What looked like gravitational waves was just interstellar dust

Planck view of BICEP2 field (Image ESA/Planck Collaboration. Acknowledgment: M.-A. Miville-Deschênes, CNRS – Institut d’Astrophysique Spatiale, Université Paris-XI, Orsay, France)
Planck view of BICEP2 field (Image ESA/Planck Collaboration. Acknowledgment: M.-A. Miville-Deschênes, CNRS – Institut d’Astrophysique Spatiale, Université Paris-XI, Orsay, France)

In March 2014, the announcement that the BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) experiment had detected gravitational waves in the perturbations in the cosmic microwave background radiation existing in the universe was sensational. This echo of cosmic inflation occurred shortly after the Big Bang was an extraordinary discovery. Unfortunately, a collaboration between the BICEP2 experiment and the team of ESA’s Planck space telescope has determined that those weren’t gravitational waves but probably emissions caused by galactic dust.

The search for gravitational waves was carried out in recent years examining the map of the Cosmic Microwave Background Radiation – CMB or CMBR – the residue of the earliest stages of the universe. ESA published the best map ever created using data collected by the Planck Space Telescope in March 2013.

The BICEP2 experiment is instead located in Antarctica and uses different types of instruments to collect data. After last year’s announcement, the analysis of the data collected started and initially they appeared to provide a degree of probability of being correct extremely high. However, very soon someone started noticing that the BICEP2 experiment’s instruments could have been deceived by the interstellar dust in this galaxy.

The problem is caused by the fact that this interstellar dust emits polarized light at frequencies similar to the cosmic microwave background radiation. BICEP2 has a limited ability to distinguish the various sources because it works on a single frequency, 150 GHz, in the microwave field.

The doubts raised led to a collaboration between the BICEP2 team, the Planck team and also Keck Array team, another experiment located in Antarctica which consists of five polarimeters that collected data on polarized light in the same area and at the same frequency of BICEP2.

The Planck space telescope observed the cosmos on nine frequency channels. This allowed it to make very sophisticated detections that separated the emissions of the cosmic microwave background radiation from those of the interstellar dust.

Putting together the data of the various experiments, it was found that the interstellar dust component in the area observed by BICEP2 was such as to add an amount of emissions greater than expected. John Kovac, one of the astrophysicists who directed the research presented last year, explained that the choice of the area to be observed was made using emission models of galactic dust available before it started.

Basically, the idea of the BICEP2 team was to investigate an area where interference were supposed to be minimal. Unfortunately, in a field where very sophisticated instruments are needed for in-depth analysis, only recent discoveries allowed to understand the actual contribution of interstellar dust to electromagnetic emissions.

The result is that what seemed an almost certain evidence was instead a measurement distorted by an instrumental limit. This shows once again the importance of the scientific method with the repetition of the experiments without taking anything for granted.

In astrophysics, having adequate instruments is crucial. In this case, a proof of the existence of gravitational waves was invalidated but that doesn’t mean they don’t exist. Other instruments will be needed such as the BICEP3 proposed to replace BICEP2 and perhaps a new space telescope even more sophisticated than Planck to have an answer.



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