How dark matter aff ects the cosmic microwave background

9.11 How dark matter aff ects the cosmic microwave background

The concentrations of dark matter that existed at the time the CMB originated have an observable effect due to the fact that if radiation has to ‘climb out’ of a gravi- tational potential well it will suffer a type of red shift called the ‘gravitational red

Cosmology – the Origin and Evolution of the Universe

clumped would have had longer wavelengths than those that left regions with less dark matter. This causes the effective black body temperature of photons coming from denser regions of dark matter to be less than those from sparser regions – thus giving rise to the temperature fl uctuations that are observed. As such observa- tions can directly tell us about the universe as it was just 380 000 or so years after its origin it is not surprising that they are so valuable to cosmologists!

This is no easy matter. The CMB needs to be observed at millimetre radio wave- lengths that are masked by emission from water vapour in the Earth’s atmosphere. Consequently, experiments have been fl own in satellites (COBE and WMAP), bal- loons (Boomerang and Maxima) or located at high dry sites on Earth such as the Atacama Desert in Chile at a height of 16 000 ft (5000 m) or on the fl anks of Mount Teide in Tenerife (the CBI and VSA experiments, respectively) (Figure 9.9). Another very good site, where the DASI experiment is located, is at the South Pole where it is so cold that the water vapour is largely frozen out of the atmosphere!

Observations of the CMB also enable us to measure the curvature of space. The photons that make up the CMB have travelled across space for billions of years and will thus have been affected by the curvature of space. We can calculate the spatial form of the fl uctuations at the time of their origin and it is possible to simulate the expected pattern of fl uctuations if space were negatively curved, positively curved or fl at (Figure 9.10) so that, if astronomers could map these fl uctuations

accurately, then it would be possible to measure the curvature of space. Consider this analogy. Imagine looking at a distant wall covered with a repeating pattern of wallpaper. Looking directly at the wall we could measure the observed angular spacing of the items that make up the pattern. This is analogous to observing through fl at space. Suppose now we observed through a

Introduction to Astronomy and Cosmology

Figure 9.10 Boomerang observed fl uctuations in the CMB that are consistent with space being ‘fl at’. –(Above) Boomerang map. (Below) What would be observed with positively curved, fl at and negatively curved space. Image: The international BOOMERANG consortium.

concave lens. The pattern would appear smaller and the angular separation less; analogous to observing through negatively curved space. If we then observed through a convex lens (as used in a magnifying glass) the angular separation would increase; analogous to observing through positively curved space. Thus, by comparing the observed fl uctuation pattern with that impressed on the CMB at its origin allows us to measure the curvature of space. The results of these observa- tions confi rm, without exception, that space is fl at to within 1–2%: Ω ⫽ 1.