Description
It was noted in arXiv:0905.4720v2 [hep-th] that as a general feature "String theory suggests the simultaneous presence of many ultralight axions, possibly populating each decade of mass down to the Hubble scale 10-33eV". Axion like particles (ALPs) with masses of between $m=10^{-33}\mathbf{eV}$ and m=10^{-22}\mathbf{eV}$ can affect the growth of structure in the Universe through the matter power pectrum. We compute these effects and present models for how they depend on the mass and fraction of the critical density in one ultra light ALP. Finally we briefly discuss possible observations.
