Pedagoguery

Imagine a gigantic orchestra playing for 14 billion years. At first, all sounds harmonious, but as you listen closely, you notice that the tuba and bass sections are playing a different song very softly. In a way, it appears that this picture describes our universe. When detailed looks are taken at the microwave background radiation, it appears that the lowest frequency modes are much smaller than predicted by the otherwise highly successful inflationary theory.

You have probably seen at some point the standard map of the microwave background radiation: an oval depicting the whole sky, with red and blue speckles on it representing areas that are warmer or cooler than average. That speckling can be represented by what are called spherical harmonics. Harmonics are represented by a series of integers, starting with 0. The 0 harmonic, or l = 0 is what is called the monopole: the whole sphere pulsing in and out. In the context of the microwave background radiation, it represents the average temperature: 2.725 Kelvins. The l = 1 value is what is referred to as a dipole: one hemisphere getting warmer while the other gets cooler. In this context, it represents the motion of the Earth relative to the CBR (Cosmic Background Radiation). When l = 2, it is called the quadrupole, l = 3 is the octopole, and so on. As l gets progressively larger, the areas that pulse get progressively smaller. Any pattern on a sphere can be reduced to a series of these harmonics added together. On the Earth, for example, the smaller values of l would represent oceans and continents, and as the values of l increased, we would move down to islands, mountain chains, lakes, hills, and so on to progressively smaller and smaller terrain features.

Similar mappings have been done with the CBR. In particular, a probe called the Wilkinson Microwave Anisotropy Probe (WMAP) has taken the most accurate and detailed measurements of the temperature fluctuations of the CBR. These have been compared to the predictions of a particular inflationary theory called the lambda cold dark matter theory. Overall, just like prior probes, the observations are in excellent agreement with the theory. The problem lies with the two lowest modes – the quadrupole and octopole – which are anomalously low in power; well outside theoretical predictions.

There are other strangenesses with these two harmonics. When looking at the correlation of temperatures in separated areas of the sky, theory predicts that there should be high correlation at low separations (a few degrees), rapidly dropping to zero at about 45 degrees. Between 45 and 125 degrees there should be a slight negative correlation (meaning that areas separated by that amount should tend to have opposite from each other). Above 125 degrees, there should once again be a slight positive correlation. The observed data follows the predictions until we get to separations of about 60 degrees. Above that , there is essentially zero correlation.

Another oddity is that the quadrupole and ocotopole are aligned with local features. They appear to be closely aligned with the ecliptic, which is the plane of our solar system, as well as the dipole, the motion of our solar system relative to the CBR. Two of the points are also on something called the supergalactic plane, which holds the Milky Way and most of its neighboring galaxies. This alignment is so close that it is highly unlikely to have happened by chance. Does this therefore solve our problem? Is there something in our local neighborhood that masks the harmonics? If anything, it makes our problem bigger, because after subtracting out any local influence, it gives us an even bigger discrepancy between theory and observation. Any interference that canceled out to give us what we observe would be highly unlikely. The other alternative is that the theory is wrong. Indeed, an inflationary theory can be contrived to fit the data, but such a theory would be just that: a contrivance, similar to Ptolemy adding epicycles to his geocentric cosmology to account for the observed retrograde motion of the planets against the stars.

One possible explanation is that the universe has an unusual topology. For example if the universe were finite, and wrapped into a structure resembling a torus or a pretzel, it could produce what we observe. Unfortunately, if the structure were large enough that it was beyond our horizon, it would be very hard for us to verify that it was the case. In the end, the solution will only come from more data. WMAP is continuing to make observations, and it will be joined next year by the European Space Agency's Planck satellite. Between the two of them, they should be able to better ascertain any foreground features that may be masking the background.

Next issue: Why do so many galaxies show spiral structure?