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.
Highlights
How, during that brief time interval, did the energy density of the universe come to be dominated by relativistic particles? In other words, what set the conditions at the start of Weinberg’s analysis?
In the following we review various possible causes and consequences of deviations from radiation domination in the early Universe – taking place either before or after Big Bang Nucleosynthesis (BBN) – and the constraints on them, as they have been discussed in the literature during the recent years
We have reviewed several causes and consequences of deviations from radiation domination in the early Universe – taking place either before or after Big Bang Nucleosynthesis – and the constraints on them, as they have been discussed in the literature during the recent years
Summary
More than forty years ago, Steven Weinberg famously summarized the state of early-universe cosmology in his book, The First Three Minutes [1]. Constraints on the effects of neutrinos on the CMB, as well as Big Bang Nucleosynthesis (BBN) constraints, reveal that the Universe had to be radiation dominated (RD) by the time neutrinos decoupled from the thermal plasma, at the energy scale O(1) MeV [15, 16, 17, 18, 19] Between these two epochs stretches a period of cosmic history that is not well constrained by observations (see Fig. 1). A common lore is that the Universe must be radiation dominated prior to BBN This is due to the fact that the abundances of light elements are extremely sensitive to the expansion rate, thermalization of neutrino background, and the time-evolution of the neutron-to-proton ratio and their measured values agree very well.
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