Only over the past few years, gravitational waves have been directly observed. It took merging black holes and neutron stars to make that possible. However, a direct detection of the inflationary gravitational wave background, for example by means of an extremely long baseline gravitational wave detector or an array of pulsars, is still far to come. In the meantime, polarisation of the Cosmic Microwave Background (CMB) offers an alternate, and promising, indirect observational window.
The CMB is a faint glow that pervades the entire sky. Dating back to 380 000 years after the Big Bang, it is in its own right the most ancient light in the cosmos. Before then, the universe was too hot and dense for photons to travel without scattering off matter, and in particular electrons. It was only when the universe cooled down to the point that electrons could bind with protons and neutrons, thus forming neutral atoms, that the light was released. From that moment on the CMB has been able to travel (almost) freely across the universe till hitting our detectors.
Early observations have pointed out that the CMB radiation is greatly isotropic, while subsequent more accurate measurements revealed tiny variations in its intensity, of the order of one part over 100000, from a direction in the microwave sky to another. They are conventionally termed anisotropies and are interpreted as the fingerprint of small fluctuations in the energy density of the early universe, from which, by gravitational amplification, the wealth of structures we observe in the sky originated.
The CMB radiation is expected to be polarized, meaning it is expected to show a preferred orientation perpendicularly to the direction of propagation (see polarisation box). This is a consequence of the interaction between photons and free electrons, known as Thomson scattering.
In the scattering process the electromagnetic fields transverse to the outgoing direction pass through unimpeded, while those parallel are stopped. Thus, if the incident radiation varies with the incoming direction on angular separations of 90 degrees, meaning that the incoming field has a so-called quadrupolar anisotropy, then the emerging radiation is linearly polarized.
However, as long as photons are tightly coupled to matter, as in the primeval phases of our universe, any polarization created locally gets erased by subsequent scatterings. In fact, it turns out that a net polarization can only be imprinted on the CMB radiation field in a relatively short time during the radiation-matter decoupling, and therefore only a small fraction of the CMB, about 10%, is actually polarized. Nonetheless, there is also another epoch in the universe’s history when polarization can be generated, and that is cosmic reionization. Several hundred million years after the Big Bang the first luminous sources formed and started to emit ultraviolet radiation that reionized the neutral regions around them. This process frees electrons which can then interact with CMB photons. Polarization generated at the two different epochs will correspond to signals on different angular scales in the sky, and LiteBIRD has been designed to accurately map both angular scale regimes.
The reason why CMB polarization can provide the missing piece of evidence for inflation is that gravitational waves, which do have a handedness, can also imprint curl-like patterns in the polarization field, the B-modes (see figure). On the contrary, density fluctuations, which make the dominant contribution to the CMB polarization signal, can only source a particular type of polarization pattern, the so-called E-modes. The main goal of LiteBIRD will be to hunt for primordial B-modes.
If the CMB temperature anisotropy pattern has been measured and characterised with exquisite precision by three space missions (COBE, WMAP, Planck) and quite a number of sub-orbital experiments, and it has led the way in shaping the standard model of cosmology, the potential of its polarised counterpart is yet to be released.