Unwinding Inflation

TL;DR

Unwinding inflation presents a novel mechanism in which higher-form flux threads both our 3+1 dimensions and compact extra dimensions, and flux discharge proceeds via a nucleated brane bubble that unwinds the flux one unit at a time. A four-dimensional effective description with a DBI-type inflaton $z$ captures the background evolution and perturbations, predicting a high-scale, long-duration inflation with a scale-invariant spectrum augmented by persistent oscillations, sizable tensor modes, and characteristic non-Gaussianity. The framework naturally situates inflation within the string landscape, offering automatic initial conditions through bubble formation and leading to potentially observable signatures that survive after inflation. While the authors provide concrete prototype and stabilized-compactification examples and discuss string-theory parametrizations, substantial work remains to embed the scenario in fully stabilized compactifications and to fully map its phenomenology across the string-theory landscape.

Abstract

Higher-form flux that extends in all 3+1 dimensions of spacetime is a source of positive vacuum energy that can drive meta-stable eternal inflation. If the flux also threads compact extra dimensions, the spontaneous nucleation of a bubble of brane charged under the flux can trigger a classical cascade that steadily unwinds many units of flux, gradually decreasing the vacuum energy while inflating the bubble, until the cascade ends in the self-annihilation of the brane into radiation. With an initial number of flux quanta Q_{0} \simgeq N, this can result in N efolds of inflationary expansion while producing a scale-invariant spectrum of adiabatic density perturbations with amplitude and tilt consistent with observation. The power spectrum has an oscillatory component that does not decay away during inflation, relatively large tensor power, and interesting non-Gaussianities. Unwinding inflation fits naturally into the string landscape, and our preliminary conclusion is that consistency with observation can be attained without fine-tuning the string parameters. The initial conditions necessary for the unwinding phase are produced automatically by bubble formation, so long as the critical radius of the bubble is smaller than at least one of the compact dimensions threaded by flux.

Figures (9)

• Figure 1: The setup for unwinding inflation in the case $p=4$.
• Figure 2: The mechanism of flux discharge cascade on $dS_{4}\times S_{1}$, where $z \simeq z+l$ is the coordinate on the $S_{1}$. The amount of flux is indicated by $Q, Q-1, \ldots$, while the dashed arrows represent the direction of the electric force and the velocity of the branes. The squiggly lines indicate strings stretched between sections of brane; their mass depends on the separation and changes with time. The figure is not to scale; usually $l <1/H, R$ where $R$ is the radius of curvature of the branes.
• Figure 3: Left panel: The pressure $V'$ from \ref{['eq:Vpp']}, constant between collisions and changing in steps at each collision. Right panel: The potential $V$ around the zero flux minimum; the smooth line is the quadratic approximation.
• Figure 4: From left to right: The brane position $z$, the Lorentz factor $\gamma$, the oscillations around the smooth approximation of the potential, zoomed in to the region corresponding to the CMB quadrupole. All plots use the set of parameters: $g_s=.01$, $l=19.7/m_s$, $d=2/m_s$, $Q_0=400$, for a wrapped $(p=4)$-brane expanding on an $S_1$, see Sec. \ref{['stringsec']}. As is apparent from the plots the oscillations in $\gamma$ and $V$ are very small in this case, but the power spectrum may have larger oscillations depending on the degree of string production.
• Figure 5: Stabilization of $S_1$. Left panel: Effective potential for the radion field as the number of flux units decreases in the direction of the arrow. Right panel: Change in the minimum of the radion as the number of flux units varies.