A stringy perspective on the coincidence problem

Abstract

We argue that, for string compactifications broadly consistent with swampland constraints, dark energy is likely to signal the beginning of the end of our universe as we know it, perhaps even through decompactification, with possible implications for the cosmological coincidence problem. Thanks to the scarcity (absence?) of stable de Sitter vacua, dark energy in string theory is assumed to take the form of a quintessence field in slow roll. As it rolls, a tower of heavy states will generically descend, triggering an apocalyptic phase transition in the low energy cosmological dynamics after at most a few hundred Hubble times. As a result, dark energy domination cannot continue indefinitely and there is at least a percentage chance that we find ourselves in the first Hubble epoch. We use a toy model of quintessence coupled to a tower of heavy states to explicitly demonstrate the breakdown in the cosmological dynamics as the tower becomes light. This occurs through a large number of corresponding particles being produced after a certain time, overwhelming quintessence. We also discuss some implications for early universe inflation.

A stringy perspective on the coincidence problem

“…Without the clockwork, the dark energy dynamics should also be spoilt. Whilst this idea is appealing we have not been able to identify a stringy realisation of the clockwork dark energy model, at least within the context of perturbative type IIA and IIB supergravity [56].…”

“…A complete analysis of this phenomena requires a detailed numerical study of particle creation on the dynamical background, taking into account the effect of the time varying mass and the de Sitter cosmology. This work [49] is now underway but beyond the scope of the current letter. To get some immediate insight into what might happen we neglect the curvature of the background spacetime and focus on the particle production due to the time varying mass on a Minkowski geometry.…”

Quadratic curvature corrections to stringy effective actions and the absence of de Sitter vacua

“…The difficulty of finding de Sitter vacua could make the idea of a dynamical model of dark energy in the bulk of moduli space more appealing. Such models could also offer a solution to the cosmological coincidence problem [20]. However, it should be noted that engineering a viable quintessence model building has many difficulties [21][22][23][24][25], and it is likely that quintessence could be even harder to obtain than a true de Sitter vacuum [23,25].…”

A New Parameterized Interacting Holographic Dark Energy

“…Nevertheless, this fact poses a problem because it requires very specific initial conditions in the early Universe. This is the so-called coincidence problem [8,9]. Our uncertainty about the nature of the cosmological constant and the associated theoretical problems have motivated an extensive study of alternative DE models [10][11][12].…”