Photon Propagation as a Probe for Fundamental Physics

In this thesis we show how light propagation can be used in order to constrain particle physics models beyond the Standard Model. We study in particular the effects of non standard physics on the polarization of cosmic microwave background radiation and on the flux of ultrahigh energy γ-rays (Eγ > 1019 eV). In the first part we discuss the effects on cosmic microwave background polarization of coupling between photons and pseudoscalar fields acting as dark matter (e.g. axions) or as dark energy (e.g. ultralight pseudo Nambu- Goldstone bosons). In particular we describe how the public code CAMB can be modified in order to take into account the rotation of the linear polarization plane from last scattering surface to nowadays, produced by photon propagation in a cosmological background of pseudoscalar particles. Polarization power spectra are compared with the ones obtained in the widely used approximation in which the rotation angle is assumed constant in time. We show how polarization-polarization and temperature-polarization angular power spectra can be very useful to constrain the coupling constant gφ between photons and pseudoscalars. In the second part of this thesis we use the current upper limits on the flux of ultrahigh-energy photons in order to constrain Lorentz invariance violating terms in the dispersion relations for elementary particles. Theories trying to unify quantum mechanics with general relativity and many supersymmetry models predict indeed that Lorentz symmetry has to be modified at energies of the order of the Planck scale (1028 eV). If standard dispersion relations of elementary particles are modified, then the propagation and therefore also the energy spectrum of ultrahigh-energy cosmic rays can be considerably changed. We study in particular how it is possible to constrain Lorentz invariance violating terms for photons and electrons (suppressed both at first and second order of the Planck mass) improving current constraints by several orders of magnitude. The main results are summarized in the following papers: • F. Finelli and M. Galaverni, “Rotation of Linear Polarization Plane and Circular Polarization from Cosmological Pseudoscalar Fields,” arXiv:0802.4210 [astro-ph], submitted. • M. Galaverni, F. Finelli, “Systematics of Cosmic Microwave Background Polarization,” Internal Report IASF-BO 454/2007. • M. Galaverni and G. Sigl, “Lorentz Violation in the Photon Sector and Ultra-High Energy Cosmic Rays,” Phys. Rev. Lett. 100, 021102 (2008) [arXiv:0708.1737 [astro-ph]]. • M. Galaverni and G. Sigl, “Lorentz Violation and Ultrahigh-Energy Photons,” Phys. Rev. D 78, 063003 (2008) [arXiv:0807.1210 [astroph]].