Foregrounds and removal

LiteBIRD chases the signature of primordial gravitational waves in the cosmic microwave background (CMB). The CMB is the most distant light we can observe. From one hand, this means that it carries invaluable information about how the universe began. On the other, it can be obscured by any light emitted in the intervening universe.  The main foregrounds for LiteBIRD are diffuse emissions from our galaxy.

The space between its stars is not empty and contains the so-called interstellar dust — material processed and ejected by previous generations of stars.  Dust absorbs starlight, thus increasing its temperature and therefore re-emitting in the far-infrared, with some contribution in the microwaves. Moreover, this emission can be polarized due to the alignment of the dust grains with the galactic magnetic fields.

The interstellar medium is also pervaded by high energy cosmic ray electrons. As they spiral around magnetic field lines, they produce synchrotron radiation, which is also polarized. The synchrotron radiation peaks in the radio frequencies, but has significant contribution in the microwaves.

The signal targeted by LiteBIRD is so faint that it is covered by foreground emissions in any region of the sky and any observational frequency. However, their spectrum is different from the one of the CMB and, therefore, we can use multi-frequency observations to disentangle the contribution of our target signal from that of the foregrounds. This process — called components separation — will be one of the critical steps in the data analysis. The LiteBIRD number and range of frequencies were designed to make this cleaning process as effective and robust as possible, with a key role played by the extreme channels. In particular, LiteBIRD highest frequencies represent an invaluable piece of information, as they are not accessible from the ground due to the opaqueness of the atmosphere.