Axion-Electromagnetic Waves

We extend the duality symmetry between the electric and the magnetic fields to the case in which an additional axion-like term is present, and we derive the set of Maxwell's equations that preserves this symmetry. This new set of equations allows for a gauge symmetry extending the ordinary symmetry in the classical electrodynamics. We obtain explicit solutions for the new set of equations in the absence of external sources, and we discuss the implications of a new internal symmetry between the axion field and the electromagnetic gauge potential.Comment: 11pages, accepted for publication on Modern Physics Letters A 28, 35 (2013

Condensation of Galactic Cold Dark Matter

We consider the steady-state regime describing the density profile of a dark matter halo, if dark matter is treated as a Bose-Einstein condensate. We first solve the fluid equation for "canonical" cold dark matter, obtaining a class of density profiles which includes the Navarro-Frenk-White profile, and which diverge at the halo core. We then solve numerically the equation obtained when an additional "quantum pressure" term is included in the computation of the density profile. The solution to this latter case is finite at the halo core, possibly avoiding the "cuspy halo problem" present in some cold dark matter theories. Within the model proposed, we predict the mass of the cold dark matter particle to be of the order of M_chi c2 = 10^-24 eV, which is of the same order of magnitude as that predicted in ultra-light scalar cold dark matter models. Finally, we derive the differential equation describing perturbations in the density and the pressure of the dark matter fluid.Comment: 19 pages, 4 figures, Version accepted for publication on JCA

Observational Constraints on Monomial Warm Inflation

Warm inflation is, as of today, one of the best motivated mechanisms for explaining an early inflationary period. In this paper, we derive and analyze the current bounds on warm inflation with a monomial potential $U\propto \phi^p$, using the constraints from the PLANCK mission. In particular, we discuss the parameter space of the tensor-to-scalar ratio $r$ and the potential coupling $\lambda$ of the monomial warm inflation in terms of the number of e-folds. We obtain that the theoretical tensor-to-scalar ratio $r\sim 10^{-8}$ is much smaller than the current observational constrain $r \lesssim 0.12$, despite a relatively large value of the field excursion \Delta \phi \sim 0.1\MP. Warm inflation thus eludes the Lyth bound set on the tensor-to-scalar ratio by the field excursion.Comment: 21 pages, 4 figure

Light axion-like dark matter must be present during inflation

Axion-like particles (ALPs) might constitute the totality of the cold dark matter (CDM) observed. The parameter space of ALPs depends on the mass of the particle $m$ and on the energy scale of inflation $H_I$ , the latter being bound by the non-detection of primordial gravitational waves. We show that the bound on HI implies the existence of a mass scale $m_\chi = 10 {\rm \,neV} {\div} 0.5 {\rm \,peV}$, depending on the ALP susceptibility $\chi$, such that the energy density of ALPs of mass smaller than $m_\chi$ is too low to explain the present CDM budget, if the ALP field has originated after the end of inflation. This bound affects Ultra-Light Axions (ULAs), which have recently regained popularity as CDM candidates. Light ($m < m_\chi$) ALPs can then be CDM candidates only if the ALP field has already originated during the inflationary period, in which case the parameter space is constrained by the non-detection of axion isocurvature fluctuations. We comment on the effects on these bounds from additional physics beyond the Standard Model, besides ALPs.Comment: 12 pages, 3 figures. Improved reference

Probing the Early Universe with Axion Physics and Gravitational Waves

We show results for the expected reach of the network of experiments that is being set up globally with the aim of detecting the "invisible" axion, in light of a non-standard thermal history of the universe. Assuming that the axion is the dark matter, we discuss the reach of a successful detection by a given experimental setup in a particular axion mass window for different modifications of the cosmological background before primordial nucleosynthesis occurred. Results are presented both in the case where the present energy budget in cold axions is produced through the vacuum realignment mechanism alone, or in the case in which axionic strings also provide with additional contributions to the axion energy density. We also show that in some cosmological models, the spectrum of gravitational waves from the axionic string network would be within reach of the future network of detectors like LISA and DECIGO-BBO. We conclude that some scenarios describing the early universe can be probed jointly by the experimental efforts on axion detection and by gravity wave multi-messenger astronomy.Comment: 18 pages, 4 figures. Accepted in PRD. Matches published versio