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BICEP2 (for “Background Imaging of Cosmic Extragalactic Polarization”) is the name of an astrophysical experiment (their webpage) which released its data in March 2014. The experiment claims to have detected a pattern called the “B-mode” in the polarization of the cosmic microwave background (CMB).


This data, if confirmed, would have been likely due to a gravitational wave mode created during the period of cosmic inflation by a vacuum fluctuation in the field of gravity which then at the era of decoupling left the characteristic B-mode imprint on the CMB. This fact alone was regarded as further strong evidence for the already excellent experimental evidence for cosmic inflation as such (competing models did not predict such gravitational waves to be strong enough to be detectable in this way).

What would have singled out the BICEP2 result over previous confirmations of cosmic inflation is that the data also gives a quantitative value for the energy scale at which cosmic inflation happened (the mass of the hypothetical inflaton), namely at around 10 1610^{16}GeV. This is noteworthy as being only two order of magnituded below the Planck scale, and hence 12 or so orders of magnitude above energies available in current accelerator experiments (the LHC). Also, it is at least a curious coincidence that this is precisely the hypothetical GUT scale.

It was thought that this value rules out a large number of variant models of cosmic inflation and favors the model known as chaotic inflation.


Further measurement by the Planck collaboration showed that much of the signal claimed by BICEP2 is naturally explained just by galactic dust forground, see below. Further measurements seem to be needed to clarify the situation.