String Excitation by Initial Singularity of Inflation

TL;DR

The paper investigates how initial singularities in inflation models can excite string oscillation modes, indicating inevitable stringy effects in early universe scenarios, especially for strong singularities.

Contribution

It demonstrates that strong initial singularities in certain inflation models lead to excitation of string modes, highlighting the significance of stringy corrections in early cosmology.

Findings

Infinitely heavy string modes get excited at strong singularities.

Initial singularities in Starobinsky and hill top inflation are strong, implying string effects.

Hill top inflation's singularity can be weak in non-slow roll cases.

Abstract

We discuss excitation of string oscillation modes by an initial singularity of inflation. The initial singularity of inflation is known to occur with a finite Hubble parameter, which is generally lower than the string scale, and hence it is not clear that stringy effects become significant around it. With the help of Penrose limit, we find that infinitely heavy oscillation modes get excited when a singularity is strong in the sense of Krolak's classification. We demonstrate that the initial singularities of Starobinsky and hill top inflation, assuming the slow roll inflation to the past infinity, are strong. Hence stringy corrections are inevitable in the very early stage of these inflation models. We also find that the initial singularity of the hill top inflation could be weak for non-slow roll case.