Notes from the Astronomy Underground- Astropalooza

According to the tagline in Ridley Scott’s 1979 blockbuster Alien, “In space, no one can hear you scream.” It’s true that sound waves, unlike light, need a medium- some kind of substance to carry their energy across a distance. And space is a vacuum, which, save the occasional solar system, fuzzy nebula, or bizarre stellar end product, is devoid of any respectable amount of matter. No matter, no sound, right?

 

Well, almost. Space is not completely empty. There are about one or two hydrogen molecules per square centimeter in the sparsest of regions. It beats our clumsy, terrestrial vacuum chambers handedly, but it’s not a vacuum in the strictest connotation of the word. Sound waves can still propagate through space, but so slowly and ineffectively that it would be pointless for aerophilic humans to do anything about it. Unless of course, we had ears many millions of times larger than we do now, and could hear frequencies millions of billions of times lower (and slower) than our current 20 Hz limit. Only then could we collect enough sluggishly disturbed atoms that listening to the cosmos would be feasible.

 

Given that sound is not the exclusive domain of terrestrial existence, how might we tune into this ambient “flicker noise,” nature’s otherwise inaudible music? Scientists have come up with a couple of different ways to observe the instruments of the cosmic orchestra and indirectly pinpoint what kind of music they would make.

 

 

Starting from our local solar neighborhood, it became apparent a decade ago that the closest planets emitted their own inaudible hum. For the Earth, if we throw out the din of human civilization, the dizzying menagerie of life around us, and subsurface tremors of earthquakes, we’re left with a residual white noise coming from the planet itself. In 1997, Naoki Kobayashi at the Tokyo Institute of Technology proposed that the lower atmosphere tugging and pushing the ground beneath it generates multiple echos within the planet of frequencies 0.01 Hz or less. Though this is far below the threshold for human ears, it’s technically loud enough that, if it were several octaves higher, would drown out everything else around us.

 

The other rocky planets in our solar system drone their own tune, each at different strengths that are set by their different atmospheric pressures. Mars, Earth, and Venus have fairly comparable internal densities, so their waves begin with similar amplitudes. But the thin atmosphere of Mars severely attenuates the sound strength by the time it reaches space, while Venus’ thick, choking blanket of carbon dioxide keeps the music alive with greater ease.

 

How much of a role do planetary atmospheres play in shaping these sounds? Several years ago, researchers at Penn State were morbidly intrigued by the Alien tagline, and decided to investigate how far a scream could theoretically be heard on Mars. They found that, while an average shriek could be heard for ¾ mile on our world, it would only go 53 feet in the atmosphere of Mars, which is 1% the density of ours. Its carbon dioxide air would noticeably lower the pitches produced by human vocal chords and geology alike.

 

More recently, scientists were able to hear actual sounds from Mars. Last May, the European Mars Express Satellite relayed the NASA Phoenix lander’s audio recordings it made during its descent to the planet’s surface. The probe is now settled comfortably in the artic wastelands of the Martian North Pole, hunting for frozen subsurface water. In orbit around Saturn, the Cassini spacecraft performed a similar task for Huygens in 2005, which recorded its own descent onto the moon Titan.

 

Our next and most prominent noisy neighbor is the sun, always a tumultuous hotbed of magnetically intricate flares, sunspots, and prominences. In October 2003, David Gurnett at the University of Iowa decided to take a particularly violent season of solar activity and scale the frequencies to the threshold of human hearing. Compressing hours of observations into a 15 second solar sound byte, the sun sounds more like a low-flying jet and hissing cockroach than a star.

 

But these are genuine sound waves. Electrons ejected in solar flares or the more massive coronal mass ejections move at a third of the speed of light, catching up with the surrounding solar “wind” of slower particles. Collisions between the two produce oscillations in the solar wind, observationally manifested as radio waves. These waves then decrease in frequency as the density of their medium decreases further away from the sun.  

 

 

Moving out to interstellar distances, astronomers also deduce sound waves in the gaseous nebulae that pepper our Milky Way Galaxy. The oft-imaged Orion, Horsehead, and Lagoonnebulae are particularly photogenic examples of these nurseries of star formation, dense agglomerations of interstellar gas, dust, and often shockingly complex organic molecules. Overall, these enormous, 10¹⁶ km- sized clouds maintain their shape through a delicate balancing act between the force of gravity, which wants to push everything together, and thermal pressure, which threatens to blow it all apart.

 

The particles in the nebulae, while at a chilly 10 degrees or so above absolute zero, move randomly and at varying speeds, creating momentarily denser regions where more particles have clustered together, and less dense regions where they have been evacuated. If these differences in particle density, or perturbations, are sufficiently small, they translate to transient disturbances in the nebula’s ambient pressure. This is the definition of sound. In this case it’s another messy static, with frequencies even lower than in moaning planets.

 

But if these perturbations are lucky, they may be large enough for gravity to win the war against thermal pressure, and trigger a local gravitational collapse that accelerates until a new star is born. These are the “failed” sound waves of which Astronomers are so fond. Then the freshly minted starstuff has the option of reverberating in all those exotic solar modes, not just listlessly wailing in the interstellar medium.

 

Other stars with masses 10 to 25 times that of the sun have a few more musical options. According to Adam Burrows at the University of Arizona, sounds waves generated in the death throes of the massive stars may provide the energy for supernovae, stellar explosions that are known to outshine entire galaxies at a time.

 

When a dying star has exhausted its nuclear fuel and fused its contents into iron, its interior is bereft of thermal pressure to support it against gravity, which demands its immediate collapse. New simulations predict that, within a second or two after this begins, the inner core of the star vibrates up to 200-400 Hz. Any observers not crushed by the gravitational stresses of a deflating star would be treated to a ubiquitous, ringing middle C. That is, until the sound waves propagate up to the outer layers of the star, heating them enough to blow them apart in a fiery supernova detonation.

 

But even then the symphony continues. The more massive stellar explosions may leave behind compact, spacetime-bending black holes, which will incite overtures in any interstellar “food” they’re offered.

 

 

The orbiting X-ray satellite Chandra in 2003 observed concentric shells of higher-pressure gas in the Perseus galaxy cluster, 250 million light years away from a suspected black hole. As intergalactic gas and dust spirals to its doom into the black hole, it is accelerated and compressed, causing it to radiate its energy outward in both energetic X-rays and physical compression waves, i.e. sound. Andrew Fabian and Steve Allen of the Institute of Astronomy in Cambridge found the shells produced by the latter phenomenon correspond to a B flat, but one so deep that it is 57 octaves below what we can hear.

 

This is no feeble tone, however. This black hole has been singing for an estimated 2.5 billion years, unleashing a total energy of 100 million exploding suns in the process. Astronomers theorize that this may be the heating mechanism responsible for keeping the tenuous gas they observe between galaxies so anomalously hot. Along with playful gravitational interactions between galaxies, this blistering gas provides the impetus for blue, star forming galaxies like our own to turn into quiescent red elliptical galaxies over time. As galaxies run across this intergalactic medium, their own star forming fuel is stripped away, leaving them to fade and redden as fewer new blue stars are born within them.

 

Astronomers can also synthesize music from the murmurings of the largest scales of the universe around us, stemming from the tiniest quantum-sized fluctuations billions of years in the past.

 

In practice, Astronomers can only see back to 380,000 years after the Big Bang, some 13.7 billion years ago. At this time of “recombination,” the expanding universe cooled enough for atoms to form from protons and neutrons. The light from this event has since expanded with the universe and cooled to a lukewarm 3 degrees above absolute zero. But by making some assumptions about the geometry and content of the Universe before this time, when the cosmos was too opaque to see anything, Mark Whittle at the University of Virginia made a sound clip chronicling our spacetime fabric’s first million years.

 

Compressing this time into a 5 second recording, the big bang sounds nothing at all like the awesome explosion many expect. Its stunning silence is a testament to its initial radial expansion, when no pressure waves yet congealed in the matter-energy soup. As quantum fluctuations begin to froth in this plasma of primordial matter, the cosmos adopt a major chord, switching to a minor third as time goes on.

 

Mathematically, this much like what occurs in the interstellar medium. The main difference here is that bigger and bigger disturbances are allowed as the scale of the universe increases. These pressure waves oscillate until atoms form, at which point the prevailing sound speed of the material in the Universe plummets. These waves effectively “stall out” and freeze at a distance scale of 150 million parsecs, or 4.6 x 10²¹ km.

 

Scientists can correlate these veritable wavesicles to the 3 degree Cosmic Microwave background, or CMB measured by projects like WMAP or BOOMERANG. The bubbly patches in CMB maps arise from temperature (or density) fluctuations at recombination, whose characteristic sizes tell us about the universe’s geometry and composition. Along with deep imaging surveys like Sloan and the 2 Degree Sky Survey, these “anisotropies” have hinted that the Universe is geometrically flat like some kind of three dimensional pancake, it has an unseen Dark Matter component that outweighs visible matter by ten to one, and an enigmatic, repulsive Dark Energy that outweighs both kinds of matter combined.