A new approach to going back to the Big Bang

Active observatories around the globe are targeting regions of the sky characterized by low galactic radiation contamination, looking for the imprint of cosmological gravitational waves (CGWs) produced during inflation, the mysterious phase of quasi- exponential from space to the very beginning of the universe. A new study of the POLARBEAR collaboration, conducted by SISSA for the part concerning the interpretation for Cosmology and published in the Astrophysical Journalprovides a new correction algorithm that allows researchers to nearly double the amount of reliable data acquired at such observatories, opening up uncharted territory in the signal produced by CGWs and bringing us closer to the Big Bang.

“According to current knowledge in cosmology, just after the Big Bang, the Universe was very small, dense and hot. In 10^-35 seconds it stretched by a factor of 10³⁰“, explains Carlo Baccigalupi, coordinator of the astrophysics and cosmology group at SISSA. “This process, known as inflation, produced cosmic gravitational waves (CGW) which can be detected by the polarization of the cosmic microwave background (CMB), the radiation remnants of the Big Bang. The POLARBEAR experiment, of which SISSA is a part, searches for such signals using the Huan Tran telescope in the Atacama Desert in northern Chile, in the Antofagasta region.”

The analysis of the data acquired by the POLARBEAR Observatory is a complex pipeline where the reliability of the measurements represents one of the most delicate and key factors. “CGWs only excite a tiny fraction of the CMB bias signal, better known as B-modes,” explain SISSA researcher Nicoletta Krachmalnicoff and Davide Poletti, previously at the same institute. “They are very difficult to measure, especially due to signal contamination from emissions from diffuse galactic gas. This must be eliminated with exquisite precision to isolate the unique contribution of CGWs.”

Over the past two years, Anto. I. Lonappan, Ph.D. student at SISSA, and Satoru Takakura of the University of Boulder, Colorado, characterized the quality of an extensive dataset from the POLARBEAR collaboration, tracing all uncertainties and instrumental systematics and known physics. “We have implemented an algorithm that assigns accuracy to measurements in the ‘Large Patch’, a region spanning approximately 670 square degrees in the southern celestial hemisphere, where our sounder reveals data in agreement with other probes looking in the same place, such as the BICEP2/Keck Array located at the South Pole”, they explain. The study has just been published in the Astrophysical Journal.

“This is an important step on a long road to observing CGWs. The new approach allows us to probe the sky with unprecedented precision, doubling the amount of reliable data and, therefore, accessible information. This is a crucial step for the entire community now that new telescopes are being prepared for operations,” the scientists add.

Major developments are underway from an experimental point of view. A system of three improved POLARBEAR telescopes, known as the Simons Array, is in the works. The Simons Observatory, a new system of small and large aperture telescopes, funded by the Simons Foundation, will be operational at nearby Atacama with first light in 2023. Later in this decade, the LiteBIRD satellite will fly, and a An extensive network of ground-based observatories, which are located in the Atacama Desert and the South Pole, known as “Stage IV”, will complement these observations.

“All of these efforts will lead to the ultimate measurement of CGWs, at the same time revealing the most important clues to the cosmological components of dark energy and matter,” concludes Baccigalupi. “Through SISSA’s main mission as a doctoral school, to train students to become young researchers, our Institute contributes and will contribute significantly to major contemporary challenges in physics, such as this one, targeting gravitational waves at from a tiny fraction of a second after the Big Bang.”