Cosmic voids are magnetized at the level of at least 10⁻¹⁷ G on Mpc scales, as implied by blazar observations. We show that an electrically conducting plasma is present in the voids, and that, because of the plasma, diffusion into the voids of galactic fields generated by a mean-field dynamo is far too slow to explain the present-day void magnetization. Indeed, we show that even in the presence of turbulence in the voids, dynamo-generated galactic fields diffuse out to a galactocentric radius of only 200–400 kpc. Therefore, it is challenging to meet the required volume filling-factor of the void magnetic field. We conclude that a primordial origin remains the most natural explanation to the space-filling weak fields in voids.

In an electron-positron plasma, an imbalance in the number of right-and left-chiral particles can lead to the growth of a helical magnetic field through a phenomenon called the chiral plasma instability (CPI). In the early Universe, scattering reactions that violate chirality come into thermal equilibrium when the plasma cools below a temperature of approximately 80 TeV. Since these reactions tend to relax any preexisting chiral asymmetry to zero as the system approaches equilibrium, the standard lore is that primordial magnetogenesis via the CPI is not viable below 80 TeV. In this work, we propose that the presence of a source for chirality can allow the CPI to operate even below 80 TeV, we explore the implications of this scenario, and we derive predictions for the resultant magnetic field helicity using a combination of analytical methods and direct numerical simulation.

The inverse cascade in MHD turbulence plays a crucial role in various astrophysical processes such as galaxy cluster formation, solar and stellar dynamo mechanisms, and the evolution of primordial magnetic fields in the early Universe. A standard numerical approach involves injecting magnetic helicity at intermediate length scales to generate a secondary, time-dependent spectral peak that gradually propagates toward larger scales. Previous simulations have already suggested a resistive dependence of inverse transfer rates and demonstrated the significant influence of magnetic helicity flux density & varepsilon;H on this process. On dimensional grounds, we have for the spectral envelope EM(k,t)<= CH & varepsilon;H2/3k-1 , where CH represents a potentially universal dimensionless coefficient analogous to the Kolmogorov constant. We present a summary of the 25 distinct simulations conducted with the Pencil Code, systematically varying the forcing wavenumber kf, magnetic Prandtl number PrM, grid resolution N3, and Lundquist number Lu. We obtained CH and corresponding error bars by calculating the compensated spectrum and investigated its dependence with Lu and kf. For the CH-Lu relationship, we observe strong correlations with a power-law exponent around unity. In contrast, we find no significant correlation between CH and kf. We also present evidence for a nonresistive scaling of the form EM(k,t)<= C2/3vA2k-1 , where vA is the rms Alfv & eacute;n speed and C2/3 approximate to 0.4 for all of our runs.

Silica fume is a valuable by-product of silicon and ferrosilicon production. It is therefore important to understand the impact on the silica fume quality when converting the furnace feed from fossil-based to renewable reduction materials. Using self-consistent numerical simulations of the nucleation and condensation process, we present a detailed study of the silica fume formation process. It is found that the most critical physical effect that determines the final particle size distribution is coalescence due to Brownian motion. Furthermore, it is crucial to use appropriate thermophysical parameters in order to reproduce reliable particle size distributions. Contrary to what has been done in previous studies on the same topic, this is now done by using reasonable expressions for surface energy, saturation pressure and the nucleation pre-exponential factor. It is also found that under conditions relevant to furnaces, the liberation of latent heat leads to an explosive chain reaction of particle nucleation and condensation when the first particles nucleate and start growing due to condensation. This process continues until the relative saturation pressure of silicon dioxide is reduced to unity. Finally, it is found that the Lagrangian approach for particle tracking is more flexible and accurate, and also more CPU efficient, than the Eulerian approach.

Turbulent flows are known to produce enhanced effective magnetic and passive scalar diffusivities, which can fairly accurately be determined with numerical methods. It is now known that, if the flow is also helical, the effective magnetic diffusivity is reduced relative to the nonhelical value. Neither the usual second-order correlation approximation nor the various τ approaches have been able to capture this. Here we show that the helicity effect on the turbulent passive scalar diffusivity works in the opposite sense and leads to an enhancement. We have also demonstrated that the correlation time of the turbulent velocity field increases with the kinetic helicity. This is a key point in the theoretical interpretation of the obtained numerical results. Simulations in which helicity is being produced self-consistently by stratified rotating turbulence resulted in a turbulent passive scalar diffusivity that was found to be decreasing with increasing rotation rate.

We estimate the magnetic field in the jets of the recently discovered 7 Mpc long Porphyrion system. We used nondetection of the system in gamma-rays to derive a lower bound on the co-moving magnetic field strength at the level of 10 nG (comoving). This value is consistent with recent estimates of magnetic fields in the filaments of the large-scale structure.We discuss the possibility that instead of being the extreme case of a radio jet formation scenario, Porphyrion actually traces a very high-energy -ray beam emitted by an active galactic nucleus. In such a model, jets do not need to spread into the voids of the large-scale structure to appear straight on a very large distance range, and several anomalies of the standard radio jet scenarios can be solved at once.

Powerful lasers may be used in the future to produce magnetic fields that would allow us to study turbulent magnetohydrodynamic inverse cascade behaviour. This has so far only been seen in numerical simulations. In the laboratory, however, the produced fields may be highly anisotropic. Here, we present corresponding simulations to show that, during the turbulent decay, such a magnetic field undergoes spontaneous isotropisation. As a consequence, we find the decay dynamics to be similar to that in isotropic turbulence. We also find that an initially pointwise non-helical magnetic field is unstable and develops magnetic helicity fluctuations that can be quantified by the Hosking integral. It is a conserved quantity that characterises magnetic helicity fluctuations and governs the turbulent decay when the mean magnetic helicity vanishes. As in earlier work, the ratio of the magnetic decay time to the Alfv & eacute;n time is found to be approximately $50$ in the helical and non-helical cases. At intermediate times, the ratio can even reach a hundred. This ratio determines the endpoints of cosmological magnetic field evolution.

Understanding the origin and evolution of magnetic fields on cosmological scales opens up a window into the physics of the early Universe. Numerical simulations of such fields require a careful treatment to faithfully solve the equations of magnetohydrodynamics (MHD) without introducing numerical artefacts. In this paper, we study the growth of the magnetic fields in controlled kinematic dynamo set-ups using both smoothed particle hydrodynamics implementations in the swift code. We assess the quality of the reconstructed solution in the Roberts flow case against the reference implementation in the pencil code and find generally a good agreement. Similarly, we reproduce the known features of the more complex Arnold-Beltrami-Childress (ABC) flow. Using a simple induction-diffusion balance model to analyse the results, we construct an 'overwinding' trigger metric to locally detect regions where the magnetic diffusion cannot counteract the expected induction because of limitations in the method's ability to resolve magnetic field gradients. This metric is then used to identify the necessary resolution and resistivity levels to counteract the overwinding problem. We finally apply this metric to adiabatic cosmological simulations and discuss the resolution requirements needed to resolve the growth of the primordial fields without artefacts.

We numerically study axion-U(1) inflation, focusing on the regime where the coupling between axions and gauge fields results in significant backreaction from the amplified gauge fields during inflation. These amplified gauge fields not only generate high-frequency gravitational waves (GWs), but also enhance spatial inhomogeneities in the axion field. GWs serve as key probe for constraining the coupling strength between the axion and gauge fields. We find that, when backreaction is important during inflation, the constraints on the coupling strength due to GW overproduction are relaxed compared to previous studies, in which backreaction matters only after inflation. Moreover, our results suggest that the probability density function (PDF) of axion fluctuations tends toward a Gaussian distribution even in cases where gauge field backreaction is important only after inflation. This aligns with previous studies where the same effect was observed for cases with strong backreaction during inflation. This finding can be crucial for future studies of primordial black hole (PBH) formation, which can further constrain the coupling strength. We also calculate the spectrum of the produced magnetic fields in this model and find that their strength is compatible with the observed lower limits.

The question of whether a dynamo can be triggered by gravitational collapse is of great interest, especially for the early Universe. Here, we employ supercomoving coordinates to study the magnetic field amplification from decaying turbulence during gravitational collapse. We perform 3D simulations and show that for large magnetic Reynolds numbers, there can be exponential growth of the comoving magnetic field with conformal time before the decay of turbulence impedes further amplification. The collapse dynamics only affect the nonlinear feedback from the Lorentz force, which diminishes more rapidly for shorter collapse times, allowing nearly kinematic continued growth. We confirm that helical turbulence is more efficient in driving dynamo action than nonhelical turbulence, but this difference decreases for larger collapse times. We also show that for nearly irrotational flows, dynamo amplification is still possible, but it is always associated with a growth of vorticity—even if it still remains very small. In nonmagnetic runs, the growth of vorticity is associated with viscosity and grows with the Mach number. In the presence of magnetic fields, vorticity emerges from the curl of the Lorentz force. During a limited time interval, an exponential growth of the comoving magnetic field with conformal time is interpreted as clear evidence of dynamo action.