Brane Inflation : String Theory viewed from the Cosmos

TL;DR

The paper investigates brane inflation within string theory, arguing it is a natural and testable realization of inflation in warped throat geometries. It details how a D3-brane moving toward an anti-D3-brane in a warped compactification, with DBI kinetics, yields inflation with robust e-folds across various regimes and predicts distinctive signatures including cosmic strings and potential non-Gaussianity. It analyzes the graceful exit through reheating, the production and properties of cosmic strings, and their evolution and detectability via gravitational waves, lensing, and CMB signatures, highlighting that current data already constrain parameter space while future observations could validate key stringy predictions. The work emphasizes the interplay between string compactifications, throat geometry, and cosmological observables as a pathway to testing string theory with cosmology.

Abstract

Brane inflation is a specific realization of the inflationary universe scenario in the early universe within the brane world framework in string theory. The naturalness and robustness of this realistic scenario is explained. Its predictions on the cosmological observables in the cosmic microwave background radiation, especially possible distinct stringy features, such as large non-Gaussianity or large tensor mode that deviates from that predicted in the slow roll scenario, are discussed. Stringy KK modes as hidden dark matter is also a possibility. Another generic consequence of brane inflation is the production of cosmic strings towards the end of inflation. These cosmic strings are nothing but superstrings stretched to cosmological sizes. The properties of these cosmic superstrings and their subsequent cosmological evolution into a scaling network open up their possible detections in the near future, via cosmological, astronomical and/or gravitational wave measurements. At the moment, cosmological data is already imposing strong constraints on the details of the scenario. Finding distinctive stringy signatures in cosmological observations will go a long way in revealing the specific brane inflationary scenario and validating string theory as well as the brane world picture. Precision measurements may even reveal the structures of the flux compactification. Irrespective of the final outcome, we see that string theory is confronting data and making predictions.

Figures (8)

• Figure 1: (a) The brane world scanrio. Here, as light open string modes with each end of an open string ending on a brane, the standard model particles are stuck to the branes, while closed string modes such as a graviton are free to roam the bulk. (b) During brane inflation, a tiny region of the branes (i.e., our universes) grows by an exponentially large factor. Fluctuations such as defects, radiation or matter will be inflated away. Also, the differences in spacing between branes as well as the curvature decreases rapidly.
• Figure 2: A pictorial sketch of the compactified bulk. Besides some warped throats, there are $D$7-branes wrapping 4-cycles. The $D$3-${\bar{D}}$3-brane inflationary scenario in a generic flux compactfied 6-dimensional bulk. The blue dots stand for mobile $D$3-branes while the red dots are ${\bar{D}}$3-branes sitting at the bottoms of throats. After inflation and the annihilation of the last $D$3-brane with the ${\bar{D}}$3-brane in $A$-throat, the remaining ${\bar{D}}$3-branes in $S$-throat may be the standard model branes.
• Figure 3: The predictions of the slow-roll brane inflationary scenario Kachru:2003sxFirouzjahi:2005dh : the cosmic string tension $\mu$, the power spectrum index $n_{s}$, the ratio $r$ of the tensor to the scalar density perturbations and the running of $n_{s}$.
• Figure 4: (a) The exchange of closed strings between two branes. In the dual channel, this describes the one-loop radiative effect of the open strings stretching between 2 branes. (b) The potential $V(y)$ between the $D$3-brane and the ${\bar{D}}$3-brane due to the diagram (a), as a function of the separation $y$ for the brane pair, where $\alpha^{\prime}=$1 Sarangi:2003sg. The dashed curve is the imaginary part of $V(y)$. The thick line is the real part of $V(y)$. The Coulombic potential (the thin red curve) is shown for comparison. (c) The potential $V(\phi, T)$ as a function of the inflaton $y \sim \phi$ and the tachyon expectation value $T$Jones:2002si. Brane inflation is a hybrid inflationary scenario.
• Figure 5: The shape of the 3-point correlation function in the DBI model Chen:2006nt. For comparison, the (negative) of the 3-point correlation function from a standard slow-roll model is shown at the upper left corner.