The Supercooled Universe

TL;DR

The paper analyzes how a TeV-scale, strongly-coupled sector can induce a long period of supercooling in the early universe due to a suppressed confinement transition. It shows that QCD effects at low temperatures can trigger the exit from inflation, with broad cosmological consequences including entropy dilution of relics, potential baryon-dark matter coincidence, modified QCD axion dynamics, and potentially strong gravitational-wave signals from bubble collisions. The work combines holographic/large-N modeling of the new sector with cosmological calculations of tunneling rates, reheating, and relic abundances, highlighting testable predictions for future gravitational-wave observatories and axion/dark-matter experiments. Overall, it provides a coherent framework connecting TeV-scale new physics, QCD dynamics, and observable cosmological imprints in a long-duration supercooling scenario.

Abstract

Strongly-coupled theories at the TeV can naturally drive a long period of supercooling in the early universe. Trapped into the deconfined phase, the universe could inflate and cool down till the temperature reaches the QCD strong scale. We show how at these low temperatures QCD effects are important and could trigger the exit from the long supercooling era. We also study the implications on relic abundances. In particular, the latent heat released at the end of supercooling could be the reason for the similarities between dark matter and baryon energy densities. The axion abundance could also be significantly affected, allowing for larger values of the axion decay constant. Finally, we discuss how a long supercooling epoch could lead to an enhanced gravitational wave signal.

Figures (5)

• Figure 2: Free energy along the tunneling path described in the main text. In the deconfined phase (left side) it is a function of the local temperature $T_{loc}$, while in the confined phase it is a function of $\mu$. The two phases are identified at the point $T_{loc}=0=\mu$. On the left, we show the free energy profile for $T\lesssim T_c$ (solid line) and for $T\ll T_c$ (dot-dashed), to show in particular that a much larger region in $\mu$ space becomes energetically accessible as temperature decreases. On the right, we show the dilaton potential.
• Figure 4: Contours of $\Delta N_e$, the number of efolds after the QCD phase transition, in the scenario described in Sec. \ref{['sub:qcdnew']}. We take $N=7$ and $N_e=\ln(T_c/\Lambda_{\rm QCD}^{\rm dec})\simeq 4.6$.
• Figure 5: Constraints on $F_a$ and $N_{e}$ from DM overproduction. We have taken $\Lambda^{\rm dec}_{\rm QCD}=0.33~(10)~\rm{GeV}$ in the left (right) plot, as well as $N=5$. In the blue region we have $3H>2m_a$, in the green region $3H\leq 2m_a$, and in the white region $3H\leq m_a$. In the orange region the supercooling era ends before the condition $3H=m_a$ is satisfied. Contours of $\Omega_{a}=1,\, 0.1,\, 0.01\, \Omega_{\text{CDM}}$ are respectively shown as thick, dashed and dotted black lines respectively. The vertical orange dashed line shows the number of efolds corresponding to the critical QCD temperature during supercooling.
• Figure 7: Relic abundance of gravitational waves from the collision of cosmic bubbles in the supercooling scenario (thick lines, as in the legend), according to Eq. (\ref{['eq:gwbubbles']}). The sensitivity curves of several future ground- and space-based interferometers are also shown (see Caprini:2015zlo for the sensitivity of different configurations of LISA).
• Figure 8: Models of type \ref{['typeI']} (left) and type \ref{['typeII']} (right).