Neutrino Density Research Articles

Cosmological Tests of the Hypothesis of Dark Matter as Bound Neutrinos

A modest extension of the Standard Model allows a family-specific, feeble form of SU(3) that causes rapid binding of neutrinos at neutrino decoupling. These bound neutrinos become nonrelativistic well before recombination. Neutrinos are bound when all three neutrino flavors are present at densities expected in the early universe. Consistency is examined against observationally-inferred data, including free-streaming lengths, dark matter interaction rates, neutrinos from SN1987A, big-bang nucleosynthesis, and the cosmic microwave background. Consistency with galactic haloes and halo interactions was studied in a companion paper. The theory yields a ratio of dark matter density to neutrino density of 147, calculated in two different ways, agreeing with the current value of 158 assuming the sum of the masses of neutrino mass eigenstates is 0.07 eV/c2. This yields a ratio of dark matter to total matter of 83.2% with a relative uncertainty of at least ±8%. A free-streaming length of about 1 kpc is obtained for hard-sphere self-scattering, and about 115 kpc for 1/r-potential self-scattering, where r is the particle separation. A BBN analysis agrees with observationally-inferred abundances of He and Li, but not the latest deuterium measurements. The latter disagreement is the only identified potential inconsistency with current cosmological measurements. Both the standard SU(3) adapted to the neutrino family and a modest extension of SU(3) give good agreement with most observations. The extension provides a means to estimate dark matter parameters whereas the standard SU(3) does not. This explanation for dark matter does not require any new fundamental particles or forces.

Dispelling a myth on dense neutrino media: fast pairwise conversions depend on energy

Pairwise conversions of neutrinos have complicated the challenging goal of quantifying the relevance of neutrino microphysics in compact astrophysical objects, limiting the ability to perform numerical simulations and encouraging the employment of semi-analytical tools, such as the linear stability analysis. Given the high neutrino density, the dependence of pairwise conversions on the neutrino energy has been deemed to play a negligible role. We show that fast pairwise conversions are affected by the neutrino energy, especially in the non-linear regime. An earlier onset of flavor conversions and a higher oscillation frequency are found as the vacuum frequency increases (i.e., the neutrino energy decreases); the oscillation periodicity, otherwise present, is gradually destroyed as the vacuum frequency increases. Such effects are, however, not further exacerbated by the inclusion of spectral energy distributions for neutrinos, contrary to what was found for “slow” neutrino-neutrino conversions. In addition, our findings highlight the dependence of fast pairwise conversions on the neutrino mass ordering for realistic values of the neutrino vacuum frequency. Our findings highlight the limitations intrinsic to widely adopted approximations and extrapolations based on the linear regime of fast conversions. In order to gauge their possible impact on the source physics, a more sophisticated modeling of pairwise conversions is necessary.

Sterile neutrino self-interactions: H0 tension and short-baseline anomalies

Sterile neutrinos with a mass in the eV range have been invoked as a possible explanation of a variety of short baseline (SBL) neutrino oscillation anomalies. However, if one considers neutrino oscillations between active and sterile neutrinos, such neutrinos would have been fully thermalised in the early universe, and would be therefore in strong conflict with cosmological bounds. In this study we first update cosmological bounds on the mass and energy density of eV-scale sterile neutrinos. We then perform an updated study of a previously proposed model in which the sterile neutrino couples to a new light pseudoscalar degree of freedom. Consistently with previous analyses, we find that the model provides a good fit to all cosmological data and allows the high value of H0 measured in the local universe to be consistent with measurements of the cosmic microwave background. However, new high ℓ polarisation data constrain the sterile neutrino mass to be less than approximately 1 eV in this scenario. Finally, we combine the cosmological bounds on the pseudoscalar model with a Bayesian inference analysis of SBL data and conclude that only a sterile mass in narrow ranges around 1 eV remains consistent with both cosmology and SBL data.

A Lagrangian perturbation theory in the presence of massive neutrinos

We develop a Lagrangian Perturbation Theory (LPT) framework to study the clustering of cold dark matter (CDM) in cosmologies with massive neutrinos. We follow the trajectories of CDM particles with Lagrangian displacements fields up to third order in perturbation theory. Once the neutrinos become non-relativistic, their density fluctuations are modeled as being proportional to the CDM density fluctuations, with a scale-dependent proportionality factor. This yields a gravitational back-reaction that introduces additional scales to the linear growth function, which is accounted for in the higher order LPT kernels. Through non-linear mappings from Eulerian to Lagrangian frames, we ensure that our theory has a well behaved large scale behavior free of unwanted UV divergences, which are common when neutrino and CDM densities are not treated on an equal footing, and in resummation schemes that manifestly break Galilean invariance. We use our theory to construct correlation functions for both the underlying matter field, as well as for biased tracers using Convolution-LPT. Redshift-space distortions effects are modeled using the Gaussian Streaming Model. When comparing our analytical results to simulated data from the \ extsc{Quijote} simulation suite, we find good accuracy down to r=20 Mpc h−1 at redshift z=0.5, for the real space and redshift space monopole particle correlation functions with no free parameters. The same accuracy is reached for the redshift space quadrupole if we additionally consider an effective field theory parameter that shifts the pairwise velocity dispersion. For modeling the correlation functions of tracers we adopt a simple Lagrangian biasing scheme with only density and curvature operators, which we find sufficient to reach down to r=20 Mpc h−1 when comparing to simulated halos.

Planck2018 results

We report on the implications for cosmic inflation of the 2018 release of thePlanckcosmic microwave background (CMB) anisotropy measurements. The results are fully consistent with those reported using the data from the two previousPlanckcosmological releases, but have smaller uncertainties thanks to improvements in the characterization of polarization at low and high multipoles.Plancktemperature, polarization, and lensing data determine the spectral index of scalar perturbations to bens = 0.9649 ± 0.0042 at 68% CL. We find no evidence for a scale dependence ofns, either as a running or as a running of the running. The Universe is found to be consistent with spatial flatness with a precision of 0.4% at 95% CL by combiningPlanckwith a compilation of baryon acoustic oscillation data. ThePlanck95% CL upper limit on the tensor-to-scalar ratio,r0.002 < 0.10, is further tightened by combining with the BICEP2/Keck Array BK15 data to obtainr0.002 < 0.056. In the framework of standard single-field inflationary models with Einstein gravity, these results imply that: (a) the predictions of slow-roll models with a concave potential,V″(ϕ) < 0, are increasingly favoured by the data; and (b) based on two different methods for reconstructing the inflaton potential, we find no evidence for dynamics beyond slow roll. Three different methods for the non-parametric reconstruction of the primordial power spectrum consistently confirm a pure power law in the range of comoving scales 0.005 Mpc−1 ≲ k ≲ 0.2 Mpc−1. A complementary analysis also finds no evidence for theoretically motivated parameterized features in thePlanckpower spectra. For the case of oscillatory features that are logarithmic or linear ink, this result is further strengthened by a new combined analysis including thePlanckbispectrum data. The newPlanckpolarization data provide a stringent test of the adiabaticity of the initial conditions for the cosmological fluctuations. In correlated, mixed adiabatic and isocurvature models, the non-adiabatic contribution to the observed CMB temperature variance is constrained to 1.3%, 1.7%, and 1.7% at 95% CL for cold dark matter, neutrino density, and neutrino velocity, respectively.Planckpower spectra plus lensing set constraints on the amplitude of compensated cold dark matter-baryon isocurvature perturbations that are consistent with current complementary measurements. The polarization data also provide improved constraints on inflationary models that predict a small statistically anisotropic quadupolar modulation of the primordial fluctuations. However, the polarization data do not support physical models for a scale-dependent dipolar modulation. All these findings support the key predictions of the standard single-field inflationary models, which will be further tested by future cosmological observations.

A precision calculation of relic neutrino decoupling

We study the distortions of equilibrium spectra of relic neutrinos due to the interactions with electrons, positrons, and neutrinos in the early Universe. We solve the integro-differential kinetic equations for the neutrino density matrix, including three-flavor oscillations and finite temperature corrections from QED up to the next-to-leading order O(e3) for the first time. In addition, the equivalent kinetic equations in the mass basis of neutrinos are directly solved, and we numerically evaluate the distortions of the neutrino spectra in the mass basis as well, which can be easily extrapolated into those for non-relativistic neutrinos in the current Universe. In both bases, we find the same value of the effective number of neutrinos, Neff=3.044, which parameterizes the total neutrino energy density. The estimated error for the value of Neff due to the numerical calculations and the choice of neutrino mixing parameters would be at most 0.0005.

Neutrino currents in wakes of cosmic strings

Neutrinos rotating around Abelian Higgs strings will generate a neutral current close to the string. As the string moves through the cosmic plasma, the velocity kick generated by the motion of the string will enhance the neutrino current in the wake region. The neutrino current density depends on its distance from the string and is oscillatory in nature. This leads to neutrino density gradients in the plasma. Such a flux of neutrinos with periodic density fluctuations will lead to electron currents in the plasma. The current will act like a cross-perturbation across the cosmic string wake. The perturbation as well as the high Reynolds number of the plasma will result in the generation of magnetic fields in the wake of the cosmic string.

Spectral swaps in a two-dimensional neutrino ring model

Neutrinos emitted deep within a supernova explosion experience a self-induced index of refraction. In the stationary, one-dimensional (1D) supernova "bulb model", this self-induced refraction can lead to a collective flavor transformation which is coherent among different neutrino momentum modes. Such collective oscillations can produce partial swaps of the energy spectra of different neutrino species as the neutrinos stream away from the proto-neutron star. However, it has been demonstrated that the spatial symmetries (such as the spherical symmetry in the bulb model) can be broken spontaneously by collective neutrino oscillations in multi-dimensional models. Using a stationary, 2D neutrino ring model we demonstrate that there exist two limiting scenarios where collective oscillations may occur. In one limit, the collective flavor transformation begins at a radius with relatively high neutrino densities and develops small-scale flavor structures. The loss of the spatial correlation in the neutrino flavor field results in similar (average) energy spectra for the anti-neutrinos of almost all energies and the neutrinos of relatively high energies. In the other limit, the flavor transformation starts at a radius where the neutrino densities are smaller (e.g., due to the suppression of the high matter density near the proto-neutron star). Although the spatial symmetry is broken initially, it is restored as the neutrino densities decrease, and the neutrinos of different flavors partially swap their energy spectra as in the 1D bulb model. This finding may have interesting ramifications in other aspects of supernova physics.

Local density of relic neutrinos with minimal mass

Local density of relic neutrinos with minimal mass

Neutrino clustering in the Milky Way and beyond

The standard cosmological model predicts the existence of a Cosmic Neutrino Background, which has not yet been observed directly. Some experiments aiming at its detection are currently under development, despite the tiny kinetic energy of the cosmological relic neutrinos, which makes this task incredibly challenging. Since massive neutrinos are attracted by the gravitational potential of our Galaxy, they can cluster locally. Neutrinos should be more abundant at the Earth position than at an average point in the Universe. This fact may enhance the expected event rate in any future experiment. Past calculations of the local neutrino clustering factor only considered a spherical distribution of matter in the Milky Way and neglected the influence of other nearby objects like the Virgo cluster, although recent N-body simulations suggest that the latter may actually be important. In this paper, we adopt a back-tracking technique, well established in the calculation of cosmic rays fluxes, to perform the first three-dimensional calculation of the number density of relic neutrinos at the Solar System, taking into account not only the matter composition of the Milky Way, but also the contribution of the Andromeda galaxy and the Virgo cluster. The effect of Virgo is indeed found to be relevant and to depend non-trivially on the value of the neutrino mass. Our results show that the local neutrino density is enhanced by 0.53% for a neutrino mass of 10 meV, 12% for 50 meV, 50% for 100 meV or 500% for 300 meV.

Neutrino quantum decoherence due to entanglement with magnetic field

Neutrino oscillations in a constant magnetic field are considered. The backward influence of neutrinos on the environment is accounted for using the density matrix formalism. The entanglement of neutrinos with the magnetic field can destroy quantum coherent superposition of different neutrino states and thus lead to the suppression of neutrino oscillations. The master equation for the neutrino density matrix accounting for the effects of quantum decoherence due to entanglement with the magnetic field is derived.

On the Occurrence of Crossings between the Angular Distributions of Electron Neutrinos and Antineutrinos in the Supernova Core

Abstract Neutrino fast pairwise conversions have been postulated to occur in the dense core of a core-collapse supernova (SN), possibly having dramatic consequences on the SN mechanism and the observable neutrino signal. One crucial condition favoring pairwise conversions is the presence of crossings between the electron neutrino and antineutrino angular distributions (i.e., electron neutrino lepton number crossings, ELN crossings). A stationary and spherically symmetric SN toy model is constructed to reproduce the development of the neutrino angular distributions in the dense SN core in the absence of perturbations induced by hydrodynamical instabilities. By iteratively solving the neutrino Boltzmann equations including the collisional term, our model predicts that ELN crossings can develop only in the proximity of the decoupling region and for a sharp radial evolution of the baryon density, when the electron neutrino and antineutrino number densities are comparable. Such conditions are likely to occur only in the late SN stages. Interestingly, flavor instabilities induced by spatial or temporal perturbations are unlikely to generate ELN crossings dynamically within our simplified setup.

Nonlinear flavor development of a two-dimensional neutrino gas

We present a numerical survey of the nonlinear flavor development of dense neutrino gases. This study is based on the stationary, two-dimensional ($x$ and $z$), two-beam, monochromatic neutrino line model with a periodic boundary condition along the $x$ direction. Similar to a previous work, we find that small-scale flavor structures can develop in a neutrino gas even if the physical conditions are nearly homogeneous along the $x$ axis initially. The power diffusion from the large-scale to small-scale structures increases with the neutrino density and helps to establish a semi-exponential dependence of the magnitudes of the Fourier moments on the corresponding wave numbers. The overall flavor conversion probabilities in the neutrino gases with small initial sinusoidal perturbations reach certain equilibrium values at large distances which are mainly determined by the neutrino-antineutrino asymmetry. Similar phenomena also exist in a neutrino gas with a localized initial perturbation, albeit only inside an expanding flavor conversion region. Our work suggests that a statistical treatment may be possible for the collective flavor oscillations of a dense neutrino gas in a multi-dimensional environment.

First constraint on the neutrino-induced phase shift in the spectrum of baryon acoustic oscillations

The existence of the cosmic neutrino background is a fascinating prediction of the hot big bang model. These neutrinos were a dominant component of the energy density in the early universe and, therefore, played an important role in the evolution of cosmological perturbations. The energy density of the cosmic neutrino background has been measured using the abundances of light elements and the anisotropies of the cosmic microwave background (CMB). A complementary and more robust probe is a distinct shift in the temporal phase of sound waves in the primordial plasma which is produced by fluctuations in the neutrino density and has recently been detected in the CMB. In this paper, we report on the first constraint on this neutrino-induced phase shift in the spectrum of baryon acoustic oscillations (BAO) of the BOSS DR12 data. Constraining the acoustic scale using Planck data while marginalizing over the effects of neutrinos in the CMB, we find a non-zero phase shift at greater than 95\% confidence. We also demonstrate the robustness of this result in simulations and forecasts. Besides providing a new test of the cosmic neutrino background, our work is the first application of the BAO signal to early universe physics and a non-trivial confirmation of the standard cosmological history.

First Detection of Scale-Dependent Linear Halo Bias in N-Body Simulations with Massive Neutrinos.

Using N-body simulations with massive neutrino density perturbations, we detect the scale-dependent linear halo bias with high significance. This is the first time that this effect is detected in simulations containing neutrino density perturbations on all scales, confirming the same finding from separate universe simulations. The scale dependence is the result of the additional scale in the system, i.e., the massive neutrino free-streaming length, and it persists even if the bias is defined with respect to the cold dark matter plus baryon (instead of total matter) power spectrum. The separate universe approach provides a good model for the scale-dependent linear bias, and the effect is approximately 0.25f_{ν} and 0.43f_{ν} for halos with bias of 1.7 and 3.5, respectively. While the size of the effect is small, it is not insignificant in terms of f_{ν} and should therefore be included to accurately constrain neutrino mass from clustering statistics of biased tracers. More importantly, this feature is a distinct signature of free-streaming particles and cannot be mimicked by other components of the standard cosmological model.

Tree-level bispectrum in the effective field theory of large-scale structure extended to massive neutrinos

We compute the tree-level bispectrum of dark matter in the presence of massive neutrinos in the mildly non-linear regime in the context of the effective field theory of large-scale structure (EFTofLSS) . For neutrinos, whose typical free streaming wavenumber (kfs) is longer than the non-linear scale (kNL), we solve a Boltzmann equation coupled to the effective fluid equation for dark matter. We solve perturbatively the coupled system by expanding in powers of the neutrino density fraction (fν) and the ratio of the wavenumber of interest over the non-linear scale (k/kNL) and add suitable counterterms to remove the dependence from short distance physics. For equilateral configurations, we find that the total-matter tree-level bispectrum is approximately 16fν times the dark matter one on short scales (k > kfs). The largest contribution stems from the back-reaction of massive neutrinos on the dark matter growth factor. On large scales (k < kfs) the contribution of neutrinos to the bispectrum is smaller by up to two orders of magnitude.

Simple method of diagnosing fast flavor conversions of supernova neutrinos

Neutrinos emitted from a supernova may undergo flavor conversions almost immediately above the core, with possible consequences for supernova dynamics and nucleosynthesis. However, the precise conditions for such fast conversions can be difficult to compute and require knowledge of the full angular distribution of the flavor-dependent neutrino fluxes, that is not available in typical supernova simulations. In this paper, we show that the overall flavor evolution is qualitatively similar to the growth of a so-called `zero mode', determined by the background matter and neutrino densities, which can be reliably predicted using only the second angular moments of the electron lepton number distribution, i.e., the difference in the angular distributions of $\nu_e$ and $\bar{\nu}_e$ fluxes. We propose that this zero mode, which neither requires computing the full Green's function nor a detailed knowledge of the angular distributions, may be useful for a preliminary diagnosis of possible fast flavor conversions in supernova simulations with modestly resolved angular distributions

Evaluating radiation transport errors in merger simulations using a Monte Carlo algorithm

Neutrino-matter interactions play an important role in the post-merger evolution of neutron star-neutron star and black hole-neutron star mergers. Most notably, they determine the properties of the bright optical/infrared transients observable after a merger. Unfortunately, Boltzmann's equations of radiation transport remain too costly to be evolved directly in merger simulations. Simulations rely instead on approximate transport algorithms with unquantified modeling errors. In this paper, we use for the first time a time-dependent general relativistic Monte-Carlo (MC) algorithm to solve Boltzmann's equations and estimate important properties of the neutrino distribution function ~10ms after a neutron star merger. We do not fully couple the MC algorithm to the fluid evolution, but use a short evolution of the merger remnant to critically assess errors in our approximate gray two-moment transport scheme. We demonstrate that the analytical closure used by the moment scheme is highly inaccurate in the polar regions, but performs well elsewhere. While the average energy of polar neutrinos is reasonably well captured by the two-moment scheme, estimates for the neutrino energy become less accurate at lower latitudes. The two-moment formalism also overestimates the density of neutrinos in the polar regions by ~50%, and underestimates the neutrino pair-annihilation rate at the poles by factors of 2-3. Although the latter is significantly more accurate than one might have expected before this study, our results indicate that predictions for the properties of polar outflows and for the creation of a baryon-free region at the poles are likely to be affected by errors in the two-moment scheme, thus limiting our ability to reliably model kilonovae and gamma-ray bursts.

Isocurvature initial conditions for second order Boltzmann solvers

We study how to set the initial evolution of general cosmological fluctuations at second order, after electron-positron annihilation. We compute approximate initial solutions for the transfer functions of all the relevant cosmological variables sourced by quadratic combinations of adiabatic and isocurvature modes. We perform these calculations in synchronous gauge, assuming a Universe described by the ΛCDM model and composed of neutrinos, photons, baryons and dark matter. We highlight the importance of mixed modes, which are solutions sourced by two different isocurvature or adiabatic modes and do not exist at the linear level. In particular, we investigate the so-called compensated isocurvature mode and find non-trivial initial evolution when it is mixed with the adiabatic mode, in contrast to the result at linear order and even at second order for the unmixed mode. Non-trivial evolution also arises when this compensated isocurvature is mixed with the neutrino density isocurvature mode. Regarding the neutrino velocity isocurvature mode, we show it unavoidably generates non-regular (decaying) solutions at second order. Our results can be applied to second order Boltzmann solvers to calculate the effects of isocurvatures on non-linear observables.

Scale-dependent bias and bispectrum in neutrino separate universe simulations

Cosmic background neutrinos have a large velocity dispersion, which causes the evolution of long-wavelength density perturbations to depend on scale. This scale-dependent growth leads to the well-known suppression in the linear theory matter power spectrum that is used to probe neutrino mass. In this paper, we study the impact of long-wavelength density perturbations on small-scale structure formation. By performing separate universe simulations where the long-wavelength mode is absorbed into the local expansion, we measure the responses of the cold dark matter (CDM) power spectrum and halo mass function, which correspond to the squeezed-limit bispectrum and halo bias. We find that the scale-dependent evolution of the long-wavelength modes causes these quantities to depend on scale and provide simple expressions to model them in terms of scale and the amount of massive neutrinos. Importantly, this scale-dependent bias reduces the suppression in the linear halo power spectrum due to massive neutrinos by 13 and 26% for objects of bias $\bar{b}=2$ and $\bar{b} \gg1$, respectively. We demonstrate with high statistical significance that the scale-dependent halo bias ${\it cannot}$ be modeled by the CDM and neutrino density transfer functions at the time when the halos are identified. This reinforces the importance of the temporal nonlocality of structure formation, especially when the growth is scale dependent.