Non-Gaussianity from inflation: theory and observations

Magnetic fields appear everywhere in the universe From stars and galaxies, all the way to galaxy clusters and remote protogalactic clouds magnetic fields of considerable strength and size have been repeatedly observed Despite their widespread presence, however, the origin of cosmic magnetic fields is still a mystery The galactic dynamo is believed capable of amplifying weak magnetic seeds to strengths like those measured in ours and other galaxies, but the question is where do these seed fields come from? Are they a product of late, post-recombination, physics or are they truly cosmological in origin? The idea of primordial magnetism is attractive because it makes the large-scale magnetic fields, especially those found in early protogalactic systems, easier to explain As a result, a host of different scenarios have appeared in the literature Nevertheless, early magnetogenesis is not problem free, with a number of issues remaining open and a matter of debate We review the question of primordial magnetic fields and consider the limits set on their strength by the current observational data The various mechanisms of pre-recombination magnetogenesis are presented and their advantages and shortcomings are debated We consider both classical and quantum scenarios, that operate within as well as outside the standard model, and also discuss how future observations could be used to decide whether the large-scale magnetic fields we see in the universe today are truly primordial or not

Primordial magnetogenesis

No, it is simply not plausible that cosmic acceleration could arise within the context of general relativity from a back-reaction effect of inhomogeneities in our universe, without the presence of a cosmological constant or 'dark energy'. We point out that our universe appears to be described very accurately on all scales by a Newtonianly perturbed FLRW metric. (This assertion is entirely consistent with the fact that we commonly encounter δρ/ρ > 1030.) If the universe is accurately described by a Newtonianly perturbed FLRW metric, then the back-reaction of inhomogeneities on the dynamics of the universe is negligible. If not, then it is the burden of an alternative model to account for the observed properties of our universe. We emphasize with concrete examples that it is not adequate to attempt to justify a model by merely showing that some spatially averaged quantities behave the same way as in FLRW models with acceleration. A quantity representing the 'scale factor' may 'accelerate' without there being any physically observable consequences of this acceleration. It also is not adequate to calculate the second-order stress–energy tensor and show that it has a form similar to that of a cosmological constant of the appropriate magnitude. The second-order stress–energy tensor is gauge dependent, and if it were large, contributions of higher perturbative order could not be neglected. We attempt to clear up the apparent confusion between the second-order stress–energy tensor arising in perturbation theory and the 'effective stress–energy tensor' arising in the 'shortwave approximation'.

https://iopscience.iop.org/article/10.1088/0264-9381/23/1/012/pdf

Can the acceleration of our universe be explained by the effects of inhomogeneities

https://researchportal.port.ac.uk/portal/files/128942/0611537.pdf

Cosmology with inhomogeneous magnetic fields

The authors propose a new covariant and gauge-invariant (GI) treatment of perturbations in a Robertson-Walker universe dominated by a classical scalar field phi . They first set up the formalism, based on the natural slicing of the problem by the surfaces phi =constant, and introduce a set of covariantly defined GI variables. In their approach the whole inhomogeneity of the matter field is incorporated in the GI spatial fluctuations of the momentum psi of phi ; then the GI density perturbations are simply proportional to the momentum perturbations. The inhomogeneity of the geometry is characterized by GI fluctuations of the 3-curvature scalar of the surfaces phi =constant. The time evolution of the matter and curvature perturbations are coupled by a pair of first-order linear differential equations. Correspondingly, each GI variable satisfies a second-order linear homogeneous differential equation. When the background curvature vanishes, k=0, the curvature variable is conserved for perturbation scales larger than the horizon, but this is no longer true in general if k not=0. They discuss simple examples, including the case when more than one scalar field is present, recovering standard results for inflationary universe models. They also demonstrate that in coasting solutions with k=-1, inhomogeneities are damped out on all scales.

https://iopscience.iop.org/article/10.1088/0264-9381/9/4/010/pdf

Minoru Yamamoto

Papers

Steady Distributions in Aggregation Process of Sticky Particles