Theory HEP Seminar

Dark Radiation vs Hagedorn Strings

Andrew Frey

University of Winnipeg

The contribution to cosmological energy density from new light degrees of freedom is constrained by Big Bang nucleosynthesis and the cosmic microwave background. These constraints can be particularly problematic in string compactifications, which may have many axions or dark gauge sectors, but string theory itself may point to a solution. The existence of an open string Hagedorn phase for the visible sector (while the dark radiation is not in this phase) suppresses the dark radiation contribution to cosmological energy density. We will review string thermodynamics and demonstrate this suppression factor. Then we will discuss a class of models where string theory realizes this mechanism in cosmology.