| Summary: | © 2019 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/" Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP 3 . While the classification of α′ corrections of string inspired effective theories remains an unsolved problem, we show how to classify duality invariant α′ corrections for purely time-dependent (cosmological) backgrounds. We determine the most general duality invariant theory to all orders in α′ for the metric, b-field, and dilaton. The resulting Friedmann equations are studied when the spatial metric is a time-dependent scale factor times the Euclidean metric and the b-field vanishes. These equations can be integrated perturbatively to any order in α′. We construct nonperturbative solutions and display duality invariant theories featuring string-frame de Sitter vacua.
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