Despite earlier reports of a possible detection, a joint analysis of data from ESA’s Planck satellite and the ground-based BICEP2 and Keck Array experiments has found no conclusive evidence of primordial gravitational waves.
The Universe began about 13.8 billion years ago and evolved from an extremely hot, dense and uniform state to the rich and complex cosmos of galaxies, stars and planets we see today.
An extraordinary source 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.
ESA’s Planck satellite observed this background across the whole sky with unprecedented accuracy, and a broad variety of new findings about the early Universe has already been revealed over the past two years.
But astronomers are still digging ever deeper in the hope of exploring even further back in time: they are searching for a particular signature of cosmic ‘inflation’ — a very brief accelerated expansion that, according to current theory, the Universe experienced when it was only the tiniest fraction of a second old.
This signature would be seeded by gravitational waves, tiny perturbations in the fabric of space-time, that astronomers believe would have been generated during the inflationary phase. Interestingly, these perturbations should leave an imprint on another feature of the cosmic background: its polarization.
When light waves vibrate preferentially in a certain direction, we say the light is polarized.
The CMB is polarized, exhibiting a complex arrangement across the sky. This arises from the combination of two basic patterns: circular and radial (known as E-modes), and curly (B-modes).
Different phenomena in the Universe produce either E- or B-modes on different angular scales and identifying the various contributions requires extremely precise measurements. It is the B-modes that could hold the prize of probing the 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 polarization 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.
Planck is not alone in this search. In early 2014, another team of astronomers presented results based on observations of the polarized CMB on a small patch of the sky performed 2010–12 with BICEP2, an experiment located at the South Pole. The team also used preliminary data from another South Pole experiment, the Keck Array.
They found something new: curly B-modes in the polarization observed over stretches of the sky a few times larger than the size of the full Moon.
The BICEP2 team presented evidence favoring the interpretation that this signal originated in primordial gravitational waves, sparking an enormous response in the academic community and general public.
However, there is another contender in this game that can produce a similar effect: interstellar dust in our Galaxy, the Milky Way.
About this image of BICEP2
This image shows a patch of the southern sky and is based on observations performed by ESA’s Planck satellite at microwave and sub-millimetre wavelengths.
The color scale represents the emission from dust, a minor but crucial component of the interstellar medium that pervades our Milky Way galaxy. The texture, instead, indicates the orientation of the Galactic magnetic field. It is based on measurements of the direction of the polarized light emitted by the dust.
The highlighted region shows the position of a small patch of the sky that was observed with two ground-based experiments at the South Pole, BICEP2 and the Keck Array, and yielding a possible detection of curly B-modes in the polarisation of the Cosmic Microwave Background (CMB), the most ancient light in the history of the Universe.
However, a joint analysis of data from BICEP2, the Keck Array, and Planck has later shown that this signal is likely not cosmological in nature, but caused by dust in our Galaxy.
The image shows that dust emission is strongest along the plane of the Galaxy, in the upper part of the image, but that it cannot be neglected even in other regions of the sky. The small cloud visible in red, to the upper right of the BICEP2 field, shows dust emission from the Small Magellanic Cloud, a satellite galaxy of the Milky Way.