Neutrino Viscosity Research Articles

Magnetorotational instability in neutron star mergers: impact of neutrinos

The merger of two neutron stars may give birth to a long-lived hypermassive neutron star. If it harbours a strong magnetic field of magnetar strength, its spin-down could explain several features of short gamma-ray burst afterglows. The magnetorotational instability (MRI) has been proposed as a mechanism to amplify the magnetic field to the required strength. Previous studies have, however, neglected neutrinos, which may have an important impact on the MRI by inducing a viscosity and drag. We investigate the impact of these neutrino effects on the linear growth of the MRI by applying a local stability analysis to snapshots of a neutron star merger simulation. We find that neutrinos have a significant impact inside the hypermassive neutron star, but have at most a marginal effect in the torus surrounding it. Inside the hypermassive neutron star, the MRI grows in different regimes depending on the radius and on the initial magnetic field strength. For magnetic fields weaker than $10^{13}-10^{14}\,{\rm G}$, the growth rate of the MRI is significantly reduced due to the presence of neutrinos. We conclude that neutrinos should be taken into account when studying the growth of the MRI from realistic initial magnetic fields. Current numerical simulations, which neglect neutrino viscosity, are only consistent, i.e. in the adopted ideal regime, if they start from artificially strong initial magnetic fields above $\sim10^{14}\,{\rm G}$. One should be careful when extrapolating these results to lower initial magnetic fields, where the MRI growth is strongly affected by neutrino viscosity or drag.

Can Neutrino Viscosity Drive the Late Time Cosmic Acceleration?

In this paper it has been shown that the neutrino bulk viscous stresses can give rise to the late time acceleration of the universe. It is found that a number of spatially flat FRW models with a negative deceleration parameter can be constructed using neutrino viscosity and one of them mimics a $\Lambda$CDM model. This does not require any exotic dark energy component or any modification of gravity.

Dead zone formation and non-steady hyperaccretion in collapsar disks

AbstractIn ultra dense and hot region realized in stellar core-collapse, neutrino takes major role in energy and momentum transports. We investigate the growth of magnetorotational instability (MRI) in neutrino viscous matter by using linear theory. It is found from the local linear analysis that the neutrino viscosity can suppress the MRI in the regime of weak magnetic field (B ≪ 1014G). This suggest that MHD turbulence sustained by the MRI might not be driven efficiently in the neutrino viscous media. Applying this result to collapsar disk, which is known as the central engine of gamma-ray burst (GRB), we find that the MRI can be suppressed only in its inner region. Based on this finding, a new evolutionary scenario of collapsar disk, “Episodic Disk Accretion Model” are proposed.

Dead Zone Formation and Nonsteady Hyperaccretion in Collapsar Disks: A Possible Origin of Short‐Term Variability in the Prompt Emission of Gamma‐Ray Bursts

The central engine of gamma-ray bursts (GRBs) is believed to be a hot and dense disk with hyperaccretion onto a few solar-mass black hole. We investigate where the magnetorotational instability (MRI) actively operates in the hyperaccretion disk, which can cause angular momentum transport in the disk. The inner region of hyperaccretion disks can be neutrino opaque, and the energy- and momentum-transport by neutrinos could affect the growth of the MRI significantly. Assuming reasonable disk models and a weak magnetic field $B \lesssim 10^{14} \rm{G}$, it is found that the MRI is strongly suppressed by the neutrino viscosity in the inner region of hyperaccretion disks. On the other hand, the MRI can drive active MHD turbulence in the outer neutrino-transparent region regardless of the field strength. This suggests that the baryonic matter is accumulated into the inner dead zone where the MRI grows inactively and the angular momentum transport is inefficient. When the dead zone gains a large amount of mass and becomes gravitationally unstable, intense mass accretion onto the central black hole would occur episodically through the gravitational torque. This process can be a physical mechanism of the short-term variability in the prompt emission of GRBs. Finally, the origin of flaring activities in the X-ray afterglow is predicted in the context of our episodic accretion scenario.

Viscosity and Rotation in Core‐Collapse Supernovae

We construct models of core-collapse supernovae in one spatial dimension, including rotation, angular momentum transport, and viscous dissipation employing an alpha-prescription. We compare the evolution of a fiducial 11 M_sun non-rotating progenitor with its evolution including a wide range of imposed initial rotation profiles (1.25<P_0<8 s, where P_0 is the initial, approximately solid-body, rotation period of the iron core). This range of P_0 covers the region of parameter space from where rotation begins to modify the dynamics (P_0~8 s) to where angular velocities at collapse approach Keplerian (P_0~1 s). Assuming strict angular momentum conservation, all models in this range leave behind neutron stars with spin periods <10 ms, shorter than those of most radio pulsars, but similar to those expected theoretically for magnetars at birth. A fraction of the gravitational binding energy of collapse is stored in the free energy of differential rotation. This energy source may be tapped by viscous processes, providing a mechanism for energy deposition that is not strongly coupled to the mass accretion rate through the stalled supernova shock. This effect yields qualitatively new dynamics in models of supernovae. We explore several potential mechanisms for viscosity in the core-collapse environment: neutrino viscosity, turbulent viscosity caused by the magnetorotational instability (MRI), and turbulent viscosity by entropy- and composition-gradient-driven convection. We argue that the MRI is the most effective. We find that for rotation periods in the range P_0<~5 s, and a range of viscous stresses, that the post-bounce dynamics is significantly effected by the inclusion of this extra energy deposition mechanism; in several cases we obtain strong supernova explosions.

Hydromagnetic Instabilities in Proto–Neutron Stars

The stability properties of newly born neutron stars, or proto-neutron stars (PNSs), are considered. We take into account dissipative processes, such as neutrino transport and viscosity, in the presence of a magnetic field. In order to find the regions of the star subject to different sorts of instability, we derive the general instability criteria and apply them to evolutionary models of PNSs. The influence of the magnetic field on instabilities is analyzed, and the critical magnetic field stabilizing the star is obtained. In light of our results, we estimate the maximum poloidal magnetic field that might be present in young pulsars or magnetars.

Quantitative analysis of the thermal damping of coherent axion oscillations.

Unruh and Wald have recently discussed a new mechanism for damping coherent axion oscillations, ``thermal damping,'' which occurs due to the temperature dependence of the axion mass and neutrino viscosity. We investigate the effect quantitatively and find that the present energy density in axions can be written as ${\ensuremath{\rho}}_{a}$=${\ensuremath{\rho}}_{a0}$/(1+${J}_{\mathrm{UW})}$, where ${\ensuremath{\rho}}_{a0}$ is what the axion energy density would be in the absence of the thermal-damping effect and ${J}_{\mathrm{UW}}$ is an integral whose integrand depends upon (${\mathrm{dm}}_{a}$/dT${)}^{2}$. As a function of ${f}_{\mathrm{PQ}(\mathrm{\ensuremath{\equiv}}}$Peccei-Quinn symmetry-breaking scale) ${J}_{\mathrm{UW}}$ achieves its maximum value for ${f}_{\mathrm{PQ}\mathrm{\ensuremath{\simeq}}3\ifmmode\times\else\texttimes\fi{}{10}^{12}}$ GeV; unless the axion mass turn-on is very sudden, \ensuremath{\Vert}(T/${m}_{a}$)(${\mathrm{dm}}_{a}$/dT)\ensuremath{\Vert}\ensuremath{\gg}1, ${J}_{\mathrm{UW}}$ is \ensuremath{\ll}1, implying that this damping mechanism is not significant.

Effects of Anisotropy and Dissipation on the Primordial Light-Isotope Abundances

Account is taken of the dissipation of anisotropy due to neutrino viscosity and nearly collisionless radiation in a Bianchi type-I cosmology during primordial nucleosynthesis. For experimentally allowed cross sections, and for moderate anisotropy, the final $^{4}\mathrm{He}$ mass fraction is significantly less than in the standard model, while the D fraction increases much less than would be expected in a low-helium isotropic cosmology.

Neutrino viscosity and isotropization of anisotropic-curvature cosmological models

We investigate the evolution of Bianchi spaces of type ${\mathrm{VII}}_{h}$, VIII, and IX during the lepton era, and find that a large entropy amplification occurs due to neutrino viscosity for large initial anisotropies. The curvature has a weak influence on the dissipative phenomena. However, following the adiabatic evolution in subsequent epochs we find that models with large anisotropic curvature and high dissipation are not consistent with the isotropy of the Hubble expansion inferred from the observations of the cosmic background radiation. We conclude that our universe is either remarkably flat or nondissipative, the only exception being the particular case of isotropically curved open spaces.

Neutrino viscosity and isotropization of Bianchi type-VIII and type-IX cosmological models

We investigate the isotropization of Bianchi type-VIII and type-IX spaces due to neutrino viscosity. We find that, owing to a peculiar coupling between curvature and anisotropy, neutrino viscosity does not guarantee the present-day isotropy of the cosmological expansion in such spaces. The observed isotropy of the background radiation imposes stringent limits either on the initial anisotropy or on the curvature of space, in contrast with the principle of chaotic cosmology. On the other hand, the amplification of the radiation entropy depends weakly on curvature and may be as large as 1012.

The isotropization of low-density cosmological models

We analyse the evolution of Bianchi type-VII h cosmological models during the lepton era and the subsequent adiabatic epochs. We find that these model cannot generally attain to the low-density regime with a reasonable anisotropy level because of a curvature—anisotropy coupling. The primordial damping due to neutrino viscosity, although very effective for the entropy production, cannot help to match to isotropy requirement at the present epoch.

Neutrino viscosity in Bianchi type-V universes

We extend the analysis of neutrino viscosity to open cosmological models. We find that in type-V spaces, contrary to the results found in closed spaces, viscous dissipation guarantees the isotropy of the universe at the present epoch.

Neutrino viscosity in Bianchi type IX universes

We investigate the isotropization of anisotropic closed spaces and the entropy production due to neutrino viscosity.

On neutrino viscosity in collapsing stellar cores

If, as current numerical models show, neutrinos get trapped and become degenerate in the central region of collapsing stellar cores, then they should significantly contribute to their shear viscosity. The magnitude of neutrino viscosity (approx.10/sup 11/ cm/sup 2/ s/sup -1/) is such that it would tend to make a fast rotating stellar core stable beyond the point of bifurcation. The bulk viscosity of such systems is also found to be big, and of the same order of magnitude. The above effects, along with thermal conduction, will tend to damp out acoustical waves generated in a collapsing stellar core after its hydrodynamic bounce.

Weak interactions in astrophysics and cosmology

There are many problems in astrophysics and cosmology in which the form of the weak interactions, their strength or the number of weakly interacting particles, is very important. It is possible that astronomical observations may give some information about the weak interactions. In the conventional hot big bang cosmological theory the number of leptons with associated neutrinos influences the speed of expansion of the Universe and the chemical composition of pre-galactic matter. The strength of the weak interaction, as exemplified by the half-life of the neutron, has a similar effect. In addition, the form of the weak interactions will determine how effectively neutrino viscosity can smooth out irregularities in the early Universe. Because neutrinos have a very long mean free path, they can escape from the central region of stars whereas photons can only escape from the surface. In late stages of stellar evolution, neutrino luminosity is often believed to be much greater than photon luminosity.

The Evolution of Anisotropy in Nonrotating Bianchi Type V Cosmologies

view Abstract Citations (44) References (5) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Evolution of Anisotropy in Nonrotating Bianchi Type V Cosmologies Matzner, Richard A. Abstract We consider the effect of neutrino viscosity and of free neutrinos in universes of Bianchi types I and V. We discuss the collisionless Boltzmann equation (which gives the evolution of the neutrino-distribu~ tion function) for these types and compare the type V results with those for type I previously given by Misner. \7tje assume that dissipative effects lower pa/p (the ratio of the anisotropy energy density to the matter energy density, both measured by their gravitational effect on the expansion) to a number of order unity before the electron neutrinos become collisionless. We are unable to integrate the null geodesics completely in type V, so we are unable to present a complete numerical example of the dynamics of the model with collisionless radiation. We do, however, present an approximate model universe of type V which begins its free evolution with p~ = p at a tem- perature of `-~-`1O'° ° K. The approximation lies in an assumption of small j3 for the subsequent motion (where j3 is a parameter describing the anisotropy). This model predicts L~.T/T `-~` 2 X 1O-~ for the varia- tion in the primordial radiation temperature, if one assumes a switchover from radiation domination to matter domination at T `-S-' 3000 K. This variation L~T/T is of the same order as that found previously for type I and type IX by Misner. There is, however, a new feature, since there is an asymmetry (in the strength of the anisotropy) between opposite hemispheres. Depending on the current relative sizes of p~ (the matter density) and R* (the 3-space curvature), this asymmetry can be quite strong. An estimate taking parameters consistent with current observations suggests an excess of < 5-10 in the amplitude of the anisotropy in the hemisphere 0> ir/2, compared with the hemisphere 8 <ir/2, for the epoch when p~1 R* `~ 1. (The current epoch has Pm1 R* <1. Publication: The Astrophysical Journal Pub Date: September 1969 DOI: 10.1086/150138 Bibcode: 1969ApJ...157.1085M full text sources ADS |

Non-Equilibrium Processes in the Early Universe

In a recent paper Misner (1968) has suggested that neutrino viscosity is a highly efficient process for removing shear anisotropy during the early stages of the universe, so that the remarkable degree of isotropy observed in the microwave background would be expected to occur whatever the initial conditions. In this paper we suggest that the early universe may not have been in near thermal equilibrium as Misner assumed, so that under a wide range of initial conditions, arbitrarily large anisotropy could occur at any epoch. Therefore, the observed anisotropy does place some restrictions on early conditions in the universe.