Chapter 15: Primordial Sound

As we saw in the last chapter, in the 1970s, the Universe was believed to be homogeneous. The photons have been traveling since recombination; they have been affected by the roughness of the Universe on the scale of the cosmological horizon. Overall the level of inhomogeneity is very mild. If space-time were rough, it would be so warped that it would be populated by black holes. Rarely do we see black holes on small scales, which means that space-time cannot be too rough on small scales. When we look at galaxies on a scale, which correspond to sizes between thousands and millions of light years, it is clear that there is structure. This means that between the smoothness of large scales and the mild roughness of small scales, there is inhomogeneity on intermediate scales. Inhomogeneity has some randomness.

The existence of sound waves in the Universe was predicted in the 1970s and 1980s. The expedition of the COBE satellite revealed the discovery of inhomogeneities on large angular scales. A number of experiments were designed to detect sound waves from the primordial universe. To effectively measure sound, powerful telescopes were needed that could block out sound from contaminating sources. Contaminating sources, including the atmosphere, absorb and emit radiation. One way to get around the contamination from other sources, such as the atmosphere, is to send telescopes up in balloons to a height of about 40 kilometres. To accurately measure, it is essential to get high enough in the atmosphere to escape contaminating factors like variable weather conditions and other uncontrollable effects that might affect the interpretation of the relic sky.