THE COSMIC SOUND

An auditive illustration of Planck data processing and WMAP map

francais Version francaise

Here are two ways of showing the Cosmic Microwave Background with sounds rather than images.
First, sounds can be used in the Planck data processing.
Second, the WMAP data can be "listened to".

1. Planck Timeline Sounds

The steps in the Planck data processing can be illustrated with the following auditive rendition. 
While waiting for the true data we hope to receive from the satellite in 2009, here is a realistic computer simulation.

The detector output as a function of time is named a TOI (time-ordered information). 

As the Planck spacecraft spins at a constant rate (1 revolution per minute = 1 rpm), the beam sweeps the sky and reads a pointing position over a 5 millisecond integration period.

We can transform the signal into a sound. The sky signal fundamental frequency is however inaudible (infrasound at 17 milliHz). Here we decide to scale it up by a given factor (20000) so that we can now
clearly hear the fundamental at 333 Hz. Two days worth of data are thus compressed into an eight second sound.

The signal that we obtain with the telemetry coming from the antenna of the satellite (1.5 million km away from Earth) sounds terrible,  like garbage

The raw signal after demodulating the telemetry sounds like that , in which at least we can hear the fundamental plus a lot of other things (sounds like an old scratching vinyl). In the Data Processing Center (DPC), the data are massaged so as to "hear" the first image emitted by the Universe 380000 years after the Big Bang. The clean sound should be like that. The processing involves removing the glitches (due to cosmic rays hitting the detector at random intervals) and the noise (by averaging).

The data are calibrated in temperature units using the dipole that is produced by our motion through the Universe.  This can be heard as a perfect sine sound at the fundamental frequency.

The emission from our Galaxy (the Milky Way) shows up as the red band in the figure above. This sound must be removed somehow to hear the Big Bang. On the other hand, it happens to be useful to locate regions where stars are born right now in our Galaxy.

2nd. Listening to WMAP map


The best map of the Cosmic Microwave Background (CMB) has so far been obtained with the WMAP mission.WMAP 5-years ILC
This whole sky map (projection of the celestial sphere) can be decomposed into modes of various angular scales. The first mode is the dipole and has been removed from this map







The second mode and others can be obtained via a spherical harmonic transform. Here are some examples (l=2, 5, & 10).
Wmap mode 2Wmap mode 5Wmap mode 10

This film shows the first modes from the WMAP map (flash version here)

Now comes the sound: let's make each mode oscillates with a frequency proportional to the mode number (l). Here we take 1Hz for l=40.
The oscillations look like this film (or  flash version here)

Now if we increase the scaling to 400 Hz for
l=40, then we can
"listen" to one particular location on the oscillating map: a so-called "Cosmic Drum"
In that sound, the location where we listen to, is changed every 3 seconds.

First Warning: although the Universe density did oscillate during its warm phase in an acoustic sense, the large-scale modes did not have time to span a single period before the "photo" (CMB) was taken (380 000 years after the Big Bang). So the real oscillations did take place but only for modes above the so-called first acoustic peak (l=220, here at 2200 Hz) before the Universe became transparent.

Second Warning: no sound can travel in the transparent Universe. Here we simply work by analogy with a drum oscillation and take our ear to the surface of the drum.

v1.0 October 2008
v1.1 Dec 2008 (French version added)
v2.0 Feb 2009: now includes the cosmic drum sounds that would reflect the so-called "acoustic peaks" in a loose sense.
       Healpix software and the Planck-HFI computers are acknowledged.
v2.1 To be done: Improve the films
v3.0 To be done: Show the different sounds produced by Universes with a different cosmology

Author: F.-Xavier Désert, LAOG


Original idea is from A. Riazuelo et al.
Image Credits: ESAand WMAP