Adding helicity to inflationary magnetogenesis

The most studied mechanism of inflationary magnetogenesis relies on the time-dependence of the coefficient of the gauge kinetic term $F_{\mu\nu}\,{F}^{\mu\nu}$. Unfortunately, only extremely finely tuned versions of the model can consistently generate the cosmological magnetic fields required by observations. We propose a generalization of this model, where also the pseudoscalar invariant $F_{\mu\nu}\,\tilde{F}^{\mu\nu}$ is multiplied by a time dependent function. The new parity violating term allows more freedom in tuning the amplitude of the field at the end of inflation. Moreover, it leads to a helical magnetic field that is amplified at large scales by magnetohydrodynamical processes during the radiation dominated epoch. As a consequence, our model can satisfy the observational lower bounds on fields in the intergalactic medium, while providing a seed for the galactic dynamo, if inflation occurs at an energy scale ranging from $10^5$ to $10^{10}$ GeV. Such energy scale is well below that suggested by the recent BICEP2 result, if the latter is due to primordial tensor modes. However, the gauge field is a source of tensors during inflation and generates a spectrum of gravitational waves that can give a sizable tensor to scalar ratio $r={\cal O}(0.2)$ even if inflation occurs at low energies. This system therefore evades the Lyth bound. For smaller values of $r$, lower values of the inflationary energy scale are required. The model predicts fully helical cosmological magnetic fields and a chiral spectrum of primordial gravitational waves.Comment: 17 pages, 4 figures. Minor changes to match the version accepted for publication in JCA

Dynamical Four-Form Fields

We present an example of a gauge-invariant Lagrangian that contains four derivatives and describes one massive, non-ghostlike, degree of freedom.Comment: 5 page

Dynamics of localized Kaluza-Klein black holes in a collapsing universe

The Clayton Antitrust Act of 1914 prohibits corporate mergers that would result in certain highly undesired end states. We study an exact solution of the Einstein equations describing localized, charged Kaluza-Klein black holes in a collapsing deSitter universe and seek to demonstrate that a similar effect holds, preventing a potentially catastrophic black hole merger. As the collapse proceeds, it is natural to expect that the black hole undergoes a topological transition, wrapping around the shrinking compact dimension to merge with itself and form a black string. However, the putative uniform charged black string end state is singular and such a transition would violate (a reasonable notion of) cosmic censorship. We present analytic and numerical evidence that strongly suggests the absence of such a transition. Based on this evidence, we expect that the Kaluza-Klein black hole horizon stays localized, despite the increasingly constraining size of the compact dimension. On the other hand, the deSitter horizon does change between spherical and cylindrical topologies in a simple way.Comment: 25 pages, 6 figure