Fluctuations in the CMB (2)

Fluctuations In the CMB (2) ...

A Lagrange Point with a View

The Wilkensen Microwave Anisotropy Probe (WMAP) is located at the L 2 Lagrange point for the Earth and Sun (recall our discussion of Lagrange points for a planet and the Sun in Chapter 16 when considering the Trojan asteroids). The L 2 Lagrange point is on the Earth-Sun line about 1.5 million kilometers further from the Sun than the Earth. This location has advantages over that of COBE, which is in relatively low Earth orbit. The distance from Earth helps both to cut down on microwave noise and to keep the Earth from blocking part of the sky. Therefore, WMAP can make more precise measurements and sees the full sky more continuously than COBE could.

WMAP High-Precision Microwave Anisotropies

The pioneering COBE results on microwave anisotropies have been extended to much higher precision in subsequent satellite, ground-based, and balloon measurements. The most comprehensive results are from the Wilkensen Microwave Anisotropy Probe (WMAP), which is much more sensitive than COBE and has uninterrupted full-sky coverage at five different microwave frequencies (see the adjacent box; the coverage at five different frequencies helps WMAP distinguish between microwave emission from the galaxy and the CMB).

WMAP has an angular resolution of about 12 arc minutes, which is much better than COBE. The figure below compares the full-sky map of microwave anisotropies measured by COBE with newer results from WMAP. As before, these maps represent tiny temperature variations in the CMB in different directions in the sky, plotted in galactic coordinates. Red and yellow regions are hotter, blue regions are cooler, and green regions are in between. With its higher angular resolution, WMAP can see much finer structure than COBE could. Detailed comparisons indicate, on the other hand, that the structures seen in the lower-resolution COBE results are completely consistent with the more detailed picture from WMAP.

You may verify this for yourself by examining some features in the two maps. For example, compare the large green region near the right boundary of the COBE map with the corresponding one for WMAP. The regions have the same shape and size, but WMAP fills it in with much more detailed variation.

Acoustic Peaks

The above CMB maps are pretty, but what do they mean? To answer that, we must consider in more detail what causes these fluctuations. As we have already noted, they represent anisotropies in the microwave background at the time of decoupling between radiation and matter (which occurred, according to the WMAP data, about 380,000 years after the big bang; see below). The hot and cold spots in the WMAP and COBE temperature maps from above correspond to regions that had slightly higher or slightly lower mass density than the average at the time of decoupling. These variations in density were produced by sound or acoustic waves moving through the plasma before the time of decoupling. As for normal sound waves, these were waves of compression and decompression that caused small variations in the density and motion of particular regions of the plasma. The motions associated with these acoustic waves, and the density variations associated with them, gave rise to slightly different red or blue shifts for the radiation from different regions (with the shifts arising from both Doppler effects and gravitational redshifts). It is these slightly different redshifts that acount for the tiny variations in temperature displayed above.

The general plasma physics that describes these acoustic waves is thought to be well understood. But the details of the acoustic waves that formed the temperature anisotropies in the CMB depend on basic cosmological parameters. This means that the temperature anisotropies carry information about these fundamental parameters and that by comparing a theoretical description of the acoustic oscillations assuming different values of these parameters with the data one can determine the values of the unknown cosmological parameters. The following figure illustrates schematically for the parameters determining the curvature of spacetime.

As illustrated in this figure, the observed anisotropies in the CMB depend on the curvature of spacetime. The curvature acts as a kind of lens distorting the pattern of anisotropies in different ways if the curvature is negative than if it is positive, and not distorting it at all if the Universe is flat. (The Technically Speaking box at the bottom of this page contains a more extensive discussion of how curvature is related to the CMB anisotropies.)