand how the universe’s largest structures, called galaxy clusters, develop. Moreover, the sound waves seem to be heating gas in the Perseus galaxy cluster, probably solving an ancient mystery about why the gas surrounding this cluster and others does not chill out as presented theory predicts. The gas is seemingly dancing excitedly to the hum of a deep B-flat.

What’s interesting is that the astronomers were not surprised to find the super massive black hole making a strong sound. Although these best known matter-sinks are by nature dark and invisible, they create bright and frenzied environments in which many forms of radiation - from radio waves to visible light to X-rays - have been recorded. These electromagnetic waves all travel at the speed of light.

Sound waves are similar, but they travel far more slowly and are more physical in nature. For example the sound you hear can be produced by the visible compression and expansion of a stereo speaker. The waves physically condense the stuff through which they move, be it air, water, or hot inter-stellar gas.

Other studies have shown that the violent activity around black holes — where gas is accelerated to nearly light-speed — produces many notes that are, all together, much like music. Collectively, the cosmos produce, scientists believe, a cacophonic masterpiece of tunes, which are impossible to hear.

Musical production appears to be ever-present in Nature. Scientists often call it “flicker noise”, and it has also been detected in the X-ray outputs of magnetic fields within our solar system. Even Earth hums its own tune. Musical analogies are found in everything from seascapes to brainwaves.

That is the reason why the 53 hours of Chandra observations revealed a note that is more than a million billion times deeper than what you can hear. “We have observed the prodigious amounts of light and heat created by black holes,” said Andrew Fabian of the Institute of Astronomy in Cambridge, England, and the leader of the study. “Now we have detected the sound.”

Scientists had previously observed large amounts of hot gas saturating clusters. Given what’s known, the gas should cool over time, however. Cooler gas would create areas of lower pressure near the center of a cluster, causing border gas to fall inward. In the process, trillions of stars would form. But this isn’t what astronomers see when they look at clusters.

The Perseus cluster is the brightest known in X-rays, making it a good target for study. It has two large, bubble-shaped cavities that extend away from a central black hole. The cavities are formed by streams of material ejected from the black hole’s surroundings, and the streams have been suspected of heating the outlying gas. But scientists couldn’t see how.

A special image-processing technique was used to bring out delicate changes in brightness that revealed the presence of ripples - the sound waves. Fabian and his colleague Steve Allen figure the sound waves, observed spreading out from the cavities, heat the gas. The amount of energy involved is amazing, equal to what would be produced if 100 million stars exploded.

A single, long-sounding note is produced by a sound wave in which the waves are the same size and shape continuously. The newfound note has been sounding, the researchers say, for about 2.5 billion years.

So the sound waves carry immense amounts of energy away from the black hole, heating up the surrounding gas. Previously the satellite revealed that there are two vast bubbles in this gas, apparently originating from the galaxy’s massive central black hole. The researchers also calculate that the sound waves’ heating effect could balance the steady cooling of the gas. Each bubble would have to be blown continuously for around ten million years before shedding from the galaxy core and moving through the gas.

“The Perseus sound waves are much more than just an interesting form of black hole acoustics,” said Allen. “These sound waves may be the key in figuring out how galaxy clusters … grow.”

The writer contributes regularly to Dawn ScienceDotcom on science-related issues

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