Observing Brane Inflation

TL;DR

The paper demonstrates that D3-$\bar{D}$3 brane inflation in warped throats can generically yield enough e-folds across slow-roll, intermediate, and ultra-relativistic (DBI) regimes by leveraging warp-induced flattening and the DBI kinetic term. It introduces DBI-specific parameters $\epsilon_D$, $\eta_D$, and $\kappa_D$ to unify the treatment and analyzes how observables such as $n_s$, $r$, $dn_s/d\ln k$, and $f_{NL}$ vary with the throat parameters and the inflaton mass $m$. The results show that COBE normalization can be met while allowing broad parameter flexibility, including regions where either the tensor-to-scalar ratio $r$ or the non-Gaussianity $f_{NL}$ saturate current bounds (but not both), with cosmic strings potentially contributing to $G\mu$. This work provides a framework to map string-theoretic compactifications to CMB observables, highlighting testable predictions for future observations to constrain the string landscape.

Abstract

Linking the slow-roll scenario and the Dirac-Born-Infeld scenario of ultra-relativistic roll (where, thanks to the warp factor, the inflaton moves slowly even with an ultra-relativistic Lorentz factor), we find that the KKLMMT D3/anti-D3 brane inflation is robust, that is, enough e-folds of inflation is quite generic in the parameter space of the model. We show that the intermediate regime of relativistic roll can be quite interesting observationally. Introducing appropriate inflationary parameters, we explore the parameter space and give the constraints and predictions for the cosmological observables in this scenario. Among other properties, this scenario allows the saturation of the present observational bound of either the tensor/scalar ratio r (in the intermediate regime) or the non-Gaussianity f_NL (in the ultra-relativistic regime), but not both.

Figures (10)

• Figure 1: The tensor/scalar ratio $r$ versus the scalar power index $n_{s}$. The shaded region is covered by the model. A few sample points are shown: stars are slow-roll points, triangles and diamonds are relativistic. Most of the region is filled out by relativistic points, where higher $m^2$ in the potential and/or large $\gamma$ lead to points further to the left (smaller $n_s$). The outlined (i.e., open) symbols have $\gamma$ too high to satisfy the current non-Gaussianity bound. To understand the slope of the left boundary, note that large $r$ requires a steeper potential (out of the slow-roll regime), which also corresponds to $n_s<1$.
• Figure 2: The running of the scalar index $\frac{d n_{s}}{d \ln k}$ versus the scalar index $n_{s}$. The shaded region is covered by our model. A few sample points are shown: stars are slow-roll points, triangles and diamonds are relativistic. Most of the region is filled out by relativistic points, where higher $m^2$ in the potential and/or large $\gamma$ lead to points further to the left (smaller $n_s$). The outlined symbols have $\gamma$ too high to satisfy the current non-Gaussianity bound.
• Figure 3: The running of the scalar index $\frac{d n_{s}}{d \ln k}$ versus the tensor/scalar ratio $r$. The shaded region is covered by our model. A few sample points are shown: stars are slow-roll points, triangles and diamonds are relativistic. Most of the region is filled out by relativistic points, where higher $m^2$ in the potential and/or large $\gamma$ fills in points further to the right (larger $r$). The outlined symbols have $\gamma$ too high to satisfy the current non-Gaussianity bound.
• Figure 7: The value of $\phi$ (units of $\alpha^{\prime-1/2}$) that gives 55 e-folds as a function of $m/M_p$. The dashed lines shows the result for the full potential, while the upper and lower dot-dashed lines show potentials with quadratic and constant + Coulombic terms respectively.
• Figure 8: Range of values of the $D$3 charge (N) that give working inflation models. The dashed line shows the trend of the points, and the bars show the range $N$.