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A<sc>bstract</sc> We explore the possibility that our universe’s current accelerated expansion is explained by a quintessence model with an exponential scalar potential,V=V0e−λ ϕ, keeping an eye towardsλ≥$$ \sqrt{2} $$ and an open universe, favorable to a string theory realisation and with no cosmological horizon. We work out the full cosmology of the model, including matter, radiation, and optionally negative spatial curvature, for allλ> 0, performing an extensive analysis of the dynamical system and its phase space. The minimal physical requirements of a past epoch of radiation domination and an accelerated expansion today lead to an upper boundλ≲$$ \sqrt{3} $$ , which is driven slightly up in the presence of observationally allowed spatial curvature. Cosmological solutions start universally in a kination epoch, go through radiation and matter dominated phases and enter an epoch of acceleration, which is only transient forλ>$$ \sqrt{2} $$ . Field distances traversed between BBN and today are sub-Planckian. We discuss possible string theory origins and phenomenological challenges, such as time variation of fundamental constants. We provide theoretical predictions for the model parameters to be fitted to data, most notably the varying dark energy equation of state parameter, in light of recent results from DES-Y5 and DESI.
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This content will become publicly available on May 1, 2026
Single versus multifield scalar potentials from string theory
A<sc>bstract</sc> In this work, we investigate the properties of string effective theories with scalar field(s) and a scalar potential. We first claim that in most examples known, such theories aremultifield, with at least 2 non-compact field directions; the few counter-examples appear to be very specific and isolated. Such a systematic multifield situation has important implications for cosmology. Characterising properties of the scalar potentialVis also more delicate in a multifield setting. We provide several examples of string effective theories withV> 0, where the latter admits an asymptotically flat direction along an off-shell field trajectory: in other words, there exists a limit$$\widehat{\varphi }\to \infty $$for which$$\frac{\left|{\partial }_{\widehat{\varphi }}V\right|}{V}\to 0$$. It is thus meaningless to look for a lower bound to this single field quantity in a multifield setting; the complete gradient ∇Vis then better suited. Restricting to on-shell trajectories, this question remains open, especially when following the steepest descent or more generally a gradient flow evolution. Interestingly, single field statements in multifield theories seem less problematic forV< 0.
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- Award ID(s):
- 2210271
- PAR ID:
- 10651231
- Publisher / Repository:
- Springer
- Date Published:
- Journal Name:
- Journal of High Energy Physics
- Volume:
- 2025
- Issue:
- 5
- ISSN:
- 1029-8479
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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