Dark radiation and the Hagedorn phase
Andrew R. Frey (Department of Physics and Winnipeg Institute for Theoretical Physics, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba R3B 2E9, Canada); Ratul Mahanta (Harish-Chandra Research Institute, A CI of Homi Bhabha National Institute, Allahabad 211019, India); Anshuman Maharana (Harish-Chandra Research Institute, A CI of Homi Bhabha National Institute, Allahabad 211019, India)
We point out that if the sector associated with the Standard Model degrees of freedom entered an open string Hagedorn phase in the early universe while the dark radiation sector was not part of this plasma, then this can lead to low values of the observable (effective number of additional neutrinolike species). For explicit analysis, we focus on warped string compactifications with the Standard Model degrees of freedom at the bottom of a warped throat. If the Hubble scale during inflation is above the warped string scale associated with the throat, then the Standard Model sector will enter the Hagedorn phase. In this scenario, bulk axions are no longer dangerous from the point of view of dark radiation. While this article focuses on warped compactifications, the basic idea can be relevant to any scenario where the early universe entered a Hagedorn phase.
