Warped Reheating in Multi-Throat Brane Inflation

TL;DR

The paper analyzes reheating in multi-throat brane inflation by computing and comparing the time scales of energy transfer processes after D$ar{D}$ annihilation. It shows that warp-factor enhancements favor decays of closed strings into KK modes within the inflationary throat and that KK modes preferentially tunnel to a Standard Model throat rather than decay into bulk gravitons. Once in the SM throat, KK modes decay efficiently to Standard Model degrees of freedom, leading to high reheating temperatures on the order of $T_{RH}\sim 10^{13}$–$10^{14}$ GeV for reasonable parameter ranges. The study also argues that adding mildly warped extra throats remains compatible with successful reheating, provided late-time constraints (e.g., BBN) are respected, and highlights the broader implications for string cosmology and potential observable signatures such as cosmic strings.

Abstract

We investigate in some quantitative details the viability of reheating in multi-throat brane inflationary scenarios by estimating and comparing the time scales for the various processes involved. We also calculate within perturbative string theory the decay rate of excited closed strings into KK modes and compare with that of their decay into gravitons; we find that in the inflationary throat the former is preferred. We also find that over a small but reasonable range of parameters of the background geometry, these KK modes will preferably tunnel to another throat (possibly containing the Standard Model) instead of decaying to gravitons due largely to their suppressed coupling to the bulk gravitons. Once tunneled, the same suppressed coupling to the gravitons again allows them to reheat the Standard Model efficiently. We also consider the effects of adding more throats to the system and find that for extra throats with small warping, reheating still seems viable.

Figures (4)

• Figure 1: We will consider a KKLMMT setup with two throats, as in a.); as a simplification, we will consider the throats to be $AdS_{5}$, and glue the two throats together at a "Planck" brane, as in b.).
• Figure 2: The numerical value of the decay rate (in units of the local string scale) of a closed string into an a.) graviton and b.) KK mode, for all allowed processes, is plotted for finite oscillator level $N$ (black dotted line), which is relevant for the decay of the closed string end products from $D\overline{D}$ annihilation, and in the limit $N\rightarrow \infty$ (red solid line), which corresponds to the field theory limit of CIR, where $\Gamma \propto \sqrt{N}$ (note that we have not included any warp factor enhancement yet). The jumps in b.) are from threshold effects for the production of KK modes; see the discussion below Eq.(\ref{['eq:GraviDecayLimit']}).
• Figure 3: The numerical value of the decay rate of a $N=100$ (red) and $N=10$ (black) closed string into a KK mode for different decay processes $N_{0}=N-N'$. Notice that the decay rate is peaked at small $N_{0}$ and falls off quickly, but increasing $N$ increases the $N_{0}$ at which the rate is maximum. The large initial jump when $N_{0}\approx N_{0,max}$ is due to the decay process crossing the threshold for production of a KK mode. One can show that $N_{0,max}\sim .65 \sqrt{N}$.
• Figure 4: The warping creates a potential barrier for Kaluza-Klein states of the graviton in the Schrödinger coordinate system. States initially localized in the inflationary (left) throat must tunnel through this barrier to communicate with the Standard Model (right) throat. The Schrödinger energy of a state is $E=m^{2}$.