Vol. 551
In section 3. Cosmology

A precise and accurate determination of the cosmic microwave background temperature at z = 0.89

Gravitational lenses have a variety of uses. One of the more amusing tests of modern cosmological theory has been the use of absorption lines in the lensing galaxy to measure the temperature of the microwave background at high red shift. An example of this is the article by Muller et al. highlighted in this issue where they measure the background temperature at z=0.89 against the continuum of a background quasar. Perhaps disappointingly, the measurement is consistent to within a couple of percent with expectations.