We study limits on a primordial magnetic field arising from cosmological data, including that from big bang nucleosynthesis, cosmic microwave background polarization plane Faraday rotation limits, and large-scale structure formation. We show that the physically-relevant quantity is the value of the effective magnetic field, and limits on it are independent of how the magnetic field was generated.Comment: 7 pages, 6 figure

The Hubble tension between the ΛCDM-model-dependent prediction of the current expansion rate H0 using Planck data and direct, model-independent measurements in the local universe from the SH0ES collaboration disagree at >3.5σ. Moreover, there exists a milder ∼ 2σ tension between similar predictions for the amplitude S8 of matter fluctuations and its measurement in the local universe. As explanations relying on unresolved systematics have not been found, theorists have been exploring explanations for these anomalies that modify the cosmological model, altering early-universe-based predictions for these parameters. However, new cosmological models that attempt to resolve one tension often worsen the other. In this paper, we investigate a decaying dark matter (DDM) model as a solution to both tensions simultaneously. Here, a fraction of dark matter density decays into dark radiation. The decay rate Γ is proportional to the Hubble rate H through the constant αdr, the only additional parameter of this model. Then, this model deviates most from ΛCDM in the early universe, with αdr being positively correlated with H0 and negatively with S8. Hence, increasing αdr (and allowing dark matter to decay in this way) can then diminish both tensions simultaneously. When only considering Planck CMB data and the local SH0ES prior on H0, ∼ 1% dark matter decays, decreasing the S8 tension to 0.3σ and increasing the best-fit H0 by 1.6 km/s/Mpc. However, the addition of intermediate-redshift data (the JLA supernova dataset and baryon acoustic oscillation data) weakens the effectiveness of this model. Only ∼ 0.5% of the dark matter decays bringing the S8 tension back up to ∼ 1.5 σ and the increase in the best-fit H0 down to 0.4 km/s/Mpc.

From previous studies of the effect of primordial magnetic fields on early structure formation, we know that the presence of primordial magnetic fields during early structure formation could induce more perturbations at small scales (at present 1-10 h −1 Mpc) as compared to the usual ΛCDM theory. Matter power spectra over these scales are effectively probed by cosmological observables such as shear correlation and Lyα clouds. In this paper we discuss the implications of primordial magnetic fields on the distribution of Lyα clouds. We simulate the line-ofsight density fluctuation including the contribution coming from the primordial magnetic fields. We compute the evolution of Lyα opacity for this case and compare our theoretical estimates of Lyα opacity with the existing data to constrain the parameters of the primordial magnetic fields. We also discuss the case when the two density fields are correlated. Our analysis yields an upper bound of roughly 0.3-0.6 nG on the magnetic field strength for a range of nearly scale-invariant models, corresponding to a magnetic field power spectrum index n −3.

It has been proposed that primordial gas in early dark matter halos, with virial temperatures T vir ∼ > 10 4 K, can avoid fragmentation and undergo rapid collapse, possibly resulting in a supermassive black hole (SMBH). This requires the gas to avoid cooling and to remain at temperatures near T ∼ 10 4 K. We show that this condition can be satisfied in the presence of a sufficiently strong primordial magnetic field, which heats the collapsing gas via ambipolar diffusion. If the field has a strength above | B | ∼ > 3.6 (comoving) nG, the collapsing gas is kept warm (T ∼ 10 4 K) until it reaches the critical density n crit ≈ 10 3 cm −3 at which the roto-vibrational states of H 2 approach local thermodynamic equilibrium. H 2 -cooling then remains inefficient, and the gas temperature stays near ∼ 10 4 K, even as it continues to collapse to higher densities. The critical magnetic field strength required to permanently suppress H 2 -cooling is somewhat higher than upper limit of ∼ 2nG from the cosmic microwave background (CMB). However, it can be realized in the rare ∼ > (2 − 3)σ regions of the spatially fluctuating B-field; these regions contain a sufficient number of halos to account for the z ≈ 6 quasar BHs.

Yes, for a wide range of cosmological models (ΛCDM, non-interacting w z CDM, w z WDM, or a class of interacting DMDE). Recently there have been attempts to solve the tension between direct measurements of H 0 and σ 8 √ Ω 0m from respective low redshift observables and indirect measurements of these quantities from the CMB observations. In this work we construct a quasi-model independent framework that reduces to different classes of cosmological models under suitable parameters choices. We test this parameterization against the latest Planck CMB data combined with recent BAO, SNe and direct H 0 measurements. Our analysis reveals that a strong positive correlation between H 0 and σ 8 is more or less generic for most of the cosmological model. The present data slightly prefers a phantom equation of state for DE and a slightly negative effective equation of state for DM (a direct signature of interacting models), with a relatively high H 0 consistent with Planck+R16 data and, simultaneously, a consistent Ω 0m . Thus, even though the tensions cannot be fully resolved, a class of interacting models with phantom w DE get a slight edge over w z CDM for present data. However, although they may resolve the tension between high redshift CMB data and individual low redshift datasets, these datasets have inconsistencies between them (e.g., between BAO and H 0 , SNe and BAO, and cluster counts and H 0 ).