Primordial Non-Gaussianity from a String-Inspired Cosmology

TL;DR

This work embeds the climbing-scalar cosmology, arising from string-theory–motivated exponential dilaton potentials, into a detailed perturbative framework for early-universe fluctuations. It develops the low-energy effective actions of several non-supersymmetric 10D string models, derives the background climbing and slow-roll dynamics (including a two-exponential potential that yields a fast-roll climb and a slow-roll descent), and then constructs the full cosmological perturbation theory, including the Mukhanov-Sasaki equation and a Maldacena-like cubic action. The authors show that the climbing phase imprints a distinctive low-k suppression and oscillatory features in the power spectrum, and compute the cubic action to assess non-Gaussianities, finding two components of the bispectrum: a small post-bounce contribution and a potentially sizeable turning-point contribution, with f_NL potentially detectable for certain inflationary e-fold counts. The analysis provides a string-inspired route to pre-inflationary physics that can be confronted with CMB observations, linking high-energy theory to observable non-Gaussian signatures while carefully addressing singularities via higher-curvature regularization assumptions.

Abstract

The interplay between string theory and early-universe cosmology offers promising avenues to explore high-energy regimes where the standard single-field slow-roll model may no longer provide an accurate description. One intriguing scenario emerges from certain non-supersymmetric string models, where supersymmetry breaking induces a non-trivial vacuum energy, or more precisely an exponential potential for scalar fields, and primarily for the dilaton. This setup gives rise to the so-called "climbing scalar" phenomenon, whereby the scalar is forced to emerge from the initial singularity while climbing up the potential, if sufficiently steep. This phase precedes a turning point, and the subsequent descent can support inflation. The resulting pre-inflationary dynamics can leave imprints in cosmological observables. To begin with, it induces a low-frequency cut in the primordial power spectrum that resonates with the lack of power present in the first few CMB multipoles. The main theme of this work is to clarify its possible effect on non-Gaussianities.

Figures (27)

• Figure 1: (a) Quantum correction to open string propagation. (b) Decay of one closed string into two. Time evolves upward. Source: Polchinski:1998rq.
• Figure 2: (a) Scattering of four closed strings with sources at $x_0 = \pm \infty$. (b) Conformally equivalent picture as a sphere with punctures. Taken from Polchinski:1998rq.
• Figure 3: The torus as a periodic lattice. This picture is taken from Angelantonj:2002ct.
• Figure 4: Fundamental domain for the torus. This picture is taken from Angelantonj:2002ct.
• Figure 5: Left panel: The duality hexagon for ten-dimensional supersymmetric superstrings; Right panel: The larger duality diagram including the ten-dimensional nonsupersymmetric superstrings. The green lines identify orientifold projections, the blue boxes identify the three nontachyonic models: $HE_2$ stands for the $SO(16) \times SO(16)$ model of AlvarezGaume1986, $0'B$ for the $U(32)$ model of Sagnotti1995Sagnotti1997, and BSB for Sugimoto’s model in Sugimoto1999Antoniadis1999Angelantonj2000bAldazabal1999Angelantonj2000b . This picture is taken from MouradSagnotti2021.