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BICEP2 at the South Pole detects evidence for CMB polarisation: primordial gravitational waves?

BICEP2 at the South Pole detects evidence for CMB polarisation: primordial gravitational waves?

The BICEP2 collaboration has observed  what is probably the imprint of primordial gravitational waves on the Cosmic Microwave Background. It observed polarisation in the CMB  in the form of "B-modes?. The latter may be produced by tensor perturbations of the space-time metric due to the crossing of gravitational waves in the primordial plasma.


These perturbations induce a rotational component of the oscillation direction of electromagnetic waves, in the radiation field within the last-scattering surface of the CMB photons, 13,7 billion years ago, shortly after the big bang.


BICEP2 is a microwave telescope located at Amundsen-Scott station, at the South Pole. Its aim is to observe the polarization patterns of the Cosmic Microwave Background (CMB) radio waves, the light emitted as the universe cooled to its present state. The high altitude, dry air and large distance from radios, cellular telephone networks and other sources of radio interference make the south pole an ideal place to observe the sky.


While the indirect detection of gravitational waves is important, the true revelation of this discovery is the strength of this signal: if confirmed, it would be strong -almost compelling- evidence  of inflation, one of the building blocks of modern cosmology. "This has been like looking for a needle in a haystack, but instead we found a crowbar," says co-leader Clem Pryke (associate professor of the University of Minnesota).


Inflation would take place at extremely high energies (about 10^{12} TeV), shortly (10^{-36} s) after the Big Bang, when the Universe was dominated by a scaler field that induced an accelerating expansion.


Through this effect, inflation explains some otherwise puzzling aspects of our universe, by flattening it, ensuring its homogeneity over large scales, and driving down the number density of magnetic monopoles which it contains.


The basic product of this initial accelerated expansion of the universe is the primordial stochastic gravitational waves background revealed by BICEP2.


The amplitude of this signal depends upon the tensor-to-scalar ratio r, which measures the amount of energy generated in gravitational waves (tensor modes) versus ordinary matter-radiation density (scalar modes).


The BICEP2 discovery of r = 0.2 is a major victory for inflationary theory. This measurement is obtained after allowing a running of the scalar spectral index, since previous limits on r from Planck and other experiments appear to be in moderately strong tension with the BICEP2 B-mode measurements (interpreted as tensors). The BICEP2 measurement, if confirmed by upcoming results, would rule out many competing theories.


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