In recent years astronomers have come to realize that the Universe is somewhere between 12 and 18 billion years old. They arrive at this estimate by measuring how fast the Universe is expanding due to the Big Bang and whether the expansion is accelerating or decelerating. By tracing the cosmos back in time to an era when the entire Universe was contained in a single point, they can estimate the time elapsed since the Big Bang. Unfortunately for cosmologists, who would like to know exactly when the Big Bang happened, it's difficult to measure precisely how fast the Universe is expanding and how the rate of expansion has changed since the Big Bang.
Traditional methods lead to rather large uncertainties in the final answer.
One of the astonishing things about the CMBR is its smoothness. In 1980, the Russian physicists Sunyaev and Zeldovich suggested that tiny fluctuations in the CMBR might be found if astronomers looked in the direction of giant clusters of galaxies. Mostly clusters have atmospheres of very hot gas that we can see because of the X-rays they emit. Sunyaev and Zeldovich realized that something interesting happens when a CMBR photon passes through such a cluster. There is a good chance that it will collide with one of the electrons in the hot atmosphere.