Reheating the Universe after String Theory Inflation

TL;DR

The paper investigates reheating after string theory inflation in warped compactifications, focusing on brane-antibrane inflation within a Klebanov-Strassler throat. It identifies a cascading energy transfer: brane annihilation produces heavy closed strings, which decay into local KK modes, which then decay into open-string SM modes; warp factors suppress gravitons and help channel energy into KK modes, yielding a residual gravitational-wave background $\Omega_{GW} \sim 10^{-8}$ and a reheating temperature $T_r$ that depends sensitively on throat geometry. A key result is that, while KK modes thermalize efficiently and can reheat the SM sector, long-lived angular KK relics and intermediate-throat KK modes pose significant cosmological problems, especially in multi-throat scenarios. The paper concludes that viable reheating is possible in a single throat or certain KKLMMT-like subclasses, but many warped, multi-throat configurations face severe relic and transfer-rate constraints, guiding future model-building in string-theoretic inflation. The analysis provides concrete scaling relations for KK and open-string decays, and highlights observational signatures such as a warped-gravity-suppressed gravitational-wave background.

Abstract

In String theory realizations of inflation, the end point of inflation is often brane-anti brane annihilation. We consider the processes of reheating of the Standard Model universe after brane inflation. We identify the channels of inflaton energy decay, cascading from tachyon annihilation through massive closed string loops, KK modes, and brane displacement moduli to the lighter standard model particles. Cosmological data constrains scenarios by putting stringent limits on the fraction of reheating energy deposited in gravitons and nonstandard sector massive relics. We estimate the energy deposited into various light degrees of freedom in the open and closed string sectors, the timing of reheating, and the reheating temperature. Production of gravitons is significantly suppressed in warped inflation. However, we predict a residual gravitational radiation background at the level $Ω_{GW} \sim 10^{-8}$ of the present cosmological energy density. We also extend our analysis to multiple throat scenarios. A viable reheating would be possible in a single throat or in a certain subclass of multiple throat scenarios of the KKLMMT type inflation model, but overproduction of massive KK modes poses a serious problem. The problem is quite severe if some inner manifold comes with approximate isometries (angular KK modes) or if there exists a throat of modest length other than the standard model throat, possibly associated with some hidden sector (low-lying KK modes).

Figures (4)

• Figure 1: Radial geometry of a Klebanov Strassler throat. For most part of this paper, we consider KKLMMT-like inflation scenario where unstable D-brane system of D3 branes and anti-D3 branes near the bottom of the throat drives inflation, possibly with some leftover D3's.
• Figure 2: Identifying the channels of D-brane decay
• Figure 3: KK modes in the inflation throat can decay to another throat via quantum oscillation. While the oscillation amplitude depends on mass eigenvalue distribution in the 2nd throat, it will be also suppressed further if the state in throat 1 has a large width.
• Figure 4: KK modes in the inflation throat deposit energy to lower energy throats. The branching ratio will be largely determined by how throats are distributed in the internal manifold with respect to the inflation throat and less sensitive to the field content of each throat. Energy deposited in throat 1 will later decay to throat 2, but at much more suppressed rate because its mass scale is far lower than that of the inflation throat.