Number Of Neutrino Species Research Articles

The Influence of the Effective Number of Active and Sterile Neutrinos on the Determination of the Values of Cosmological Parameters

Neutrino, as the second most abundant known particle in the Universe, has a significant impact on its expansion rate during the radiation- and matter-dominated eras. For this reason, a change in the number of neutrino species can lead to substantial changes in the estimates of cosmological parameters, the most accurate values of which, at the moment, are obtained by analysing the anisotropy of the CMB. In the presented work we consider the influence of a hypothetical sterile neutrino (with eV-scale mass) on the determination of cosmological parameters. The possible existence of such neutrino is suggested by the analysis of a series of different experiments. If it is detected, it will be necessary to include it into the $$\Lambda\rm CDM$$ model with the fixed values of its mass $$m_{\rm s}$$ and mixing angle $$\theta_{s}$$ , which is the main method used through this paper. Apart from that, the seesaw mechanism requires there to be at least two sterile states, one of them being much heavier than the active ones. The heavier sterile state ( $$m_{s}\sim 1$$ keV) would decay and increase the effective number of active neutrinos. Therefore, the influence of a change in the effective number of relativistic neutrino species $$N_{\textrm{eff}}$$ was studied as well, which could be caused by various reasons, for example, by the decay processes of dark matter particles or the above-mentioned sterile neutrinos, as well as processes leading to an increase in the temperature of relic neutrinos $$T_{\textrm{C}\nu\textrm{B}}$$ . The effects studied in this work lead to a significant change in the estimates of the cosmological parameters, including the value of $$H_{0}$$ . It has been discovered that the accounting of the sterile neutrino with masses $$m=1$$ and $$2.7$$ eV leads to a decrease in the estimate of the current Hubble parameter value $$H_{0}$$ and, therefore, exacerbates the ‘‘ $$H_{0}$$ -tension’’ problem. An increase in the value of the effective number of relativistic neutrino species leads, on the contrary, to an increase in the $$H_{0}$$ estimate, resolving the above-mentioned problem at $$N_{\textrm{eff}}=3.0+0.9$$ , which is equivalent to an increase of the neutrino temperature up to $$T^{0}_{\textrm{C}\nu\textrm{B}}=1.95+0.14$$ K. At the same time, the rest of the cosmological parameters do not change significantly, leaving us within the framework of the standard $$\Lambda$$ CDM model.

Alternative ideas in cosmology

Some remarkable examples of alternative cosmological theories are reviewed here, ranging from a compilation of variations on the Standard Model through the more distant quasi-steady-state cosmology, plasma cosmology, or universe models as a hypersphere, to the most exotic cases including static models. The present-day Standard Model of cosmology, [Formula: see text]CDM, gives us a representation of a cosmos whose dynamics is dominated by gravity (Friedmann equations derived from general relativity) with a finite lifetime, large scale homogeneity, expansion and a hot initial state, together with other elements necessary to avoid certain inconsistencies with observations (inflation, nonbaryonic dark matter, dark energy, etc.). There are however some models with characteristics that are close to those of the Standard Model but differing in some minor aspects; we call these “variations on the Standard Model”. Many of these models are indeed investigated by some mainstream cosmologists: different considerations on CP violation, inflation, number of neutrino species, quark-hadron phase transition, baryonic or nonbaryonic dark-matter, dark energy, nucleosynthesis scenarios, large-scale structure formation scenarios; or major variations like a inhomogeneous universe, Cold Big Bang, varying physical constants or gravity law, zero-active mass (also called “[Formula: see text]”), Milne, and cyclical models. At the most extreme distance from the Standard Model, the static models, a noncosmological redshift includes “tired-light” hypotheses, which assume that the photon loses energy owing to an intrinsic property or an interaction with matter or light as it travels some distance, or other nonstandard ideas. Our impression is that none of the alternative models has acquired the same level of development as [Formula: see text]CDM in offering explanations of available cosmological observations. One should not, however, judge any theory in terms of the number of observations that it can successfully explain (ad hoc in many cases) given the much lower level of development of the alternative ones, but by the plausibility of its principles and its potential to fit data with future improvements of the theories. A pluralist approach to cosmology is a reasonable option when the preferred theory is still under discussion.

Bayesian Estimation of the D(p,γ)3He Thermonuclear Reaction Rate

Abstract Big bang nucleosynthesis (BBN) is the standard model theory for the production of light nuclides during the early stages of the universe, taking place about 20 minutes after the big bang. Deuterium production, in particular, is highly sensitive to the primordial baryon density and the number of neutrino species, and its abundance serves as a sensitive test for the conditions in the early universe. The comparison of observed deuterium abundances with predicted ones requires reliable knowledge of the relevant thermonuclear reaction rates and their corresponding uncertainties. Recent observations reported the primordial deuterium abundance with percent accuracy, but some theoretical predictions based on BBN are in tension with the measured values because of uncertainties in the cross section of the deuterium-burning reactions. In this work, we analyze the S-factor of the D(p,γ)3He reaction using a hierarchical Bayesian model. We take into account the results of 11 experiments, spanning the period of 1955–2021, more than any other study. We also present results for two different fitting functions, a two-parameter function based on microscopic nuclear theory and a four-parameter polynomial. Our recommended reaction rates have a 2.2% uncertainty at 0.8 GK, which is the temperature most important for deuterium BBN. Differences between our rates and previous results are discussed.

Thermal QCD Axions across Thresholds

Thermal axion production in the early universe goes through several mass thresholds, and the resulting rate may change dramatically across them. Focusing on the KSVZ and DFSZ frameworks for the invisible QCD axion, we perform a systematic analysis of thermal production across thresholds and provide smooth results for the rate. The QCD phase transition is an obstacle for both classes of models. For the hadronic KSVZ axion, we also deal with production at temperatures around the mass of the heavy-colored fermion charged under the Peccei-Quinn symmetry. Within the DFSZ framework, standard model fermions are charged under this symmetry, and additional thresholds are the heavy Higgs bosons masses and the electroweak phase transition. We investigate the cosmological implications with a specific focus on axion dark radiation quantified by an effective number of neutrino species and explore the discovery reach of future CMB-S4 surveys.

The Atacama Cosmology Telescope: two-season ACTPol spectra and parameters

We present the temperature and polarization angular power spectra measured by the Atacama Cosmology Telescope Polarimeter (ACTPol). We analyze night-time data collected during 2013–14 using two detector arrays at 149 GHz, from 548 deg2 of sky on the celestial equator. We use these spectra, and the spectra measured with the MBAC camera on ACT from 2008–10, in combination with planck and wmap data to estimate cosmological parameters from the temperature, polarization, and temperature-polarization cross-correlations. We find the new ACTPol data to be consistent with the ΛCDM model. The ACTPol temperature-polarization cross-spectrum now provides stronger constraints on multiple parameters than the ACTPol temperature spectrum, including the baryon density, the acoustic peak angular scale, and the derived Hubble constant. The new ACTPol data provide information on damping tail parameters. The joint uncertainty on the number of neutrino species and the primordial helium fraction is reduced by 20% when adding ACTPol to Planck temperature data alone.

CONSTRAINING WHITE DWARF STRUCTURE AND NEUTRINO PHYSICS IN 47 TUCANAE

ABSTRACT We present a robust statistical analysis of the white dwarf cooling sequence in 47 Tucanae. We combine Hubble Space Telescope UV and optical data in the core of the cluster, Modules for Experiments in Stellar Evolution (MESA) white dwarf cooling models, white dwarf atmosphere models, artificial star tests, and a Markov Chain Monte Carlo sampling method to fit white dwarf cooling models to our data directly. We use a technique known as the unbinned maximum likelihood to fit these models to our data without binning. We use these data to constrain neutrino production and the thickness of the hydrogen layer in these white dwarfs. The data prefer thicker hydrogen layers ( q H = 3.2 × 10 − 5 ) and we can strongly rule out thin layers ( q H = 10 − 6 ) . The neutrino rates currently in the models are consistent with the data. This analysis does not provide a constraint on the number of neutrino species.

The power spectrum and bispectrum of SDSS DR11 BOSS galaxies – I. Bias and gravity

We analyse the anisotropic clustering of the Baryon Oscillation Spectroscopic Survey (BOSS) CMASS Data Release 11 sample, which consists of $690 827$ galaxies in the redshift range $0.43 < z < 0.70$ and has a sky coverage of $8 498$ deg$^2$ corresponding to an effective volume of $\sim6\,\rm{Gpc}^3$. We fit the Fourier space statistics, the power spectrum and bispectrum monopoles to measure the linear and quadratic bias parameters, $b_1$ and $b_2$, for a non-linear non-local bias model, the growth of structure parameter $f$ and the amplitude of dark matter density fluctuations parametrised by $\sigma_8$. We obtain $b_1(z_{\rm eff})^{1.40}\sigma_8(z_{\rm eff})=1.672\pm 0.060$ and $b_2^{0.30}(z_{\rm eff})\sigma_8(z_{\rm eff})=0.579\pm0.082$ at the effective redshift of the survey, $z_{\rm eff}=0.57$. The main cosmological result is the constraint on the combination $f^{0.43}(z_{\rm eff})\sigma_8(z_{\rm eff})=0.582\pm0.084$, which is complementary to $f\sigma_8$ constraints obtained from 2-point redshift space distortion analyses. A less conservative analysis yields $f^{0.43}(z_{\rm eff})\sigma_8(z_{\rm eff})=0.584\pm0.051$. We ensure that our result is robust by performing detailed systematic tests using a large suite of survey galaxy mock catalogs and N-body simulations. The constraints on $f^{0.43}\sigma_8$ are useful for setting additional constrains on neutrino mass, gravity, curvature as well as the number of neutrino species from galaxy surveys analyses (as presented in a companion paper).

Three P’s in Cosmology: Progress, Problems, and Perspectives

A review on the development of cosmology for a diverse audience is presented. The first historical part is devoted to the works done half a century ago with an emphasize to those performed in Russia (or more precisely, in the Soviet Union). Then the recent data on the cosmic microwave background raditation is discussed and the impact of these data on the neutrino mass and the number of neutrino species is considered. The latter is compared with the results obtained from the big bang nucleosynthesis. Next, a short description and history of the cosmological inflation is presented. The basics of the stan- dard cosmological model and the problems of dark matter and dark energy are considered at the end of the fist section entitled Progress. In the next short section, Problems, one can find a list of the unsolved cosmological and astrophysical problems, which is by no means fully comprehensive. The last supershort section contains a few words about the perspectives of future development.

The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: weighing the neutrino mass using the galaxy power spectrum of the CMASS sample

We measure the sum of the neutrino particle masses using the three-dimensional galaxy power spectrum of the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS) Data Release 9 (DR9) CMASS galaxy sample. Combined with the cosmic microwave background (CMB), supernova (SN) and additional baryonic acoustic oscillation (BAO) data, we find upper 95 percent confidence limits of the neutrino mass $\Sigma m_{\nu}<0.340$ eV within a flat $\Lambda$CDM background, and $\Sigma m_{\nu}<0.821$ eV, assuming a more general background cosmological model. The number of neutrino species is measured to be $N_{\rm eff}=4.308\pm0.794$ and $N_{\rm eff}=4.032^{+0.870}_{-0.894}$ for these two cases respectively. We study and quantify the effect of several factors on the neutrino measurements, including the galaxy power spectrum bias model, the effect of redshift-space distortion, the cutoff scale of the power spectrum, and the choice of additional data. The impact of neutrinos with unknown masses on other cosmological parameter measurements is investigated. The fractional matter density and the Hubble parameter are measured to be $\Omega_M=0.2796\pm0.0097$, $H_0=69.72^{+0.90}_{-0.91}$ km/s/Mpc (flat $\Lambda$CDM) and $\Omega_M=0.2798^{+0.0132}_{-0.0136}$, $H_0=73.78^{+3.16}_{-3.17}$ km/s/Mpc (more general background model). Based on a Chevallier-Polarski-Linder (CPL) parametrisation of the equation-of-state $w$ of dark energy, we find that $w=-1$ is consistent with observations, even allowing for neutrinos. Similarly, the curvature \Omega_K and the running of the spectral index $\alpha_s$ are both consistent with zero. The tensor-to-scaler ratio is constrained down to $r<0.198$ (95 percent CL, flat $\Lambda$ CDM) and $r<0.440$ (95 percent CL, more general background model).

Experimental Prospect

This is the first conference of this series the three of the lepton mixing angles are known. The non-zero value of the third mixing angle (θ13) has been reported by νe appearance in accelerator νμ beam and by ν¯e disappearance in reactor experiments. The observation of the νe appearance in νμ beam is the first clear evidence of the flavor oscillations. The results also show that neutrino oscillations phenomena can be described by three-generation scheme to the first approximation, where mixing of neutrinos can be described by three angles and one complex phase. There are two main issues to be investigated by neutrino oscillation experiments. One is the study of CP violation in lepton secto. Namely, the measurement of CP violating complex phase δCP and the studies of the nature of CP violation phenomena in lepton sector. The other is search for neutrinos beyond three generations. There are indications of the existence of additional neutrinos, which should be tested by definitive experiments. The number of neutrino species has important implications not only in particle physics but also in physics of early Universe. Also many fundamental natures of neutrinos remain to be studied, such as absolute values of masses, nature of the particle (Majorana or Dirac particle). This article concentrates on the future experimental prospects on the study of neutrino physics by means of neutrino oscillation.

Impact of assuming flatness in the determination of neutrino properties from cosmological data

Cosmological data have provided new constraints on the number of neutrino species and the neutrino mass. However these constraints depend on assumptions related to the underlying cosmology. Since a correlation is expected between the number of effective neutrinos N_{eff}, the neutrino mass \sum m_\nu, and the curvature of the universe \Omega_k, it is useful to investigate the current constraints in the framework of a non-flat universe. In this paper we update the constraints on neutrino parameters by making use of the latest cosmic microwave background (CMB) data from the ACT and SPT experiments and consider the possibility of a universe with non-zero curvature. We first place new constraints on N_{eff} and \Omega_k, with N_{eff} = 4.03 +/- 0.45 and 10^3 \Omega_k = -4.46 +/- 5.24. Thus, even when \Omega_k is allowed to vary, N_{eff} = 3 is still disfavored with 95% confidence. We then investigate the correlation between neutrino mass and curvature that shifts the 95% upper limit of \sum m_\nu < 0.45 eV to \sum m_\nu < 0.95 eV. Thus, the impact of assuming flatness in neutrino cosmology is significant and an essential consideration with future experiments.

Phenomenology of1032dark sectors

We postulate an exact permutation symmetry acting on 10(32) standard model copies as the largest possible symmetry extension of the standard model. This setup automatically lowers the fundamental gravity cutoff down to TeV, and thus, accounts for the quantum stability of the weak scale. We study the phenomenology of this framework and show that below TeV energies the copies are well hidden, obeying all the existing observational bounds. Nevertheless, we identify a potential low energy window into the hidden world, the oscillation of the neutron into its dark copies. At the same time, proton decay can be suppressed by gauging the diagonal baryon number of the different copies. This framework offers an alternative approach to several particle physics questions. For example, we suggest a novel mechanism for generating naturally small neutrino masses that are suppressed by the number of neutrino species. The mirror copies of the standard model naturally house dark matter candidates. The general experimentally observable prediction of this scenario is an emergence of strong gravitational effects at the LHC. The low energy permutation symmetry powerfully constrains the form of this new gravitational physics and allows to make observational predictions, such as, production of micro black holes with very peculiar properties.

Neutrino masses, dark energy and the gravitational lensing of pre-galactic H i

We study the constraints which the next generation of radio telescopes could place on the mass and number of neutrino species by studying the gravitational lensing of high redshift 21 cm emission in combination with wide-angle surveys of galaxy lensing. We use simple characterizations of reionization history and of proposed telescope designs to forecast the constraints and detectability threshold for neutrinos. It is found that the degeneracy between neutrino parameters and dark energy parameters is significantly reduced by incorporating 21 cm lensing. The combination of galaxy and 21 cm lensing could constrain the sum of the neutrino masses to within ~ 0.04 eV and the number of species to within ~ 0.1. This is an improvement of a factor of 2.6 in mass and 1.3 in number over a galaxy lensing survey alone. This includes marginalizing over an 11 parameter cosmological model with a two parameter model for the dark energy equation of state. If the dark energy equation of state is held fixed at w = p/\rho=-1 the constraints improve to ~0.03 eV and 0.04. These forecasted errors depend critically on the fraction of sky that can be surveyed in redshifted 21 cm emission (25% is assumed here) and the redshift of reionization ($z=7$ is assumed here). It is also found that neutrinos with masses too small to be detected in the data could none the less cause a significant bias in the measured dark energy equation of state.

Probing the effective number of neutrino species with the cosmic microwave background

We discuss how much we can probe the effective number of neutrino species ${N}_{\ensuremath{\nu}}$ with the cosmic microwave background alone. Using the data of the WMAP, ACBAR, CBI, and BOOMERANG experiments, we obtain a constraint on the effective number of neutrino species as $0.96l{N}_{\ensuremath{\nu}}l7.94$ at 95% C.L. for a power-law $\ensuremath{\Lambda}\mathrm{CDM}$ flat universe model. The limit is improved to be $1.39l{N}_{\ensuremath{\nu}}l6.38$ at 95% C.L. if we assume that the baryon density, ${N}_{\ensuremath{\nu}}$, and the helium abundance are related by the big bang nucleosynthesis theory. We also provide a forecast for the Planck experiment using a Markov chain Monte Carlo approach. In addition to constraining ${N}_{\ensuremath{\nu}}$, we investigate how the big bang nucleosynthesis relation affects the estimation for these parameters and other cosmological parameters.

Sterile neutrino production in models with low reheating temperatures

By numerically solving the appropriate Boltzmann equations, we study the production of sterile neutrinos in models with low reheating temperatures. We take into account the production in oscillations as well as in direct decays and compute the sterile neutrino primordial spectrum, the effective number of neutrino species, and the sterile neutrino contribution to the mass density of the Universe as a function of the mixing and the reheating parameters. It is shown that sterile neutrinos with non-negligible mixing angles do not necessarily lead to Nν ~ 4 and that sterile neutrinos may have the right relic density to explain the dark matter of the Universe. If dark matter consists of sterile neutrinos produced in oscillations, X-rays measurements set a strong limit on the reheating temperature, TR 7 MeV. We also point out that the direct decay opens up a new production mechanism for sterile neutrino dark matter where cosmological constraints can be satisfied.

CMB signals of neutrino mass generation

We propose signals in the cosmic microwave background (CMB) to probe the type and spectrum of neutrino masses. In theories that have spontaneous breaking of approximate lepton flavor symmetries at or below the weak scale, light pseudo-Goldstone bosons recouple to the cosmic neutrinos after nucleosynthesis and affect the acoustic oscillations of the electron-photon fluid during the eV era. Deviations from the Standard Model are predicted for both the total energy density in radiation during this epoch, $\ensuremath{\Delta}{N}_{\ensuremath{\nu}}$, and for the multipole of the $n$'th CMB peak at large $n$, $\ensuremath{\Delta}{l}_{n}$. The latter signal is difficult to reproduce other than by scattering of the known neutrinos, and is therefore an ideal test of our class of theories. In many models, the large shift $\ensuremath{\Delta}{l}_{n}\ensuremath{\approx}8{n}_{S}$ depends on the number of neutrino species that scatter via the pseudo-Goldstone boson interaction. This interaction is proportional to the neutrino masses, so that the signal reflects the neutrino spectrum. The prediction for $\ensuremath{\Delta}{N}_{\ensuremath{\nu}}$ is highly model dependent, but can be accurately computed within any given model. It is very sensitive to the number of pseudo-Goldstone bosons, and therefore to the underlying symmetries of the leptons, and is typically in the region of $0.03&lt;\ensuremath{\Delta}{N}_{\ensuremath{\nu}}&lt;1$. This signal is significantly larger for Majorana neutrinos than for Dirac neutrinos, and, like the scattering signal, varies as the spectrum of neutrinos is changed from hierarchical to inverse hierarchical to degenerate.

Late-time entropy production from scalar decay and the relic neutrino temperature

Entropy production from scalar decay in the era of low temperatures after neutrino decoupling will change the ratio of the relic neutrino temperature to the CMB temperature, and, hence, the value of ${N}_{\mathrm{eff}},$ the effective number of neutrino species. Such scalar decay is relevant to reheating after thermal inflation, proposed to dilute massive particles, like the moduli and the gravitino, featuring in supersymmetric and string theories. The effect of such entropy production on the relic neutrino temperature ratio is calculated in a semianalytic manner, and a recent lower bound on this ratio, obtained from the WMAP satellite and 2dF galaxy data, is used to set a lower bound of $\ensuremath{\sim}1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}23}$ GeV on the scalar decay constant, corresponding to a reheating temperature of about $3.3$ MeV.

Cosmological implications of neutrinos

Massive neutrinos were the first proposed, and remain the most natural, particle candidate for the dark matter. In the absence of firm laboratory evidence for neutrino mass, considerations of the formation of large scale structure in the universe provide a sensitive, albeit indirect, probe of this possibility. Observations of galaxy clustering and large angle anisotropy in the cosmic microwave background have been interpreted as requiring that neutrinos provide ∼20% of the critical density. However the need for such “hot” dark matter is removed if the primordial spectrum of density fluctuations is tilted below scale-invariance, as is often the case in physically realistic inflationary models. This question will be resolved by forthcoming precision measurements of microwave background anisotropy on small angular scales. This data will also improve the nucleosynthesis bound on the number of neutrino species and test whether decays of relic neutrinos could have ionized the intergalactic medium.

Massive decaying τ neutrino and Big Bang nucleosynthesis

Comparing Big Bang Nucleosynthesis predictions with the light element abundances inferred from observational data, we can obtain the strong constraints on some neutrino properties, e.g. number of neutrino species, mass, lifetime. Recently the deuterium abundances were measured in high red-shift QSO absorption systems. It is expected that they are close to the primordial values, however, two groups have reported inconsistent values which are different in one order of magnitude. In this paper we show how we can constrain on τ neutrino mass and its lifetime in each case when we adopt either high or low deuterium data. We find that if 0.01sec≲τντ≲1sec and 10MeV≲mντ≲24MeV, the theoretical predictions agree with the low D/H abundances. On the other hand if we adopt the high D/H abundances, we obtain the upper bound of τ neutrino mass, mντ≲20MeV.

Exact analytic description of neutrino oscillations in matter with an exponentially varying density for an arbitrary number of neutrino species.

Exact analytical expressions in terms of generalized confluent hypergeometric functions for the transition amplitudes of neutrino oscillations in presence of matter are computed for an arbitrary number of species. The density of matter is assumed to be exponentially decaying. The results can be used for the description of matter-induced neutrino oscillations in the Sun which can take place when the solar neutrinos propagate radially from the interior to the surface. Expressions are particularly simple in the limit of infinite propagation time as is suitable for the case of detection at Earth. PACS: 14.60.Pq, 02.30.Gp, 02.30.Hq