Spatial or Angular Peaks?
Acoustic oscillations cause a spatial
variation in the CMB temperature that oscillates
An observer right around recombination will see an
essenentially isotropic CMB
(same temperature in all directions)
The picture of acoustic
oscillations in gravitational potential wells
tells us that before recombination the distribution of the temperature
of the CMB varies from place to place.
Furthermore there is a characteristic length scale for this variation that
corresponds to the distance sound can travel. Why is it then that
when we observe the CMB today we see an angular
variation in the temperature on the sky?
Consider the state of the CMB right around recombination.
Since the distance sound can travel is huge - on order 100 Megaparsecs
(comoving) or 100,000 light years (physical), the temperature around us,
the observer in Chicago, is uniform in all
In this picture we are surrounded by red in every
direction. It isn't until we get to tens to a hundred comoving Mpc
out that the temperature varies much at all.
So why is it that we
see an angular variation in the temperature