A Clearer View of a Dusty Sky

The theory of inflation hypothesizes that the Universe underwent a period of rapid expansion less than 10−32 seconds after its birth [1]. If inflation occurred, physicists believe it would have produced large-scale gravitational waves, imprinting a slight swirl-like polarization—known as B modes—on the cosmic microwave background (CMB), the light generated when the Universe was 380,000 years old [2, 3]. In March of 2014, scientists observing the CMB with the BICEP2 microwave telescope announced they had detected this faint polarization signal, setting off shockwaves in the scientific community and capturing the public’s imagination (see Ref. [4] and 19 June 2014 Viewpoint). Now, a new joint analysis [5] by the BICEP2 team and the collaboration analyzing CMB data from the European Space Agency’s Planck satellite confirms what a number of scientists had suspected: the BICEP2 signal was contaminated by interstellar dust, which can emit polarized radiation at microwave frequencies. Based on their analysis, the teams have concluded there is no significant evidence for an inflationary signal. The result is certainly a major setback for the BICEP2 team. But for current and future experiments, the analysis serves as a guide for performing sensitive measurements, leaving open the possibility that an inflation signal will ultimately be detected.