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Tuesday, March 18, 2014

First direct evidence of cosmic inflation

Credit: BICEP 2014


Researchers working on the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) project at the South Pole announced yesterday the first direct evidence of cosmic inflation with the detection of primordial gravitational waves.


"What they detected is known as primordial B-mode polarization and is important for at least two reasons. It would be the first detection of gravitational waves, which are predicted to exist under Einstein’s theory of relativity but have never before been seen. But the thing that has scientists really excited is that it could provide the first direct evidence for a theorized event called inflation that caused the universe to exponentially grow just a fraction of a fraction of a second after it was born." in wired.com
"Gravitational waves from inflation are interesting for a couple of reasons. First, we know they should be there; gravitation certainly exists, and it’s a massless field. Second, there is a way to disentangle the gravitational waves from the density fluctuations, using thepolarization of the CMB. [...] Finally, how strong the gravitational waves are at different wavelengths reveals a great deal about the details of inflation — including one magic number, the energy density of the universe during the inflationary era." in Sean Carroll website
Here is the abstract of the paper submited to arXiv website:

BICEP2 I: Detection Of B-mode Polarization at Degree Angular Scales

We report results from the BICEP2 experiment, a Cosmic Microwave Background (CMB) polarimeter specifically designed to search for the signal of inflationary gravitational waves in the B-mode power spectrum around l=80. [...] BICEP2 observed from the South Pole for three seasons from 2010 to 2012. A low-foreground region of sky with an effective area of 380 square degrees was observed to a depth of 87 nK-degrees in Stokes Q and U. In this paper we describe the observations, data reduction, maps, simulations and results. We find an excess of B-mode power over the base lensed-LCDM expectation in the range 30<l<150, inconsistent with the null hypothesis at a significance of >5σ. [...] Additionally, cross-correlating BICEP2 against 100 GHz maps from the BICEP1 experiment, the excess signal is confirmed with 3σ significance and its spectral index is found to be consistent with that of the CMB, disfavoring synchrotron or dust at 2.3σ and 2.2σ, respectively. The observed B-mode power spectrum is well-fit by a lensed-LCDM + tensor theoretical model with tensor/scalar ratio r=0.20+0.070.05, with r=0 disfavored at 7.0σ. Subtracting the best available estimate for foreground dust modifies the likelihood slightly so that r=0 is disfavored at 5.9σ.
Paper available here: http://arxiv.org/abs/1403.3985

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