Sounds of The Universe

Sound travels at 343 meters per second on Earth. We can all agree that the speed of light is "absolute" (too big of a value for us to consider "relative", and it changes very little in different mediums in this context now), however, the speed of light still depends on some medium (usually it is vacuum, close-to-perfect vacuum, water, etc). For sound, it is much more variable than the speed of light, as it is not a static, fixed speed.

For example, in hotter air, the speed of sound raises to about 355 meters per second, but cooler climates lower it to around 330 meters per second. As we ascend up in the sky, the speed becomes roughly 295 meters per second, becoming much slower. More than a hundred kilometers from the Earth's surface, "sound" cannot propagate effectively, thus, in space, no one can hear you scream.

Even though this is the case, electromagnetic waves still propagate through space. The photon particles of radio waves can be interpreted via computers as sounds, because of the movement of plasma, thus giving us the first sound of the universe, "Earth's Song".

Charged particles are propagated by the electromagnetic force. These particles are caught up in the magnetic field of Earth. As the Earth rotates, particles change, and ripples of disturbance around those particles' plasma, and this resembles sound waves in space. This plasma emits "sound" in radio frequencies, and if we shift that data to audible, human frequencies (with Python, for example) we can listen to these sounds.

These radio frequencies are "converted" to human audible frequencies, in the 20 Hz to 20 kHz sound segment. These are the so called whistler-mode waves. Whistler mode waves come in various forms throughout the magnetosphere. Electric discharge from lightning strikes leaks into space. (lightning waves)

Lightning strikes are the possible cause of Hiss waves, in the cold ionized gas, called the Plasmasphere around the Earth. Plasmapheric waves resemble radiostatic sounds, rather than whistling sounds.

Beyond the boundary of plasmasphere, we hear Dawn chorus waves, named because they mimick Earthsong, from high to low chirping sounds (chorus waves). This is caused by the acceleration of electrons.

Beyond our planet's sphere of influence, we can no longer rely on electomagnetic waves.

More recently, the Parker Probe made several solar approaches. Successfully managing the map solar frequencies closer. Shifting to audible frequencies, we hear dispersing chirp waves, whisler-mode solar waves, langmuir waves.

Scientists managed to process more than 20 years of SOHO (Solar Heliospheric Observatory) data was processed by the Hansen Experimental Physics Laboratory at Stanford University, USA. This data was processed in a way, so that the sound of the vibrating Sun was derived.

In 2012, Voyager I was pronounced the first probe to exit the Sun's heliosphere of emissions, thereby reaching interstellar space. On two occasions, for the first time at late 2012, and for the second time at mid-2013, Voyager I flew through - for the first time - interstellar plasma. Majority of it's decades old instruments were already dead by this point - the craft's capacity to send radio photons still persisted. These sounds were processed, and these radio energies are heard as if they are banshee-like screams.

Pulsars are radio signals from rotating neutron stars, which swing parallel funnels of emission around their circumferences, periodically pointing in the direction of the Earth. These signals are detectable to us as regularly timed pulses of electromagnetic waves. These waves are then converted to audible (human) frequencies.

The first pulsar that was discovered in 1967 (namely, PSR B1919+21), has a period of about 1.337 seconds, which sounds like a train.

Soon after, a new pulsar, namely the Vela Pulsar was discovered (PSR J0835-4510), which spins 11 times a second, and it sounds much like a drill.

Another of the first handful known pulsars belong in th Crab Nebula. This pulsar spins about 30 [Hz] times a second, thus giving us a higher frequency signal.

The fastest spinning pulsars are called the millisecond pulsars, which spin more than a 100 times every single second, rendering us beeps of high frequency.

The most powerful type of a neutron star is a magnetar, is responsible for a variety of exotic sources of e,ission, like Fast Radio Bursts. One example for this is the FRB 121102 magnetar. These are thought to be caused by rapid starquake eruptions from the shells of magnetars.

When two of the magnetars come together and collide with each other, they emit a sound of much different nature: gravitational waves. Gravitational waves can be heard from all across the universe. Any time two objects of considerable mass are pulled into robit, their acceleration and inward spiral catalyses gravity ripples, which stream offf omnidirectionally, thus disturb the space around them, much like how soundwaves disturb an atomic medium (some form of gases).

Though, the only echoes that we can actually hear are sounds emanating from the mergers of black holes and neutron stars. By using LIGOs (laser interferometers), even the most subtle gravitational waves can be seen wobbling the laser's light.

When a pair of neutron stars, black holes, or a combination of the two collide, just before the collision, they get sped up by the huge amounts of gravitational forces, to roughly half the speed of light. The second they collide, they emanate a huge burst of energy equivalent to several suns, thus, producing a relatively low-frequency signal that we call the gravitational waves, which can be "heard" from all across the universe. When these gravitational waves arrive to Earth, with the help of LIGO, scientists are able to map these low-frequency waves, and shift them twoards audible frequencies. Each of the mergers that have been catalogued, have their own unique "chirp" as a signature, which are used to reverse-engineer the properties of the progenitor objects that came before them.

These so mentioned chirps are a type of "sonification", which is an audible representation of an otherwise non-audio data. For gravity waves, scientists sonify time-domain oscillations. Whereas, in the Solar System, the sonification is of radio data. Since 2020s, NASA has been sonifying their stunning, image-captured data of stars/nebulas/galaxies too, called cosmic harmonies.

The main source of these electromagnetic data become the visually stunning images taken by NASA's space telescopes, such as the Hubble, Chandra, James Webb. The pixels of these images are then mapped to musical instruments, and cosmic harmonies are born.

All the sounds I've covered are beautiful and stunning. However, they are only a representation of how the universe may sound, through shifting electromagnetic waves to audible frequencies. The only real answer would be the sound of the birth of place, the CMB. The Cosmic Microwave Background is th ecold, omnipresent radiation that once was produced as the Big Bang's last light. When this phase of the universe ended, its photons were scattered, and freed to roam clear space forever more. These photons persist to this day, as the oldest, coldest light signal in the universe. But this signal is not of visible light - rather, a microwave-band radio static signal.

When we are switching between stations on an analogue TV, we are met with the good old grey, glitchy "ant war" backdrop, which is accompanied by a hissing sound. This backdrop is mostly caused by leaky Earth-bound radio transimissions, but deep down, of around 1% of it is the sound of the universe. This sound is the leftover noise from the Big Bang, from 13.7 billion years ago.