Next-generation CMB satellites: Norwegian participation in Core and LiteBIRD

At 10.51 on September 14th 2015 the US-funded LIGO experiment made the world's first direct observation of gravitational waves produced by two colliding black holes, almost exactly 100 years after the effect was predicted by Albert Einstein. Only two years later, this work was awarded the Nobel Prize in Physics. In parallel with, and independently of, this groundbreaking discovery, cosmologists and CMB analysts worldwide are currently looking for the same type of gravitational waves, but created during the creation of the universe itself, rather than by colliding black holes. According to the current standard model of cosmology, these gravitational waves were created during a short but extremely violent period in the history of the universe called inflation, a period of ~10^-34 seconds during which the universe expanded by a mind-blowing factor of 10^26. During this expansion, space itself was twisted and warped, and in the process a stochastic background of gravitational waves was created. If this picture is correct, those gravitational waves should be visible in the form of polarization in the cosmic microwave background, and cosmologists all over the world is looking for this signal, both using ground-based, balloon-based and satellite experiments. However, the signal is not expected to be more than 10-100 nK in amplitude, if not lower, and this corresponds to variations of more than eight orders of magnitude lower than the average signal of 2.7K. Extreme precision is therefore required to find the signal. To achieve such precision, space measurements are uniquely powerful. During the project period we have contributed to planning and forecasting for several future satellite concepts, each with a goal of being the first to detect signs of primordial gravitational waves in the cosmic background radiation: Europe-led CORE and PRISTINE, India-led CMB-Bharat. Japan-led LiteBIRD and US-led PICO, plus the ground-based pre-project GreenPol at Greenland. Out of all of these, LiteBIRD is currently the front runner. The Japanese space organization JAXA has chosen LiteBIRD as its next strategic L-class mission. University of Oslo was among the first outside of Japan to join the LiteBIRD Joint Study Group in 2016; during the last five years this has grown to encompass more than 300 people distributed over large parts of the globe. The Oslo LiteBIRD group has accordingly grown to count 10 people, and we are strongly involved in planning of and forecasting for LiteBIRD. University of Oslo has also suggested to host one of LiteBIRD's official data centres. Regarding CORE, the official ESA application was declined in 2018, though this work is part of the background for ESA's Voyage 2050 roadmap, where studies of the cosmic microwave background are one of few prioritized areas.

Hovedmålene for dette prosjektet har vært å utvikle nye konsepter og metoder for neste-generasjons CMB-satellitter som kommer til å lete etter gravitasjonsbølger fra Big Bang, og å etablere Norge som en internasjonalt ledende aktør. Disse målene er oppfylt, vi har vært direkte involvert i utviklingen av ikke mindre enn fem uavhengige internasjonale satellittkonsepter: CMBBharat (India-leda), CORE (ESA-leda), LiteBIRD (Japan-leda), PICO (NASA-leda), og PRISTINE (ESA-leda). Av disse er tre fortsatt under aktiv utvikling (CMBBharat, LiteBIRD og PICO), og ett (LiteBIRD) har blitt valgt som et hovedprosjekt for oppskytning i 2028 av JAXA. I LiteBIRD har Norge per i dag den nest største gruppa i verden etter Japan, målt i antall PhD-studenter og postdoktorer, og spiller en sentral rolle. Norge er pga dette prosjektet ideelt posisjonert for å bidra til det internasjonale kappløpet for å kartlegge det tidligste universet.

Among the greatest challenges in modern cosmology and particle physics is the detection of primordial gravitational waves created during inflation, a brief period of exponential cosmic expansion taking place ~10^-34 seconds after the Big Bang. Detecting such gravitational waves would not only represent a seminal validation of inflationary cosmologies, but also provide a unique view of particle physics at the Planck energy scale. The most promising observational avenue is to measure CMB B-mode polarization on angular scales larger than ~1 degree. However, according to the latest results from BICEP2+Keck+Planck, the amplitude of such B-modes is low, no more than 7% of the amplitude of normal fluctuations. It may be significantly lower, such as 0.1 or 0.01%, or possibly not detectable at all. Because of this low amplitude, B-modes may easily be obscured by several contaminants. The most problematic is thermal dust and synchrotron emission from the Milky Way. Fortunately, while the frequency spectrum of the CMB follows a near-perfect blackbody spectrum, all known foregrounds follow non-thermal spectra. To distinguish between true CMB signal and foreground contamination, one can therefore measure the microwave sky in many different frequencies, and perform a joint fit of both CMB and foreground components. This process is known as 'CMB component separation', a field in which Norwegian cosmologists have played a world-leading role during the last decade through their involvement in Planck. As a result of this success, Drs. Wehus and Eriksen at the University of Oslo have recently been invited to lead the astrophysical component separation efforts of Core+ and LiteBIRD, two fourth-generation CMB satellite mission concepts led by ESA and JAXA; the latter is about to initiate its Phase A study in 2017. The current application seeks to secure funding for this work, and thereby secure a strong Norwegian involvement in fourth-generation CMB satellite missions.