This is a review of the physics and cosmology of the cosmological constant. Focusing on recent developments, I present a pedagogical overview of cosmology in the presence of a cosmological constant, observational constraints on its magnitude, and the physics of a small (and potentially nonzero) vacuum energy.

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The Cosmological Constant

A variety of observations suggest that magnetic fields are present in all galaxies and galaxy clusters. These fields are characterized by a modest strength ${(10}^{\ensuremath{-}7}\char21{}{10}^{\ensuremath{-}5}\mathrm{G})$ and huge spatial scale $(\ensuremath{\lesssim}1\mathrm{Mpc}).$ It is generally assumed that magnetic fields in spiral galaxies arise from the combined action of differential rotation and helical turbulence, a process known as the \ensuremath{\alpha}\ensuremath{\omega} dynamo. However, fundamental questions concerning the nature of the dynamo as well as the origin of the seed fields necessary to prime it remain unclear. Moreover, the standard \ensuremath{\alpha}\ensuremath{\omega} dynamo does not explain the existence of magnetic fields in elliptical galaxies and clusters. The author summarizes what is known observationally about magnetic fields in galaxies, clusters, superclusters, and beyond. He then reviews the standard dynamo paradigm, the challenges that have been leveled against it, and several alternative scenarios. He concludes with a discussion of astrophysical and early-Universe candidates for seed fields.

https://arxiv.org/pdf/astro-ph/0207240.pdf

Origin of galactic and extragalactic magnetic fields

http://cds.cern.ch/record/570429/files/0207130.pdf

The pre-big bang scenario in string cosmology

I review various aspects of the pre-big-bang scenario and of its main open problems, with emphasis on the role played by the dilaton Since the dilaton is a compelling consequence of string theory, tests of this scenario are direct tests of string theory and also, more generally, of Planck scale physics

Phenomenological aspects of the pre-big-bang scenario in string cosmology

Cosmology, high-energy physics and astrophysics are today converging to the study of large scale magnetic fields. While the experimental evidence for the existence of large scale magnetization in galaxies, clusters and super-clusters is rather compelling, the origin of the phenomenon remains puzzling especially in light of the most recent observations. The purpose of the present review is to describe the physical motivations and the open theoretical problems related to the existence of large scale magnetic fields.

/pdf/the-magnetized-universe-1lgbf6rtzh.pdf

The magnetized universe

The existence of large-scale magnetic fields in galaxies is well established, but there is no accepted mechanism for generating a primordial field which could grow into what is observed today. We discuss a model which attempts to account for the necessary primordial field by invoking a pseudo Goldstone boson coupled to electromagnetism. The evolution of this boson during inflation generates a magnetic field; however, it seems difficult on rather general grounds to obtain fields of sufficient strength on astrophysically interesting scales.

W. Daniel Garretson

Papers

Primordial magnetic fields from pseudo Goldstone bosons.

Photon to pseudoscalar conversion in the interstellar medium

Could there be something rather than nothing

Two stage phase transition in the minimal supersymmetric model