Brain Pickings

The Birth of Sound: Why the Big Bang Was Actually Silent

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The science of why the wail of the baby Universe sounded like muffled highway traffic.

Questions of what sound is, why its digitization is a dangerous thing, and how it bleeds into other senses have long fascinated thinkers and listeners alike. In Discord: The Story of Noise (public library) from Oxford University Press, sound scholar Mike Goldsmith, former Head of the Acoustic Group at the UK National Physical Laboratory, explores the flipside of the cultural evolution of silence by tracing our relationship with noise, the history of sound, and where our auditory future might take us.

Among the many fascinating and counterintuitive facets of noise Goldsmith examines is the very dawn of sound, in a chapter titled A Silent Bang:

Despite a promising name, the Big Bang was silent — a sudden burst of energy in which time and space began, forming the Universe as it spread. With no space to expand into, there could be no medium around it into which sound waves could possibly propagate. But, in cosmic terms, the Universe was not silent for long — 380,000 years later (a mere 0.0003 per cent of its present age), it was filled with sound. And, this was not the random roar of white noise that one might perhaps expect — it was a sound with a pitch: it had a characteristic wavelength.

It would not, however, have been an audible sound to any eared creatures, could they have existed so far back in time, before even the stars were born: a vast objet like the Universe makes a very low sound indeed — about one trillionth of a hertz.

The reason that there was such a vast deep tone in the infancy of space and time is closely connected to one of the most mysterious and important aspects of the Universe’s history: structure, of which sound is a signpost. If the Universe had remained as it began, a completely homogenous, smoothed-out volume of energy, then galaxies, stars, and people could not exist today. But, for reasons that are still unclear, there was a clumpiness in the early Universe — some areas were a little denser than others, and it was these denser areas that would eventually become stars and galaxies. Density means gravity, and gravity attracted nearby matter (then in the form of plasma — a ‘gas’ of ions). he motion of that matter caused compression, heating the plasma, which in turn increased its output of radiation. The force of this radiation counteracted the gravitational force, and so the compression became an expansion — and it is this cycle of compression and expansion that formed the primordial sound waves.

But that early sound of was unlike what we typically think of as “sound,” not only because of its physical nature, but also because it fell on deaf — or, rather, non-existent — ears:

The wavelengths of the wail of the baby Universe — measured in hundreds of thousands of light years — were limited by the speed with which the pull of gravity travelled from one region to another, which is the speed of light. So, there was an ever-falling lowest possible pitch to the Universe, and consequently a gradually descending tone marked its growth.

The variation in pressure of the sound was around 1 per cent, or 11 dB, the kind of level that would be associated with motorway traffic a few metres away and over thirteen billion years later.

In the early Universe, as new generations of stars formed using the nuclear reaction products of the old, planets like ours formed with them — and sound waves surged and echoed through their structures and their atmospheres and, later, their hydrospheres too. But, as far as we know, for ten billion years there was nothing to hear them.

Though life on Earth began some four billion years ago in the ancient seas, it wasn’t until about 400 million years ago that the first amphibians crawled onto land, equipped with complex structures that could not only detect sound waves both underwater and in air, but could also estimate their strengths, pitches, and directions. So “sound” itself, or at least our conception and experience of it, is a property of evolution rather than of the physical environment.

The rest of Discord goes on to examine everything from the basic nature of sound to the war on noise pollution to scientific advances harnessing the power of sound in medicine.

Complement with Jad Abumrad’s fantastic talk on sound, science, and mystery.

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