Probing Bose-enhanced Inflaton Decay with Gravitational Waves

TL;DR

This paper investigates cosmic reheating in the presence of a transient condensate formed by bosonic inflaton decay products, leading to Bose-enhanced inflaton decay and qualitatively new energy-transfer dynamics. The authors develop a Boltzmann-equation framework incorporating Bose enhancement, derive a critical coupling μ_c governing rapid depletion, and show that BE-enhanced three-body decays can dominate graviton production. They compute the resulting stochastic gravitational-wave background from several processes, revealing distinctive spectra (monochromatic lines and continua) and potential observability at low frequencies, even for moderate inflaton masses. The work provides a novel observational window into reheating physics and highlights the interplay between reheating dynamics, gravitational interactions, and cosmological constraints such as ΔN_eff.

Abstract

We investigate cosmic reheating dynamics in the presence of a transient condensate formed by bosonic decay products of the inflaton. We show that the emergence of such a condensate and the corresponding Bose enhancement can dramatically increase the efficiency of inflaton decay, giving rise to qualitatively new reheating dynamics beyond the standard perturbative picture. As a consequence, graviton production from inflaton decay processes is significantly amplified by Bose enhancement effects, leading to a stochastic gravitational-wave background with a potentially observable amplitude, even in the low-frequency regime.

Figures (4)

• Figure 1: Schematic illustration of the evolution of the normalized comoving number densities $Y$ of inflatons ($\phi$, blue lines) and reheatons ($\varphi$, red lines) with (bottom panel, $\mu \gg \mu_c$) and without (top panel, $\mu \ll \mu_c$) Bose enhancement, as a function of the cosmic scale factor $a$.
• Figure 2: Numerical examples showing how the evolution of the normalized comoving number densities for $\phi$ and $\varphi$ are strongly affected by small variations of $\mu$ around the critical value $\mu_c$. It is assumed $m_\phi = 10^{13}$ GeV and $H_I/m_\phi = 0.1$.
• Figure 3: Critical valued of $y_c \equiv \mu_c/m_\phi$ (solid lines) and $\Gamma_\phi^{(0)} = H_I$ (dashed lines), for different values of $H_I/m_\phi = 10^{-5}$ (black), $H_I/m_\phi = 10^{-3}$ (blue), or $H_I/m_\phi = 10^{-1}$ (yellow). For $y < y_c$ (red area) the inflaton decays without a sizable Bose enhancement, while for $\Gamma_\phi^{(0)} > H_I$ (yellow area) its decay is prompt even without Bose enhancement. In between, inflatons have a Bose-enhanced decay (blue area).
• Figure 4: Top panel: Individual contributions to the GW spectrum, for $m_\phi = 10^{13}$ GeV, $\mu = 2\, \mu_c$ and $H_I/m_\phi = 10^{-3}$. Here, the subscript $B$ indicates that it is Bose-enhanced. Lower panels: Total GW spectra for $H_I/m_\phi = 10^{-1}$ (left) or $H_I/m_\phi = 10^{-5}$ (right), different inflaton masses, and $\mu = 2\, \mu_c$. Contributions from $\phi\to\varphi\varphi h$ and $\phi\phi\to hh$ are not included due to their weak signal strength---see Eq. \ref{['eq:GW_1-3_No_Bose']} and Eq. \ref{['eq:GW_phiphihh']}.