The following extract was taken from an interview with theoretical cosmologist Roberto Trotta published in Collpase II :
C: Another line of evidence is indicated by your work on accoustic oscillations of the early universe which are 'frozen' into the fabric of matter - a sort of primordial 'sound fossil'.
RT: Yes these acoustic oscillations are, in a way, a natural fossil. The relevance of the sound waves of the early universe in general for cosmological parameters is that it's relatively simple to calculate, because the universe was fairly young, and these density fluctuations which eventually grew to galaxies were still very small - actually one part in a hundred thousand. So they were so small that we can calculate them with very high accuracy, and we can follow their evolution up to the point where the CMBR was released , very accurately. And so from this we can confidently infer several properties of the time, for instance how much dark matter there was, how much visible matter there was, what were the characteristics of the seeds, how the seeds were sprinkled with scale, whether there were more seeds on small scales, on large scales, or whether they were uniformly sprinkled on all scales and so on. These sorts of things can be inferred from sound waves in the CMBR, because we know the physics very well. And so it's a nice spot between the very high energy physics of the very beginning, which we don't fully understand, and the messy, non-linear physics of gravitational collapse and evolutionary structure that we do understand, but which gets difficult to follow because it gets very complicated, as you can see from the filamentary structures you obtain through the computer simulations we discussed.
C: How do you go about reading these 'recordings' of the Big Bang?
RT: We saw that the CMBR is very homogeneous because of its cosmological origin. But now we have very sensitive detectors, telescopes and satellites that measure the background radiation to a very high degree of accuracy. And if you look carefully enough, you will see that this CMBR is not perfectly homogeneous; it has temperature differences in it. So if you look with your telescope in this direction, we see a slightly colder spot, if we look in that direction we see a slightly hotter spot. We can build a map of the sky, showing the temperature distribution of the background radiation. In order to measure the differences between the hot and cold spots with your telescope you need a sensitivity that's equivalent to the sensitivity you'd need with an optical telescope to see a mouse walking on the moon from the earth. So it's very tough. The guys who first did it in 1992 got the Nobel prize in 2006. These fluctuations you see in this map are the sound waves from the early universe, that's exactly what they are. When you throw a pebble in a pond you've got waves that go out in all directions; if you throw many pebbles in a pond you get a nice superposition of waves. In our case the pebbles were quantum fluctuations in the early universe, and they got frozen in at the moment the image was produced, and this is what we see - we really image them with our telescopes.
C: Calling them 'sound waves' is not just a figurative way of speaking, then?
RT: No, it's a technical definition: they're compression waves. The universe at this point was a plasma, that's a hot gas of electrons and protons, seperated by the temperature because the temperature was so high. So those were really accoustic waves, just like the waves in the air now as I speak, only they were travelling in the primordial plasma. And we can see them, as we can see in this map: it's real, it's been predicted and we find this fantastic agreement with our models...
