Magnetogenesis Models Research Articles

Inflationary magnetogenesis with a self-consistent coupling function

In this paper, we discuss the inflationary magnetogenesis scenario, in which the coupling function is introduced to break the conformal invariance of electromagnetic action. Unlike in conventional models, we deduce the Maxwell’s equations under the perturbed Friedmann–Robertson–Walker metric. We found that the self-consistency of the action depends on the form of the coupling function when the scalar mode perturbations have been considered. Therefore, this self-consistency can be seen as a restriction on the coupling function. In this paper, we give the restrictive equation for coupling function then obtain the specific form of the coupling function in a simple model. We found that the coupling function depends on the potential of the inflaton and thus is model-dependent. We obtain the power spectrum of electric field and magnetic field in large-field inflation model. We also found that the coupling function is an increasing function of time during slow-roll era as most of inflationary magnetogenesis models, it will lead to strong coupling problem. This issue is discussed qualitatively by introducing a correction function during the preheating.

Magnetogenesis and the Cosmic Web: A Joint Challenge for Radio Observations and Numerical Simulations

The detection of the radio signal from filaments in the cosmic web is crucial to distinguish possible magnetogenesis scenarios. We review the status of the different attempts to detect the cosmic web at radio wavelengths. This is put into the context of the advanced simulations of cosmic magnetism carried out in the last few years by our MAGCOW project. While first attempts of imaging the cosmic web with the MWA and LOFAR have been encouraging and could discard some magnetogenesis models, the complexity behind such observations makes a definitive answer still uncertain. A combination of total intensity and polarimetric data at low radio frequencies that the SKA and LOFAR2.0 will achieve is key to removing the existing uncertainties related to the contribution of many possible sources of signal along deep lines of sight. This will make it possible to isolate the contribution from filaments, and expose its deep physical connection with the origin of extragalactic magnetism.

Imprints of the post-recombination dissipation of helical magnetic field on the Cosmic Microwave Background Radiation

Astrophysical magnetic fields decay primarily via two processes, namely ambipolar diffusion and turbulence. Constraints on the strength and the spectral index of nonhelical magnetic fields have been derived earlier in the literature through the effect of the above-mentioned processes on the cosmic microwave background (CMB) radiation. A helical component of the magnetic field is also produced in various models of magnetogenesis, which can explain larger coherence length magnetic field. In this study, we focus on studying the effects of post-recombination decay of maximally helical magnetic fields through ambipolar diffusion and decaying magnetic turbulence and the impact of this decay on CMB. We find that helical magnetic fields lead to changes in the evolution of baryon temperature and ionization fraction which in turn lead to modifications in the CMB temperature and polarization anisotropy. These modifications are different from those arising due to nonhelical magnetic fields with the changes dependent on the strength and the spectral index of the magnetic field power spectra.

Constraints on primordial magnetic fields from Planck data combined with the South Pole Telescope CMBB-mode polarization measurements

A primordial magnetic field (PMF) present before recombination can leave specific signatures on the cosmic microwave background (CMB) fluctuations. Of particular importance is its contribution to the B-mode polarization power spectrum. Indeed, vortical modes sourced by the PMF can dominate the B-mode power spectrum on small scales, as they survive damping up to a small fraction of the Silk length. Therefore, measurements of the B-mode polarization at high-$\ell$ , such as the one recently performed by the South Pole Telescope (SPT), have the potential to provide stringent constraints on the PMF. We use the publicly released SPT B-mode polarization spectrum, along with the temperature and polarization data from the Planck satellite, to derive constraints on the magnitude, the spectral index and the energy scale at which the PMF was generated. We find that, while Planck data constrains the magnetic amplitude to $B_{1 \, \text{Mpc}} < 3.3$ nG at 95\% confidence level (CL), the SPT measurement improves the constraint to $B_{1 \, \text{Mpc}} < 1.5$ nG. The magnetic spectral index, $n_B$, and the time of the generation of the PMF are unconstrained. For a nearly scale-invariant PMF, predicted by simplest inflationary magnetogenesis models, the bound from Planck+SPT is $B_{1 \, \text{Mpc}} < 1.2$ nG at 95% CL. For PMF with $n_B=2$, expected for fields generated in post-inflationary phase transitions, the 95% CL bound is $B_{1 \, \text{Mpc}} < 0.002$ nG, corresponding to the magnetic fraction of the radiation density $\Omega_{B\gamma} < 10^{-3}$ or the effective field $B_{\rm eff} < 100$ nG. The patches for the Boltzmann code CAMB and the Markov Chain Monte Carlo engine CosmoMC, incorporating the PMF effects on CMB, are made publicly available.

Issues on the inflationary magnetogenesis

Recent observations of high-energy photons from blazars suggest that there exist magnetic fields with typical amplitude around 10 − 15 G ubiquitously even in void regions. This being the case, it is natural to invoke them to explain the processes occurring during inflation in the early universe. We provide a list of models of magnetogenesis during inflation and consider several problems associated with them. L'observation récente de photons ultra-énergétiques en provenance des blazars suggère l'existence de champs magnétiques cosmologiques, d'amplitude typique 10 − 15 G , omniprésents, y compris au sein des vides cosmiques. Il est dès lors naturel de les associer à la physique inflationnaire dans l'univers primordial. Nous donnons une liste de modèles de génération d'un champ magnétique pendant l'inflation et discutons un certain nombre de problèmes qu'ils posent.

A scenario for inflationary magnetogenesis without strong coupling problem

Cosmological magnetic fields pervade the entire universe, from small to large scales. Since they apparently extend into the intergalactic medium, it is tantalizing to believe that they have a primordial origin, possibly being produced during inflation. However, finding consistent scenarios for inflationary magnetogenesis is a challenging theoretical problem. The requirements to avoid an excessive production of electromagnetic energy, and to avoid entering a strong coupling regime characterized by large values for the electromagnetic coupling constant, typically allow one to generate only a tiny amplitude of magnetic field during inflation. We propose a scenario for building gauge-invariant models of inflationary magnetogenesis potentially free from these issues. The idea is to derivatively couple a dynamical scalar, not necessarily the inflaton, to fermionic and electromagnetic fields during the inflationary era. Such couplings give additional freedom to control the time-dependence of the electromagnetic coupling constant during inflation. This fact allows us to find conditions to avoid the strong coupling problems that affect many of the existing models of magnetogenesis. We do not need to rely on a particular inflationary set-up for developing our scenario, that might be applied to different realizations of inflation. On the other hand, specific requirements have to be imposed on the dynamics of the scalar derivatively coupled to fermions and electromagnetism, that we are able to satisfy in an explicit realization of our proposal.

Schwinger effect in 4D de Sitter space and constraints on magnetogenesis in the early universe

We investigate pair creation by an electric field in four-dimensional de Sitter space. The expectation value of the induced current is computed, using the method of adiabatic regularization. Under strong electric fields the behavior of the current is similar to that in flat space, while under weak electric fields the current becomes inversely proportional to the mass squared of the charged field. Thus we find that the de Sitter space obtains a large conductivity under weak electric fields in the presence of a charged field with a tiny mass. We then apply the results to constrain electromagnetic fields in the early universe. In particular, we study cosmological scenarios for generating large-scale magnetic fields during the inflationary era. Electric fields generated along with the magnetic fields can induce sufficiently large conductivity to terminate the phase of magnetogenesis. For inflationary magnetogenesis models with a modified Maxwell kinetic term, the generated magnetic fields cannot exceed 10^{-30} G on Mpc scales in the present epoch, when a charged field carrying an elementary charge with mass of order the Hubble scale or smaller exists in the Lagrangian. Similar constraints from the Schwinger effect apply for other magnetogenesis mechanisms.

Probing correlations of early magnetic fields using μ-distortion

The damping of a non-uniform magnetic field between the redshifts of about 104 and 106 injects energy into the photon-baryon plasma and causes the CMB to deviate from a perfect blackbody spectrum, producing a so-called μ-distortion. We can calculate the correlation ⟨μ T⟩ of this distortion with the temperature anisotropy T of the CMB to search for a correlation ⟨ B2ζ⟩ between the magnetic field B and the curvature perturbation ζ; knowing the ⟨ B2ζ⟩ correlation would help us distinguish between different models of magnetogenesis. Since the perturbations which produce the μ-distortion will be much smaller scale than the relevant density perturbations, the observation of this correlation is sensitive to the squeezed limit of ⟨ B2ζ⟩, which is naturally parameterized by bNL (a parameter defined analogously to fNL). We find that a PIXIE-like CMB experiments has a signal to noise S/N≈ 1.0 × bNL (B̃μ/10nG)2, where B̃μ is the magnetic field's strength on μ-distortion scales normalized to today's redshift; thus, a 10 nG field would be detectable with bNL=\U0001d4aa(1). However, if the field is of inflationary origin, we generically expect it to be accompanied by a curvature bispectrum ⟨ζ3⟩ induced by the magnetic field. For sufficiently small magnetic fields, the signal ⟨ B2 ζ⟩ will dominate, but for B̃μ≳ 1 nG, one would have to consider the specifics of the inflationary magnetogenesis model.We also discuss the potential post-magnetogenesis sources of a ⟨ B2ζ⟩ correlation and explain why there will be no contribution from the evolution of the magnetic field in response to the curvature perturbation.

Inflationary magnetogenesis without the strong coupling problem II:constraints from CMB anisotropies and B-modes

Recent observational claims of magnetic fields stronger than 10−16 G in the extragalactic medium motivate a new look for their origin in the inflationary magnetogenesis models. In this work we shall review the constraints on the simplest gauge invariant model f2(ϕ)FμνFμν of inflationary magnetogenesis, and show that in the optimal region of parameter space the anisotropic constraints coming from the induced bispectrum, due to the generated electromagnetic fields, yield the strongest constraints. In this model, only a very fine tuned scenario at an energy scale of inflation as low as 10−2 GeV can explain the observations of void magnetic fields. These findings are consistent with the recently derived upper bound on the inflationary energy scale. However, if the detection of primordial tensor modes by BICEP2 is confirmed, the possibility of low scale inflation is excluded. Assuming the validity of the BICEP2 claim of a tensor-to-scalar ratio r = 0.2+0.07−0.05, we provide the updated constraints on this model of inflationary magnetogenesis. On the Mpc scale, we find that the maximal allowed magnetic field strength from inflation is less than 10−30 G.

Critical constraint on inflationary magnetogenesis

Recently, there are several reports that the cosmic magnetic fields on Mpc scalein void region is larger than ∼ 10−15G withan uncertainty of a few orders from the current blazar observations.On the other hand, in inflationary magnetogenesis models,additional primordial curvature perturbations are inevitably produced fromiso-curvature perturbationsdue to generated electromagnetic fields.We explore such induced curvature perturbationsin a model independent wayandobtained a severe upper bound for the energy scale of inflationfrom the observed cosmic magnetic fields andthe observed amplitude of the curvature perturbation, as ρinf1/4 < 300MeV × (Bobs/10−15G)−1 where Bobs is the strength of the magnetic field at present.Therefore, without a dedicated low energy inflation model oran additional amplification of magnetic fields after inflation,inflationary magnetogenesis on Mpc scaleis generally incompatible with CMB observations.

MAGNETIC FIELDS FROM QCD PHASE TRANSITIONS

We study the evolution of QCD phase transition-generated magnetic fields in freely decaying MHD turbulence of the expanding Universe. We consider a magnetic field generation model that starts from basic non-perturbative QCD theory and predicts stochastic magnetic fields with an amplitude of the order of 0.02 $\mu$G and small magnetic helicity. We employ direct numerical simulations to model the MHD turbulence decay and identify two different regimes: "weakly helical" turbulence regime, when magnetic helicity increases during decay, and "fully helical" turbulence, when maximal magnetic helicity is reached and an inverse cascade develops. The results of our analysis show that in the most optimistic scenario the magnetic correlation length in the comoving frame can reach 10 kpc with the amplitude of the effective magnetic field being 0.007 nG. We demonstrate that the considered model of magneto-genesis can provide the seed magnetic field for galaxies and clusters.

Universal upper limit on inflation energy scale from cosmicmagnetic field

Recently observational lower bounds on the strength of cosmic magneticfields were reported, based on γ-ray flux from distant blazars. If inflation is responsible for the generation of such magneticfields then the inflation energy scale is bounded from above as ρinf1/4 < 2.5 × 10−7MPl × (Bobs/10−15G)−2 in a wide class of inflationary magnetogenesis models, where Bobs is the observed strength of cosmic magnetic fields. The tensor-to-scalar ratio is correspondingly constrained as r < 10−19 × (Bobs/10−15G)−8. Therefore, if the reported strength Bobs ⩾ 10−15G is confirmed and if anysignatures of gravitational waves from inflation are detected in thenear future, then our result indicates some tensions betweeninflationary magnetogenesis and observations.

Reheating constraints in inflationary magnetogenesis

Among primordial magnetogenesis models, inflation is a prime candidate to explain the current existence of cosmological magnetic fields. Assuming conformal invariance to be restored after inflation, their energy density decreases as radiation during the decelerating eras of the universe, and in particular during reheating. Without making any assumptions on inflation, on the magnetogenesis mechanism and on how the reheating proceeded, we show that requiring large scale magnetic fields to remain subdominant after inflation gives non-trivial constraints on both the reheating equation of state parameter and the reheating energy scale. In terms of the so-called reheating parameter, we find that ln Rrad > −10.1 for large scale magnetic fields of the order 5 × 10−15 Gauss today. This bound is then compared to those already derived from Cosmic Microwave Background (CMB) data by assuming a specific inflationary model. Avoiding magnetic field backreaction is always complementary to CMB and can give more stringent limits on reheating for all high energy models of inflation. For instance, a large field matter dominated reheating cannot take place at an energy scale lower than typically 500 GeV if the magnetic field strength today is B0 = 5 × 10−15 G, this scale going up to 1010 GeV if B0 = 10−9 G.

MAGNETIC FIELD GENERATION IN HIGGS INFLATION MODEL

We study the generation of magnetic field in Higgs inflation models where the Standard Model Higgs boson has a large coupling to the Ricci scalar. We couple the Higgs field to the electromagnetic fields via a nonrenormalizable dimension six operator suppressed by the Planck scale in the Jordan frame. We show that by choosing the Higgs coupling λ(MZ) = 0.132 (which corresponds to mh = 126 GeV in keeping with the recent measurements by ATLAS and CMS) and curvature coupling ξ(MZ) = 103 we can generate comoving magnetic fields of 10-7 Gauss at present and comoving coherence length of 100 kpc. The problem of large back-reaction which is generic in the usual inflation models of magneto-genesis is avoided as the back-reaction is suppressed by the large Higgs-curvature coupling.

Evidence for Strong Extragalactic Magnetic Fields from Fermi Observations of TeV Blazars

Magnetic fields in galaxies are produced via the amplification of seed magnetic fields of unknown nature. The seed fields, which might exist in their initial form in the intergalactic medium, were never detected. We report a lower bound B > or = 3 x 10(-16) gauss on the strength of intergalactic magnetic fields, which stems from the nonobservation of GeV gamma-ray emission from electromagnetic cascade initiated by tera-electron volt gamma rays in intergalactic medium. The bound improves as lambdaB(-1/2) if magnetic field correlation length, lambdaB, is much smaller than a megaparsec. This lower bound constrains models for the origin of cosmic magnetic fields.

Magnetogenesis, spectator fields and CMB signatures

A viable class of magnetogenesis models can be constructed by coupling the kinetic term of the hypercharge to a spectator field whose dynamics does not affect the inflationary evolution. The magnetic power spectrum is explicitly related to the power spectrum of (adiabatic) curvature inhomogeneities when the quasi-de Sitter stage of expansion is driven by a single scalar degree of freedom. Depending upon the value of the slow-roll parameters, the amplitude of smoothed magnetic fields over a (comoving) Mpc scale can be as large as 0.01–0.1 nG at the epoch of the gravitational collapse of the protogalaxy. The contributions of the magnetic fields to the Sachs–Wolfe plateau and to the temperature autocorrelations in the Doppler region compare favourably with the constraints imposed by galactic magnetogenesis. Stimulating lessons are drawn on the interplay between magnetogenesis models and their possible CMB signatures.

LARGE SCALE MAGNETOGENESIS THROUGH RADIATION PRESSURE

We present a new model for the generation of magnetic fields on large scales occurring at the end of cosmological reionisation. The inhomogeneous radiation provided by luminous sources and the fluctuations in the matter density field are the major ingredients of the model. More specifically, differential radiation pressure acting on ions and electrons gives rise to electric currents which induce magnetic fields on large scales. We show that on protogalactic scales, this process is highly efficient, leading to magnetic field amplitudes of the order of Gauss. While remaining of negligible dynamical impact, those amplitudes are million times higher than those obtained in usual astrophysical magnetogenesis models. Finally, we derive the relation between the power spectrum of the generated field and the one of the matter density fluctuations. We show in particular that magnetic fields are preferably created on large (galactic or cluster) scales. Small scale magnetic fields are strongly disfavoured, which further makes the process we propose an ideal candidate to explain the origin of magnetic fields in large scale structures.

Cosmological magnetogenesis driven by radiation pressure

The origin of large scale cosmological magnetic fields remains a mystery, despite the continuous efforts devoted to that problem. We present a new model of magnetic field generation, based on local charge separation provided by an anisotropic and inhomogeneous radiation pressure. In the cosmological context, the processes we explore take place at the epoch of the reionization of the Universe. Under simple assumptions, we obtain results (i) in terms of the order of magnitude of the field generated at large scales and (ii) in terms of its power spectrum. The amplitudes obtained $(B\ensuremath{\sim}8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\ensuremath{\mu}\mathrm{G})$ are considerably higher than those obtained in usual magnetogenesis models and provide suitable seeds for amplification by adiabatic collapse and/or dynamo during structure formation.