Abstract
The tensor-to-scalar ratio (r = 0.20+0.07−0.05) inferred from the excess B-mode power observed by the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) experiment is almost twice as large as the 95% CL upper limits derived from temperature measurements of the WMAP (r < 0.13) and Planck (r < 0.11) space missions. Very recently, it was suggested that additional relativistic degrees of freedom beyond the three active neutrinos and photons can help to relieve this tension: the data favor an effective number of light neutrino species Neff = 3.86±0.25. Since the BICEP2 ratio implies the energy scale of inflation (V*1/4 ∼ 2 × 1016 GeV) is comparable to the grand unification scale, in this paper we investigate whether we can accommodate the required Neff with three right-handed (partners of the left-handed standard model) neutrinos living in the fundamental representation of a grand unified exceptional E6 group. We show that the superweak interactions of these Dirac states (through their coupling to a TeV-scale Z′ gauge boson) lead to decoupling of right-handed neutrino just above the QCD cross over transition: 175 MeV≲TνRdec≲250 MeV. For decoupling in this transition region, the contribution of the three right-handed neutrinos to Neff is suppressed by heating of the left-handed neutrinos (and photons). Consistency (within 1σ) with the favored Neff is achieved for 4.5 TeV < MZ′ < 7.5 TeV. The model is fully predictive and can be confronted with future data from LHC14.
Highlights
The concordance model of cosmology, with dark energy (Λ), cold dark matter (CDM), baryons, and three flavors of left-handed neutrinos, provides a consistent description of the late early universe: big-bang nucleosynthesis (BBN), at ∼ 20 minutes, the cosmic microwave background (CMB), at ∼ 380 Kyr, and the galaxy formation epoch, at 1 Gyr [1]
This apparent mismatch cannot be resolved by varying parameters in this very restrictive, seven parameter model: {ΩCDMh2, Ωbh2, τ, Θs, As, ns, r}, where ΩCDMh2 is the CDM energy density, Ωbh2 is the baryon density, Θs is the ratio between the sound horizon and the angular diameter distance at decoupling, and τ is the Thomson scattering optical depth of reionized intergalactic medium
Unlike the previous results reported by the Planck Collaboration on the basis of CMB data alone the multiparameter fit favors extra r.d.o.f
Summary
The concordance model of cosmology, with dark energy (Λ), cold dark matter (CDM), baryons, and three flavors of left-handed (i.e. one helicity state νL) neutrinos (along with their right-handed antineutrinos νR), provides a consistent description of the late early universe: big-bang nucleosynthesis (BBN), at ∼ 20 minutes, the cosmic microwave background (CMB), at ∼ 380 Kyr, and the galaxy formation epoch, at 1 Gyr [1]. The observed B-mode power spectrum is well-fit by a ΛCDM +r model, with r = 0.20+−00..0075, and is inconsistent with the null hypothesis, r = 0, at a significance of 7σ Such unexpectedly large value of r corresponds to a Hubble rate, H 1.1×1014 GeV, constraining the energy scale of inflation: V∗1/4 ∼ 2 × 1016 GeV. The conflict is a result of the fact that the large angle temperature excess foreshadowed by the gravitational waves is not observed This apparent mismatch cannot be resolved by varying parameters in this very restrictive, seven parameter model: {ΩCDMh2, Ωbh, τ, Θs, As , ns, r}, where ΩCDMh2 is the CDM energy density, Ωbh is the baryon density, Θs is the ratio between the sound horizon and the angular diameter distance at decoupling, and τ is the Thomson scattering optical depth of reionized intergalactic medium. We associate the extra r.d.o.f. with the right-handed partners of three Dirac neutrinos, which interact with all fermions through the exchange of a new heavy vector meson Z
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