Planck & BICEP2/Keck collaboration: an upper limit on the intensity of primordial gravitational waves
By analysing their data jointly, the Planck and BICEP2/Keck collaborations have shown that the detection of primordial gravitational waves through the observation of the polarisation of the Cosmic Microwave Background (CMB) has not occurred yet. This result constitutes the denouement of the scientific drama that kept enthralled cosmologists and universe enthusiasts alike since March 2014: the signal, the detection of which has been reported by the BICEP2 team, is not related to the first instants of the Big Bang. It results from the combination of the Galactic signal and of gravitational distortions of the CMB during its propagation down to us.
The CMB, remnant radiation of the Big Bang, is a snapshot of the Universe, today 13.8 billion years old, as it was 380 000. Since its discovery, exactly fifty years ago, cosmologists keep on observing it in ever increasing detail in order to understand the origin and the contents of the cosmos. In the last years, ESA’s Planck satellite has measured it with an unprecedented precision, and has already delivered its lot of exceptional discoveries, even if the data have not revealed all their secrets yet. The CMB is a genuine cosmic screen on which shadows of the primordial Universe are projected in the form of tiny fluctuations. Observing the CMB allows us to explore epochs of the distant past, notably that of the so-called inflation, a brisk phase of exponential expansion where the tiny fluctuations originated from. During inflation, gravitational waves were naturally created. Their space-time distortion effect has since then left an imprint in the polarisation of the CMB. The so-called B-modes of polarisation, characteristic of primordial gravitational waves, lie at the heart of the work done by the Planck and BICEP2 collaborations.
March 2014 - detection by BICEP2
In March 2014, after several years of observation, the BICEP2 team, having their telescope at the South Pole, published a widely publicised result announcing the detection of a B-mode polarised signal in a region of the sky believed to be only weakly contaminated by the radiation of our Galaxy. The signal has been initially interpreted as a very likely detection of primordial gravitational waves. Doubt was however quick to grow within the scientific community because the BICEP observations provided only very little constraints about a possible contamination of galactic origin. Could the B-modes of BICEP be, partially at least, due to galactic foregrounds? Only the Planck team was in position to address that question thanks to its observations of the whole sky at frequency bands higher than BICEP in which the galactic signal dominates.
September 2014 – Planck analyses the galactic contribution
A first answer has been provided in September 2014 by a Planck publication demonstrating that the amplitude of the polarised emission of galactic dust is at least as strong, on the entire sky, as the signal measured by BICEP. Thus, there are no windows on the sky totally clean for the search of primordial gravitational waves. Still, this statistical result left an uncertainty about the nature of the signal measured by BICEP. Was it possible to demonstrate that perhaps a fraction at least of the signal was indeed of cosmological origin?
January 2015 – The collaboration Planck & BICEP2/Keck bears its fruits
The Planck and BICEP2 groups then decided to team up to answer that question. At the heart of their collaboration: the Planck map of the Galactic signal and the map of the signal measured by BICEP, completed by new observations obtained since March with the Keck Array (also at the South Pole). The comparison of the three datasets has now allowed the collaboration to show that the galactic contribution to the signal is dominant on the angular scales where the signal due to primordial gravitational waves is expected.
Nevertheless, a B-mode in the CMB polarisation has indeed been detected... but it corresponds to the gravitational lensing effect due to the distribution of matter along the path of light from the CMB down to us. This signal differs from that of primordial gravitational waves in its distribution across angular scales. Note that it is not the first detection of this effect, informing us about the matter distribution in the Universe, but it is by far the most precise today.
Then, after eliminating the galactic signal and the contribution of the lensing effect, the result is that current data do not allow to detect unambiguously the traces left by primordial gravitational waves. It does not mean that the expected signal does not exist! It only means that it is too weak to be detected in these datasets. However, the Planck/BICEP2/Keck collaboration was able to put an upper limit on the intensity of primordial gravitational waves. This limit is consistent with that obtained indirectly by the Planck collaboration in 2013 based on the analysis of the CMB temperature fluctuations only. The 2013 Planck result pre-dates the first publication of BICEP2, but it was not considered in contradiction because its interpretation depended on the cosmological model used in the data analysis. The new, direct measurements show that the standard cosmological model used by Planck is indeed sufficient for the description of all current results.
The quest for the B-modes of the CMB is not over. It will continue by evermore precise observations, both ground-based and balloon-borne, thanks to the fast development of detector arrays. But the Planck sky survey, obtained in frequencies inaccessible from the ground, will long remain the unavoidable reference for the separation of the cosmological and the galactic signals.
More about the Planck satellite: http://sci.esa.int/planck/
Map of the galactic dust (colours) and of the galactic magnetic field (embossed) measured by Planck at 353 GHz. The observation field of BICEP2 is delineated by the white dotted line. There is a small, but non-negligible amount of dust (the region is in blue, but not at all uniformly dark blue), and clearly the polarised emission tracing the magnetic field lines cannot be ignored.
Credits : ESA - Planck collaboration / Image by M.-A. Miville-Deschênes, CNRS – Institut d'Astrophysique Spatiale, Université Paris-Sud, Orsay, France