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.

Exploring the spectral shape of gravitational waves induced by primordial scalar perturbations and connection with the primordial black hole scenarios

“…The interpretation of a GW signal is nevertheless a non-trivial task and one needs to take into account several parameters, such as the reheating temperature and the equation of state of the early universe, as well as whether the primordial fluctuations are Gaussian or a non-negligible non-Gaussianity exists [21,24,27,34]. From a different point of view, detection of the relic GWs stochastic background is a direct probe of the very early cosmic history, which is unknown for t 1s [35]. In this context, the recent works [36][37][38] have studied the impact of different early cosmological evolution on the IGWs.…”

Phase Transitions in an Expanding Universe: Stochastic Gravitational Waves in Standard and Non-Standard Histories

“…In other words, we would like to interrogate how well a phase transition can serve as a cosmic witness. This is important, since growing evidence suggests that the standard assumption of radiation domination prior to Big Bang Nucleosynthesis may be too naive [54,55]. An early matter dominated era, for example, is motivated by the cosmological moduli problem [56][57][58][59], hints from dark matter searches [60][61][62][63][64][65][66][67][68], and perhaps even baryogenesis [69].…”

Non-thermal Hot Dark Matter from Inflaton/Moduli Decay: The Momentum Distribution and Relaxing the Cosmological Mass Bound

“…The present work together with the ongoing MCMC analysis should provide an interesting theory input for all of this. 8 It will be very interesting to see if the effect of free-streaming is reduced or enhanced in comparison with similar WDM mass produced by standard mechanism such as in [15]. Assuming WDM becomes non-relativistic in the radiation dominated epoch, in the approximation τ t nr (t nr being the time that the dark matter particle go nonrelativistic), free streaming length is known to scale as t nr /a nr .…”

“…In this context, if the inflaton decays perturbatively, then the reheating epoch is matter dominated with cold inflaton particles dominating the energy density of the universe [2]. Furthermore, in string and supergravity models, an epoch of early matter domination arising from vacuum misalignment of moduli fields is a generic feature [3][4][5][6] (see e.g [7,8] for reviews). An epoch of matter domination ends with the decay of the associated cold particles.…”