THE COSMIC SOUND
An auditive illustration of Planck data
processing and WMAP map
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.
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).
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
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