Latest results of cosmological diffuse background measurements

The universe was created about 13.8 billion years ago in a burst of light: the big bang. About 380,000 years later, after matter (mostly hydrogen) cooled enough for neutral atoms to form, light was able to pass through space freely. This light, the cosmic diffuse background (CMB) radiation, reaches us from all directions in the sky uniformly… or at least that’s what it seemed at first glance. In recent decades, astronomers have found that the radiation exhibits faint ripples and bumps at a brightness level of only one part in a hundred thousand – the seeds of future structures, like galaxies.

Astronomers conjectured that these ripples also contain traces of an initial burst of expansion – so-called inflation – that inflated the new universe by thirty-three orders of magnitude in just ten to the power of minus-33 seconds. Inflation clues should be weakly present in the way the cosmic ripples are looped, an effect due to gravitational waves in cosmic childhood that should be perhaps a hundred times or weaker than the ripples themselves.

The curling effect produces patterns in the light known as “B-mode polarization”, and it should be extremely weak. Other exotic processes are at work in the universe to make this intimidating measure even more difficult. The main one is the faint glow of light from the dust particles of our galaxy which have been aligned by magnetic fields. This light is also polarized and can be distorted by magnetic fields to produce B-mode polarization patterns. Radio waves from our galaxy can produce similar effects. About six years ago, CfA astronomers working at the South Pole reported the first evidence of such curling, “B-mode polarization,” at levels compatible with simple inflation patterns, but subsequent measurements at different frequencies (or colors) of microwave light revealed the signal was explained by galactic dust.

In the years since these early B-mode polarization measurements, astronomers continued their meticulous observations, adding powerful data from new telescopes at many different frequencies operating at the South Pole. CfA astronomers D. Barkats, H. Boenish, J. Connors, J. Cornelison, M. Dierickx, M. Eiben, DC Goldfinger, P. Grimes, S. Harrison, KS Karkare, JM Kovac, B. Racine, S. Richter , BL Schmitt, T. St. Germaine, C. Verges, CL Wong, L. Zeng and a large team of colleagues have just completed an analysis of all data from the South Pole experiments BICEP2, Keck Array and BICEP3 until 2018 , and correlate the results with the results of the CMB Planck and WMAP space missions. (Although data collection for these missions ended in 2013 and 2010, respectively, data processing continues and scientists have used the 2018 version.) The new results improve on previous best stresses on curling by approximately a factor of two, and now provide powerful advice on the kinds of inflation models that might describe the first moments of the universe.

A large class of simple models is now largely excluded. The team reports that the more fortunate of the remaining class of models are predicting primordial gravitational waves at levels that should be detected (or excluded) over the next decade with improved telescopes at the South Pole. The team is already upgrading the BICEP system and hopes to gain another factor of around three improvements within five years, enough to place strict constraints on inflationary models.

The research was published in Physical examination letters.