A robust upper limit on N_eff from BBN, circa 2011

We derive here a robust bound on the effective number of neutrinos from constraints on primordial nucleosynthesis yields of deuterium and helium. In particular, our results are based on very weak assumptions on the astrophysical determination of the helium abundance, namely that the minimum effect of stellar processing is to keep constant (rather than increase, as expected) the helium content of a low-metallicity gas. Using the results of a recent analysis of extragalactic HII regions as upper limit, we find that Delta Neff<= 1 at 95 % C.L., quite independently of measurements on the baryon density from cosmic microwave background anisotropy data and of the neutron lifetime input. In our approach, we also find that primordial nucleosynthesis alone has no significant preference for an effective number of neutrinos larger than the standard value. The ~2 sigma hint sometimes reported in the literature is thus driven by CMB data alone and/or is the result of a questionable regression protocol to infer a measurement of primordial helium abundance.Comment: 5 pages, 1 table, 1 figure. Minor improvements and extensions in the analysis, clarifications and reference added, conclusions slightly strengthened. Matches version published in Phys. Lett.

Damping the neutrino flavor pendulum by breaking homogeneity

The most general case of self-induced neutrino flavor evolution is described by a set of kinetic equations for a dense neutrino gas evolving both in space and time. Solutions of these equations have been typically worked out assuming that either the time (in the core-collapse supernova environment) or space (in the early universe) homogeneity in the initial conditions is preserved through the evolution. In these cases one can gauge away the homogeneous variable and reduce the dimensionality of the problem. In this paper we investigate if small deviations from an initial postulated homogeneity can be amplified by the interacting neutrino gas, leading to a new flavor instability. To this end, we consider a simple two flavor isotropic neutrino gas evolving in time, and initially composed by only $\nu_e$ and $\bar\nu_e$ with equal densities. In the homogeneous case, this system shows a bimodal instability in the inverted mass hierarchy scheme, leading to the well studied flavor pendulum behavior. This would lead to periodic pair conversions $\nu_e \bar\nu_e \leftrightarrow \nu_x \bar\nu_x$. To break space homogeneity, we introduce small amplitude space-dependent perturbations in the matter potential. By Fourier transforming the equations of motion with respect to the space coordinate, we then numerically solve a set of coupled equations for the different Fourier modes. We find that even for arbitrarily tiny inhomogeneities, the system evolution runs away from the stable pendulum behavior: the different modes are excited and the space-averaged ensemble evolves towards flavor equilibrium. We finally comment on the role of a time decaying neutrino background density in weakening these results.Comment: (7 pages, 5 eps figures. Figure improved. Final version appeared in PRD

Self-induced flavor instabilities of a dense neutrino stream in a two-dimensional model

We consider a simplifed model for self-induced flavor conversions of a dense neutrino gas in two dimensions, showing new solutions that spontaneously break the spatial symmetries of the initial conditions. As a result of the symmetry breaking induced by the neutrino-neutrino interactions, the coherent behavior of the neutrino gas becomes unstable. This instability produces large spatial variations in the flavor content of the ensemble. Furthermore, it also leads to the creation of domains of different net lepton number flux. The transition of the neutrino gas from a coherent to incoherent behavior shows an intriguing analogy with a streaming flow changing from laminar to turbulent regime. These finding would be relevant for the self-induced conversions of neutrinos streaming-off a supernova core.Comment: (v2: revised version: 8 pages, 7 eps figures. To appear on Physical Review D as Rapid Communication. Discussion enlarged. Two Appendices added.

Unveiling secret interactions among sterile neutrinos with big-bang nucleosynthesis

Short-baseline neutrino anomalies suggest the existence of low-mass ( m \sim O(1)~eV) sterile neutrinos \nu_s. These would be efficiently produced in the early universe by oscillations with active neutrino species, leading to a thermal population of the sterile states seemingly incompatible with cosmological observations. In order to relieve this tension it has been recently speculated that new "secret" interactions among sterile neutrinos, mediated by a massive gauge boson X (with M_X << M_W), can inhibit or suppress the sterile neutrino thermalization, due to the production of a large matter potential term. We note however, that they also generate strong collisional terms in the sterile neutrino sector that induce an efficient sterile neutrino production after a resonance in matter is encountered, increasing their contribution to the number of relativistic particle species N_ eff. Moreover, for values of the parameters of the \nu_s-\nu_s interaction for which the resonance takes place at temperature T\lesssim few MeV, significant distortions are produced in the electron (anti)neutrino spectra, altering the abundance of light element in Big Bang Nucleosynthesis (BBN). Using the present determination of $^4$He and deuterium primordial abundances we determine the BBN constraints on the model parameters. We find that $^2$H/H density ratio exclude much of the parameter space if one assume a baryon density at the best fit value of Planck experiment, \Omega_B h^2= 0.02207, while bounds become weaker for a higher \Omega_B h^2=0.02261, the 95 % C.L. upper bound of Planck. Due to the large error on its experimental determination, the helium mass fraction Y_p gives no significant bounds.Comment: v2: revised version. Minor changes: figures improved, references updated. Matches the version to appear in Phys. Rev.

Inconstant Planck's constant

Motivated by the Dirac idea that fundamental constant are dynamical variables and by conjectures on quantum structure of spacetime at small distances, we consider the possibility that Planck constant $\hbar$ is a time depending quantity, undergoing random gaussian fluctuations around its measured constant mean value, with variance $\sigma^2$ and a typical correlation timescale $\Delta t$. We consider the case of propagation of a free particle and a one--dimensional harmonic oscillator coherent state, and show that the time evolution in both cases is different from the standard behaviour. Finally, we discuss how interferometric experiments or exploiting coherent electromagnetic fields in a cavity may put effective bounds on the value of $\tau= \sigma^2 \Delta t$.Comment: To appear on the International Journal of Modern Physics

Strongest model-independent bound on the lifetime of Dark Matter

Dark Matter is essential for structure formation in the late Universe so it must be stable on cosmological time scales. But how stable exactly? Only assuming decays into relativistic particles, we report an otherwise model independent bound on the lifetime of Dark Matter using current cosmological data. Since these decays affect only the low-$\ell$ multipoles of the CMB, the Dark Matter lifetime is expected to correlate with the tensor-to-scalar ratio $r$ as well as curvature $\Omega_k$. We consider two models, including $r$ and $r+\Omega_k$ respectively, versus data from Planck, WMAP, WiggleZ and Baryon Acoustic Oscillations, with or without the BICEP2 data (if interpreted in terms of primordial gravitational waves). This results in a lower bound on the lifetime of CDM given by 160Gyr (without BICEP2) or 200Gyr (with BICEP2) at 95% confidence level.Comment: 15 pages, 5 figures. Prepared for submission to JCA

Impact of trans-Planckian quantum noise on the Primordial Gravitational Wave spectrum

We investigate the impact of stochastic quantum noise due to trans--Planckian effects on the primordial power spectrum for gravity waves during inflation. Given an energy scale Lambda, expected to be close to the Planck scale m_Pl and larger than the Hubble scale H, this noise is described in terms of a source term in the evolution equation for comoving modes k which changes its amplitude growth from early times as long as the mode physical wavelength is smaller than Lambda^-1. We model the source term as due to a gas of black holes in the trans--Planckian regime and the corresponding Hawking radiation. In fact, for energy scales larger than, or of the order of Lambda, it is expected that trapped surfaces may form due to large energy densities. At later times the evolution then follows the standard sourceless evolution. We find that this mechanism still leads to a scale-invariant power spectrum of tensor perturbations, with an amplitude that depends upon the ratio Lambda/m_Pl.Comment: 6 pages, 1 figur