The First Three Seconds: a Review of Possible Expansion Histories of the Early Universe

Abstract: It is commonly assumed that the energy density of the Universe was dominated by radiation between reheating after inflation and the onset of matter domination 54,000 years later. While the abundance of light elements indicates that the Universe was radiation dominated during Big Bang Nucleosynthesis (BBN), there is scant evidence that the Universe was radiation dominated prior to BBN. It is therefore possible that the cosmological history was more complicated, with deviations from the standard radiation domination during the earliest epochs. Indeed, several interesting proposals regarding various topics such as the generation of dark matter, matter-antimatter asymmetry, gravitational waves, primordial black holes, or microhalos during a nonstandard expansion phase have been recently made. In this paper, we review various possible causes and consequences of deviations from radiation domination in the early Universe - taking place either before or after BBN - and the constraints on them, as they have been discussed in the literature during the recent years.

• The paper reviews various nonstandard expansion histories of the early universe and examines their impacts on cosmic observables like BBN and CMB.
• The paper employs numerical constraints on reheating temperatures and post-inflationary phases to understand deviations from the standard model.
• The paper highlights implications for dark matter production, small-scale structure formation, and potential new physics beyond the standard cosmological model.

A Review of Possible Expansion Histories of the Early Universe

This paper presents a comprehensive analysis of the expansion histories of the early universe, focusing particularly on deviations from the standard model of cosmology, where the universe is predominantly radiation-dominated between reheating after inflation and the onset of matter domination approximately 54,000 years later. Although Big Bang Nucleosynthesis (BBN) suggests radiation dominance around the time of nucleosynthesis, evidence prior to BBN supporting this assumption is scant, leading to discussions on various possible nonstandard histories delivering deviations before or after BBN.

The authors explore a gamut of scenarios that suggest potential deviations, including the generation of dark matter, matter-antimatter asymmetry, gravitational waves, primordial black holes, and microhalos driven by nonstandard expansion phases. One of the primary endeavors here is reviewing theoretical proposals addressing such possibilities, with insights both into model-building and observational constraints that have arisen in recent years.

Strong Numerical Results and Bold Claims

The critical numerical results primarily relate to the constraints derived from BBN and Cosmic Microwave Background (CMB) data on several nonstandard scenarios. For example, the paper implies BBN heavily constrains the post-inflationary expansion phases by setting lower limits on the reheating temperature in various decay channels, ensuring consistent nucleosynthesis. The robustness brought by these constraints presents itself as a necessary boundary for models proposing an early matter-dominated (EMD) epoch or similar deviations stemming from phenomena like slow post-inflationary reheating or massive metastable particles.

The authors make intriguing claims regarding the roles nonstandard expansion phases may play, such as altering expected dark matter relic abundance by changing the universe's expansion rate, potentially diluting it, providing alternative nonthermal production mechanisms, or inducing small-scale structure formations.

Practical and Theoretical Implications

Practically, understanding these expansion histories affects predictions and expectations for dark matter detection or the formation of realistic cosmological structures. The theoretical discussions further extend implications for the unknown nature of inflation or dark matter creation and contribute to debates surrounding cosmological tensions, like the Hubble tension.

Furthermore, by illustrating constraints on nonstandard cosmologies, the paper emphasizes how these scenarios intertwine with particle physics, offering possibilities for new physics beyond the standard model. They also open discussions on mechanisms explaining neutrino masses or dark energy properties.

Future Directions

Moving forward, exploring this nonstandard cosmology could involve sophisticated observational campaigns targeting microhalos or primordial black holes as suggested by some models. These investigations could refine or challenge prevailing constraints on both micro and macro scales of the cosmic structure. The dynamics and scale of inflation, reheating temperatures, and the energy densities during different phases are expected areas to stimulate ongoing theoretical studies as observational technologies advance.

Summary

Overall, the paper presents a crucial review connecting multifaceted components of early universe cosmology. It serves as a springboard for interpreting existing data, especially leveraging the stringent constraints from BBN and CMB, putting these into the broader context of theoretical physics. It importantly highlights nonstandard cosmologies, both as plausible interpretations of existing anomalies and forward-looking predictions contributing to the broader narrative of cosmic evolution.