European Space Agency (ESA) has confirmed based on joint analysis of data gathered by ground based BICEP2 and Keck Array experiments and its own Planck satellite that previous reports of BICEP2’s primordial gravitational waves detection are untrue and they haven’t found any conclusive evidence that they were detected.
“This joint work has shown that the detection of primordial B-modes is no longer robust once the emission from Galactic dust is removed,” says Jean-Loup Puget, principal investigator of the HFI instrument on Planck at the Institut d’Astrophysique Spatiale in Orsay, France. “So, unfortunately, we have not been able to confirm that the signal is an imprint of cosmic inflation.”
Primordial Gravitational Waves
Scientists have been searching for evidence about the Big Bang ever since the theory has been floated with a common understanding that the Universe, as we see today, is a result of an ‘inflation’ some 13.8 billion years ago from an extremely hot, dense and uniform state.
Astronomers are looking for information about the Big Bang and the inflation and one of the extraordinary sources of information about the Universe’s history is the Cosmic Microwave Background, or CMB, the legacy of light emitted only 380 000 years after the Big Bang.
Scientists are looking for signs of inflation, which would be seeded by gravitational waves – tiny perturbations in the fabric of space-time. Scientists believe that these gravitational waves would have been generated during the inflationary phase.
Cosmic Microwave Background & Polarisation
Scientists agree that the tiny perturbations should leave an imprint on another feature of the cosmic background – its polarization. The Cosmic Microwave Background is polarized and exhibits a complex arrangement across the sky arising out of a combination of two basic patterns: circular and radial (known as E-modes), and curly (B-modes). Scientists believe that the B-modes could hold the secrets of Universe’s early inflation.
“Searching for this unique record of the very early Universe is as difficult as it is exciting, since this subtle signal is hidden in the polarisation of the CMB, which itself only represents only a feeble few percent of the total light,” says Jan Tauber, ESA’s project scientist for Planck.
A US-led team has been searching for this B-modes through a ground based experiment dubbed BICEP2 at the South Pole. The team presented its results based on observations of the polarised CMB on a small patch of the sky performed during 2010–12 with BICEP2. They found something new: curly B-modes in the polarisation observed over stretches of the sky a few times larger than the size of the full Moon.
The team claimed that they have found evidence of primordial gravitational waves through their analysis of data gathered by BICEP2 and the Keck Array. However, interstellar dust was to play spoil sport in these claims as it also emits polarised light that affects CMB polarization as well.
Because of technological limitations at the time and collection of data on single microwave frequency, one key thing BICEP2 missed out was separation of emissions coming from the dust in Milky Way and the CMB. The BICEP2 team had chosen a field where they believed dust emission would be low, and thus interpreted the signal as likely to be cosmological.
Planck’s maps of polarised emission from Galactic dust
Planck was launch in 2009 to map the sky in nine frequencies using two state-of-the-art instruments: the Low Frequency Instrument, which includes three frequency bands in the range 30–70 GHz, and the High Frequency Instrument, which includes six frequency bands in the range 100–857 GHz.
Based on the data gathered by these two instruments, Planck revealed its analysis and maps of polarised emission from Galactic dust. Fort the first time it came to light that polarised emission from dust is significant over the entire sky, and comparable to the signal detected by BICEP2 even in the cleanest regions.
This led to collaboration between BICEP2 and Planck teams and a combined analysis of polarised emissions from Galactic dust and the analysis reveals that the detection of primordial B-modes is no longer robust and the available data doesn’t confirm signal of imprint of cosmic inflation.
The hunt is still on
“While we haven’t found strong evidence of a signal from primordial gravitational waves in the best observations of CMB polarisation that are currently available, this by no means rules out inflation,” says Reno Mandolesi, principal investigator of the LFI instrument on Planck at University of Ferrara, Italy.
The joint study sets an upper limit on the amount of gravitational waves from inflation, but the level is too low to be confirmed through present analysis.
“The new upper limit on the signal due to gravitational waves agrees well with the upper limit that we obtained earlier with Planck using the temperature fluctuations of the CMB,” says Brendan Crill, a leading member of both the Planck and BICEP2 teams from NASA’s Jet Propulsion Laboratory in the USA. “The gravitational wave signal could still be there, and the search is definitely on.”