# Question about first peak of CMB

1. Sep 14, 2015

### gluon

Hello,i would like to ask something about the first peak in power spectrum of cmb giving by WMAP experiment.The y axis is temprature difference between two points and the horizontal is the angle seperation.the first peak giving as the fundamental frequency of sound wave right?
if we suppose that we have a overdense region at the time of inflation and until recombination it gets underdense and the wave have propagated at known distance which is given by d=c*t ,c is the speed of sound wave 0,6c and t is the time at recombination 375.000,which giving a distance d=225.000 ly and we know that the distance from cmb surface is 41billion ly away so we get a angle which is 0,5 degree .But if i take that the wave went spherical away from overdense point we get a diameter of 716.000 ly which is giving 1 degree angle.

i want to ask which difference in temprature we measure? 1) from the initial overdence ,which became underdense,until the region which sound wave reach?or 2) from two positions that sound wave have reached at the time of recombintaion which is seperated by distance 716.000 ly ? if we take the 1) case i understand why the temprature difference is so big,but the angle is 0,5 degree not 1.In case 2) the degree is 1 but i cant see why we have temprature difference between this points.should it have zero temprature difference or not?

Sorry for my English!

2. Sep 14, 2015