Maximal temperature of strongly-coupled dark sectors
Helena Kolešová, M. Laine, S. Procacci
Abstract
A bstract Taking axion inflation as an example, we estimate the maximal temperature ( T max ) that can be reached in the post-inflationary universe, as a function of the confinement scale of a non-Abelian dark sector (Λ IR ). Below a certain threshold Λ IR < Λ 0 ∼ 2 × 10 − 8 m pl , the system heats up to T max ∼ Λ 0 > T c , and a first-order thermal phase transition takes place. On the other hand, if Λ IR > Λ 0 , then T max ∼ Λ IR < T c : very high temperatures can be reached, but there is no phase transition. If the inflaton thermalizes during heating-up (which we find to be unlikely), or if the plasma includes light degrees of freedom, then heat capacity and entropy density are larger, and T max is lowered towards Λ 0 . The heating-up dynamics generates a gravitational wave background. Its contribution to N eff at GHz frequencies, the presence of a monotonic ∼ $$ {f}_0^3 $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>f</mml:mi> <mml:mn>0</mml:mn> <mml:mn>3</mml:mn> </mml:msubsup> </mml:math> shape at (10 − 4 – 10 2 ) Hz frequencies, and the frequency domain of peaked features that may originate via first-order phase transitions, are discussed.