Multibrane Inflation and Dynamical Flattening of the Inflaton Potential

TL;DR

The paper tackles the fine-tuning problem of brane-antibrane inflation in warped flux compactifications by highlighting the role of the antibrane-position parameter $\psi_0$ and a critical combination $b^2 = 2\sigma\epsilon/\psi_0^6$, showing that a dynamically flattened potential can arise when multiple branes tunnel out of a metastable minimum, enabling successful inflation with minimal tuning. It demonstrates that including Kahler and superpotential corrections expands viable parameter space, even permitting many mobile branes via racetrack stabilization, and analyzes inflationary observables (the scalar power spectrum, spectral index, and running) across generalized models. The results indicate a high likelihood of achieving $N_e \gtrsim 60$ e-foldings and a spectrum compatible with observations in broad regions of parameter space, with potential signatures such as tensor modes and cosmic strings. The work also discusses caveats, mapping challenges between the inflaton and brane coordinates, and how future data on the running spectral index could falsify the scenario.

Abstract

We investigate the problem of fine tuning of the potential in the KKLMMT warped flux compactification scenario for brane-antibrane inflation in Type IIB string theory. We argue for the importance of an additional parameter psi_0 (approximated as zero by KKLMMT), namely the position of the antibrane, relative to the equilibrium position of the brane in the absence of the antibrane. We show that for a range of values of a particular combination of the Kahler modulus, warp factor, and psi_0, the inflaton potential can be sufficiently flat. We point out a novel mechanism for dynamically achieving flatness within this part of parameter space: the presence of multiple mobile branes can lead to a potential which initially has a metastable local minimum, but gradually becomes flat as some of the branes tunnel out. Eventually the local minimum disappears and the remaining branes slowly roll together, with assisted inflation further enhancing the effective flatness of the potential. With the addition of Kahler and superpotential corrections, this mechanism can completely remove the fine tuning problem of brane inflation, within large regions of parameter space. The model can be falsified if future cosmic microwave background observations confirm the hint of a large running spectral index.

Figures (15)

• Figure 1: Sequence of inflaton potentials due to successive tunneling of branes toward the right.
• Figure 2: Potential energy of the Kähler modulus in the the original KKLT scenario for $N=0,1,2$ and $3$ antibranes, showing there can be at most 2 antibranes without destabilizing the Kähler modulus.
• Figure 3: Using a racetrack superpotential and a throat with stronger warping one can add a large number (28 in this example) of antibranes while keeping the Kähler modulus stable.
• Figure 4: $N_e$ versus $\log_{10}\epsilon$ and $\log_{10}2\sigma$ for $b=0.27$, in the range $N_e < 300$.
• Figure 5: $\log N_e$ versus the critical number of branes $\log N_c$, $b=0.27$. The empirical relationship (\ref{['emp']}) is also plotted.