The Cosmic Microwave Background (CMB), the fossil radiation from the hot big bang, has been used to study the universe for more than five decades. Most recently, experiments have been focusing on a hypothesized signature in the polarization of the CMB, so-called B-mode (swirly) polarization. Generated by primordial gravitational waves, this observable would offer a rare glimpse into the details of cosmic inflation (or alternative early universe models) and energy scales in the infant universe.

Dr. Jón E. Gudmundsson, a senior researcher at the Oskar Klein Centre and Stockholm University, is leading the design efforts for the mid- and high frequency telescopes of one of these new experiments: a satellite mission called LiteBIRD. The LiteBIRD satellite mission is designed specifically to search for ultra-faint polarized signatures in the cosmic microwave background.

Logo for the LiteBIRD telescope, a yellow hummingbird in front of a blue satellite dish

In addition to searching for primordial gravitational waves, the LiteBIRD satellite will also shed light on various other astrophysical phenomena. This includes probing cosmic reionization by mapping polarization on the largest observable scales. Data from ESA’s Planck satellite do not constrain this observable particularly well. Observations by LiteBIRD will help nail down this feature in the polarized microwave sky and drastically reduce the parameter space of viable cosmological models.

The Japanese Space Agency, JAXA, selected LiteBIRD as the 2nd strategic L-class mission in May 2019 and in 2020 it was selected as one of the flagship projects of Japan's science funding agency, MEXT. At the European level, among the 9 countries involved in the European LiteBIRD collaboration, the French Space Agency (CNES) entered into a LiteBIRD-leading phase-A (with responsibility of the mid- and high-frequency telescopes) in 2020, while the Italian Space Agency (ASI) committed to LiteBIRD phase-A in 2018. Concurrently, preliminary discussions are being held with the European Space Agency (ESA) to build an ESA Mission of Opportunity for LiteBIRD.

The goal is to develop optics that support the deployment of over 3,000 cryogenically cooled bolometric receivers, the most sensitive mm-wavelength receiver ever constructed. The mid- and high-frequency telescopes, which are under European leadership, use two concentric lenses, cooled to 5-Kelvin, to focus radiation from the sky down to so-called bolometric detectors. These detectors measure variations in the sky temperature by coupling the incoming radiation to a thermally isolated dielectric chip that is cooled to 100 mK! 

A rendering of what the LiteBIRD satellite might look like
Rendering of the current LiteBIRD design with the low frequency instrument (left) and the mid- and high-frequency instrument (right). The three telescopes are surrounded by a series of concentric radiation shields that block the telescopes from unwanted light.


The telescopes will also employ a metamaterial polarization modulator mounted to a superconducting bearing system. The plate will rotate continuously in order to modulate the incoming signal and separate it from spurious signals that are generated internal to the telescope — a part of the systematic mitigation strategy.

Finally, absorbers are expected to line the cryogenic optical cavities of these telescopes and play a crucial role in maximizing the sensitivity of the satellite while reducing optical systematics. As part of this effort, Gudmundsson recently received funding from the Swedish Space Agency to develop the broadband absorber technologies that will help improve the experimental sensitivity and minimize straylight radiation in the optics. Recruitment for a postdoctoral position to lead this project is ongoing.

One of the biggest challenges to the LiteBIRD mission is to understand complications from Galactic foregrounds; microscopic dust particles in our own Galaxy align themselves with large scale magnetic fields and emit polarised thermal radiation at mm-wavelengths. To combat this, the LiteBIRD satellite is designed to deploy 15 frequency bands from 30-450 GHz which roughly corresponds to 0.7–10 mm wavelength radiation. This extensive frequency coverage together with a significant improvement in detector sensitivity compared to past missions, will allow the experiment to distinguish Galactic foregrounds from any primordial signal.