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.
Highlights
There have been several proposals that touch upon aspects of this problem
The purpose of this paper is to investigate the extent to which string theory may point to a similar resolution of the coincidence problem via the swampland conjectures [27,28,29,30,31]
We have argued that string theory compactifications consistent with swampland constraints automatically prevent the dark energy era from extending beyond a few hundred Hubble times
Summary
We begin with the distance conjecture [29, 32,33,34], which is one of the most well studied and least controversial of the swampland conjectures (see [41,42,43,44,45,46,47,48, 55]). This initial displacement will grow thanks to the fact that the Hessian condition implies a tachyonic mass for the fluctuations in the dark energy field, μ2 = V (φ) < 0 with |μ2|. We cannot rule out the possibility that the current phase of acceleration is approaching a stable de Sitter configuration in which dark energy continues for an exponentially large number of Hubble epochs If this is the case, the coincidence problem is as problematic as ever. We might worry about the fact we have assumed dark energy to be a single canonical scalar We expect this to capture the generic dynamics of fields moving through moduli space, with our canonical scalar tangential to the trajectory and all the orthogonal directions stabilised
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