Inflationary Gravitational Waves and the Evolution of the Early Universe

TL;DR

The paper analyzes how inflationary gravitational waves reflect the thermal history of the early universe, including phase transitions, entropy production, and dark-radiation effects, and shows that specific spectral features can serve as probes of high-energy physics beyond the SM.Using a combination of analytic insights and illustrative toy histories, the authors quantify how horizon-entry dynamics, anisotropic stress from dark radiation, and late-time entropy production modify the present GW spectrum and introduce distinctive dips, hills, and oscillations.They connect these spectral signatures to concrete particle-physics models, notably SUSY Peccei-Quinn and SUSY majoron frameworks, demonstrating realizations of the proposed histories (Cases 1–5) and outlining observational prospects for future space-based GW detectors.Overall, the work argues that a measured inflationary GW spectrum could reveal the presence and nature of high-energy phenomena in the early universe, providing a complementary window to CMB and collider probes.

Abstract

We study the effects of various phenomena which may have happened in the early universe on the spectrum of inflationary gravitational waves. The phenomena include phase transitions, entropy productions from non-relativistic matter, the production of dark radiation, and decoupling of dark matter/radiation from thermal bath. These events can create several characteristic signatures in the inflationary gravitational wave spectrum, which may be direct probes of the history of the early universe and the nature of high-energy physics.

Figures (17)

• Figure 1: Normalization factor $C$ with the presence of dark radiation. $C_1$ accounts for the effect of increase in the amount of total radiation, while $C_3$ gives that of anisotropic stress due to the dark radiation. The left figure is for the SM, while the right is for the MSSM.
• Figure 2: Left : Normalization factor $C_3$ as a function of $f_X$, the energy fraction of dark radiation. Right : Relation between the effective neutrino number $\Delta N_{\rm eff}$ and $f_X$.
• Figure 3: GW spectrum with phase transition and instant decay into radiation. We assumed that the universe is radiation dominated before the vacuum energy dominates it, and varied the ratio of radiation energy density to the total energy density at the phase transition.
• Figure 4: GW spectrum with entropy injection. We varied the ratio of radiation energy density to the total energy density at $t=t_{\rm dec}\equiv \Gamma^{-1}$.
• Figure 5: GW spectrum with phase transition and subsequent instant decay into dark radiation $X$. We have assumed that the branching ratio to $X$ is 0 and 1 in the red-solid and green-dashed line, respectively. Also, $\Delta N_{\rm eff}=1, 2, 5$ and $100$ for the top left, top right, bottom left and bottom right figure, respectively.