String cosmology backgrounds from classical string geometry

Cosmology at the top of the α′ tower

“…Recent developments, including this current work, certainly pave the way to finding new stringy descriptions of the early universe. Beyond inflation, it may lead to developments in string gas cosmology (see [48,49]), but it would also be interesting to apply the framework to other alternative approaches, including pre-big bang cosmology (as envisioned in [71]) and complementary stringy descriptions of the highdensity state of the very early universe such as in the stringy black hole gas scenario [111]. To go beyond background models, though, lies an important challenge.…”

“…15 Note that the case where w = −(1 + λ/2)/d corresponds to saturating the SEC, and from (A.7), it corresponds to Minkowski space in the Einstein frame. This is the basis of the loitering solution studied in [48,49,78]. 16 The only exception is when λ = −2, in which case it can be shown [47] that the solution H = H0 anḋ Φ = −βH0 is stable for any constant β > 0.…”

“…Zwiebach [42,43] (generalizing [44]), who found the form of the most general action for the metric, the dilaton and the b-field when these fields only depend on time, which is invariant under O(d, d) transformations and which includes α corrections to all orders. The inclusion of coupling to other matter fields was then first explored in [45], and applications to string cosmology have been explored in a few studies so far [42,43,[45][46][47][48][49][50], often discovering new potential solutions, such as de Sitter-like or loitering backgrounds, which are otherwise often difficult to find in string cosmology (see, e.g., [51][52][53][54]). The current paper's aim is to go beyond these recent works on string-cosmology backgrounds to all orders in α .…”

“…8 The present paper shall not be concerned though with the construction of full cosmological scenarios. This has been first explored in, e.g., [48,49] and deserves to be extended in future work.…”

Crisis on infinite earths: short-lived de Sitter vacua in the string theory landscape

“…In fact, as shown in [72,73], the entire tower of corrections is necessary for having dS solutions in the string frame (with zero fluxes and a time-dependent dilaton). After including matter in the form of a perfect fluid energy-momentum tensor in the framework [74], solutions to the equations of motion were studied in [75] and a model sourced by a string gas was introduced in [76]. This model has a short TCC compatible phase of accelerated (super-exponential) expansion that last long enough to solve the hori-JHEP04(2021)037 zon problem of standard cosmology [77], depicting the phenomenological relevance of this class of backgrounds.…”