Galactic survey aids search for cosmic-inflation signatures

In March 2014, scientists with the BICEP2 collaboration announced that their telescope near the South Pole had detected the signature of gravitational waves that were generated during the inflationary epoch, the theorized rapid expansion of the newborn universe. The evidence came in the form of twisting, so-called B-mode polarization in the cosmic microwave background. But in the months that followed, the team’s claim unraveled as it became clear that the interpretation of the data had not properly accounted for the polarization imparted on the radiation by foreground sources, particularly galactic dust (see Physics Today, December 2014, page 8 ). Despite the existence of microwave polarization sky maps from Planck and the Wilkinson Microwave Anisotropy Probe (WMAP), the BICEP2 saga demonstrated the need for additional measurements to make robust cosmological conclusions.

Now help has arrived. Using five years of observations from two telescopes at the Teide Observatory in the Canary Islands, researchers with the QUIJOTE collaboration have released a series of microwave-intensity and polarization maps that characterize galactic sources of polarization. The maps of the northern sky reveal the polarization at 10–20 GHz, a frequency range that falls below the ones scanned by Planck and WMAP. The QUIJOTE frequencies are particularly useful for determining the polarization of synchrotron radiation that is generated by relativistic electrons getting pulled in curved paths by the Milky Way’s magnetic fields. Along with galactic dust emissions, synchrotron radiation is a primary contributor to local microwave polarization. The QUIJOTE researchers found that synchrotron emissions provide a relatively small contribution to B modes but also that models of those emissions need to be improved.

The maps should complement the ones from Planck and WMAP to help constrain measurements of primordial B modes. Even when future projects, including the Simons Observatory and CMB-S4, attain ever more precise B-mode measurements, they will still rely on maps such as QUIJOTE’s to remove foreground contributors.

The QUIJOTE data will also be used for purposes beyond cosmology. In a series of papers, the researchers analyzed the polarization contributions of specific sources of microwaves, including the Milky Way’s center . Polarization measurements could help evaluate dark-matter models that predict a microwave signal from the galactic center, where dark-matter density should be high. (J. A. Rubiño-Martín et al., Mon. Not. R. Astron. Soc. 519, 3383, 2023 .)