Was the Universe actually radiation dominated prior to nucleosynthesis?

Abstract

Maybe not. String theory approaches to both beyond the Standard Model and Inflationary model building generically predict the existence of scalars (moduli) that are light compared to the scale of quantum gravity. These moduli become displaced from their low energy minima in the early universe and lead to a prolonged matter-dominated epoch prior to BBN. In this paper, we examine whether non-perturbative effects such as parametric resonance or tachyonic instabilities can shorten, or even eliminate, the moduli condensate and matter-dominated epoch. Such effects depend crucially on the strength of the couplings, and we find that unless the moduli become strongly coupled the matterdominated epoch is unavoidable. In particular, we find that in string and M-theory compactifications where the lightest moduli are near the TeV-scale that a matter-dominated epoch will persist until the time of Big Bang Nucleosynthesis.1 We work with sign convention (−, +, +, +) and with the reduced Planck mass mp = 1/(8πG) 1/2 = 2.4 × 10 18 GeV. We use Greek indices to denote space-time µ = 0, 1, 2, 3 whereas latin indices imply spatial directions only k = 1, 2, 3.

Opening up the QCD axion window

“…As the axion starts oscillating in the early universe, certain modes of the gauge field become tachyonic, leading to an exponential growth of such modes. A similar mechanism has been previously used for generating primordial magnetic fields [33][34][35][36][37][38][39], for inflation and (p)reheating after inflation [40][41][42][43][44][45][46][47], for moduli decay [48], and for the relaxion [49]. Our mechanism differs from previous cases because we focus on smaller couplings of the axion to the gauge fields, which requires multiple oscillations of the axion until the gauge field grow sufficiently to influence the axion dynamics.…”

Breaking a dark degeneracy: The gamma-ray signature of early matter domination

“…Since the energy density of relativistic particles decreases more rapidly than that of nonrelativistic particles, any heavy field left over from the inflationary epoch would naturally come to dominate the energy density of the Universe, leading to an early matter-dominated era (EMDE); such a field is only required to decay into radiation before the onset of BBN. Well motivated examples of such heavy fields include hidden-sector particles [13][14][15][16][17][18][19][20][21][22][23][24][25], moduli fields in string theory [26][27][28][29][30][31][32][33], and certain spectator fields invoked to generate primordial curvature variations during inflation [34][35][36][37]. After inflation ends, the inflaton itself can * delos@unc.edu † linden.70@osu.edu ‡ erickcek@physics.unc.edu also behave as a pressureless fluid before its decay [38][39][40][41][42][43][44][45][46][47].…”

Probing the pre-BBN universe with gravitational waves from cosmic strings

“…While this assumption is made frequently (and often implicitly), it has not been tested directly. Furthermore, non-standard cosmological scenarios with an extended period of domination by something other than radiation between inflation and BBN have strong motivation from many perspectives, including dark matter, axions, string compactification, reheating, and baryogenesis [10,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. Testing the paradigm of pre-BBN cosmology is therefore of great significance in advancing our understanding of the universe.Gravitational waves (GWs) may provide a means of looking back in time beyond the BBN epoch and probing the universe in its very early stages [10,[27][28][29].…”