Do we have a Theory of Early Universe Cosmology?

TL;DR

This paper reviews the inflationary paradigm in early universe cosmology, discusses its conceptual issues, and introduces two alternative models—matter bounce and string gas cosmology—that align with observations and offer testable predictions.

Contribution

It presents alternative early universe models that match observations and addresses conceptual problems in inflationary theory.

Findings

Matter bounce and string gas cosmology predict similar large-scale structure spectra as inflation.

These models can be distinguished by future observational data.

The paper highlights conceptual issues in the inflationary paradigm.

Abstract

The inflationary scenario has become the paradigm of early universe cosmology, and - in conjuction with ideas from superstring theory - has led to speculations about an "inflationary multiverse". From a point of view of phenomenology, the inflationary universe scenario has been very successful. However, the scenario suffers from some conceptual problems, and thus it does not (yet) have the status of a solid theory. There are alternative ideas for the evolution of the very early universe which do not involve inflation but which agree with most current cosmological observations as well as inflation does. In this lecture I will outline the conceptual problems of inflation and introduce two alternative pictures - the "matter bounce" and "string gas cosmology", the latter being a realization of the "emergent universe" scenario based on some key principles of superstring theory. I will demonstrate that these two alternative pictures lead to the same predictions for the power spectrum of the observed large-scale structure and for the angular power spectrum of cosmic microwave background anisotropies as the inflationary scenario, and I will mention predictions for future observations with which the three scenarios can be observationally teased apart.