The cosmic microwave background ( CMB ) fluctuations effectively measure the basic properties of the universe during the recombination epoch .
CMB measurements fix the distance to the surface of last scatter , the sound horizon of the baryon-photon fluid and the fraction of the energy density in relativistic species .
We show that the microwave background observations can also very effectively constrain the thickness of the last scattering surface , which is directly related to the ratio of the small-scale E-mode polarization signal to the small-scale temperature signal .
The current cosmological data enables a 0.1 % measurement of the thickness of the surface of last scatter : 19 \pm 0.065 Mpc .
This constraint is relatively model-independent , so it can provide a new metric for systematic errors and an independent test of the \Lambda { CDM } model .
On the other hand , it is sensitive to models which affect the reionization history of the universe such as models with annihilating dark matter and varying fundamental constants ( e.g. , the fine-structure constant , \alpha _ { EM } , and electron rest mass , m _ { e } ) and as such can be used as a viable tool to constrain them .