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

TL;DR

This review investigates the possibility that the early Universe did not follow a simple radiation-dominated expansion between the end of inflation and BBN. It catalogs diverse mechanisms—post-inflation reheating dynamics, extra inflation stages, heavy particles and moduli, and dark sectors—that could yield nonstandard expansion histories and traces them to consequences for dark matter, baryogenesis, inflationary observables, curvaton dynamics, microhalo and primordial black hole formation, and gravitational-wave backgrounds. It then collects current constraints from BBN, CMB, large-scale structure, gravitational waves, and small-scale structure probes, highlighting how future observations could illuminate epochs prior to BBN. The authors emphasize that nonstandard histories are plausible predictions of UV-complete theories and offer multiple observational avenues to test early-Universe physics beyond the standard RD paradigm.

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.

Figures (18)

• Figure 1: The evolution of the comoving Hubble scale $1/(aH)$ as a function of the scale factor $a$. The expansion history between inflation and the subsequent, post-BBN era remains unknown. Here $a_{\rm end}$ marks the end of inflation, $a_{\rm BBN}$ denotes the commencement of BBN, and $a_{\rm eq}$ is the time of matter-radiation equality at redshift $z\simeq 3400$.
• Figure 2: The energy stored in the almost homogeneous inflaton is tranferred to its own perturbations, as well as other fields, and eventually to the SM fields and DM.
• Figure 3: For single-field inflaton potentials with quadratic minima which flatten as we move away from the minimum ($n=1$, $M\ll M_{\rm P}$ in Fig. \ref{['fig:Fig_Reheating']}), the almost homogeneous inflation field fragments rapidly into dense solitons (called oscillons) after inflation due to an attractive self-interaction. The oscillons then cluster gravitationally. The equation of state $w$ throughout this phase is approximately zero. Each oscillon has fixed physical size and amplitude, neither of which redshift with expansion. The above boxes are comoving and roughly horizon-sized initially. Lighter colors represent higher densities. Figure based on Ref. Amin:2019ums.
• Figure 4: The black curve shows the evolution of the Hubble horizon $1/H$ as a function of the scale factor $a$, whereas the gray dashed line shows, as an example, a scale that re-enters horizon after the matter-radiation equality.
• Figure 5: A schematic diagram of the processes being considered in Sec. \ref{['sec:dark_matter']}. Here $X$, the DM candidate, annihilates (possibly through intermediate processes) into pairs of metastable hidden sector particles, $Y$. If the hidden sector is heavy and extremely decoupled from the visible sector (which contains the SM), then $Y$ will be long-lived, and may eventually dominate the Universe's energy density. Upon its decay into SM particles, $Y$ reheats the visible Universe and dilutes all particle abundances, including the relic density of $X$.