Complex topologies in String Theory suggest the presence of a plenitude of axions. The Compton wavelength of string axions ranges from the size of our Universe to subnuclear scales, and their signatures appear in a wide range of astrophysical experiments. These axions affect the polarization of the Cosmic Microwave Background at a level within reach of Planck satellite, as well as the way matter is distributed in our Universe. Observations of rotating black holes also probe axions through the Penrose superradiance process. When the axion Compton wavelength is comparable to the size of the black hole, it binds to the black hole forming a gravitational atom in the sky. The number of axions bound to that atom grows exponentially into a Bose Einstein Condensate extracting angular momentum and energy from the black hole, causing the black hole to spin down. The existence of an axion is then diagnosed by the absence of rapidly spinning astrophysical black holes, and by gravity waves or photons coming from the gravitational atom that can be detected in upcoming experiments. In particular, the QCD axion can be discovered through gravity wave signals at Advanced LIGO.
Another possible consequence of complex extra dimensions is the existence of many photons. In the presence of low energy supersymmetry, these photons acquire fermionic partners called "photini". These lead to novel signatures at the Large Hadron Collider, as they can decay away from their production point, giving rise to displaced vertices, and out of time decays in the detector.