On classical de Sitter solutions and parametric control

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 towards λ ≥ $$ \sqrt{2} $$ 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 all λ > 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 bound λ ≲ $$ \sqrt{3} $$ 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 for λ > $$ \sqrt{2} $$ 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.

Exponential quintessence: curved, steep and stringy?

Andriot,

Parameswaran,

Tsimpis

et al. 2024

Almost classical de Sitter?

Horer,

Junghans

2024

The classical-dS scenario in the type II string theories proposes to search for dS vacua of orientifold flux compactifications in a regime where string corrections to the compactified effective field theory are negligible. We study a minimal extension of this scenario in which the leading string corrections to the O-plane/D-brane actions at the 4-derivative order are included but higher orders as well as string corrections in the bulk are self-consistently neglected. Our proposal is motivated by a recent debate about dS solutions with O8-planes which circumvent a classical no-go theorem due to unusual sources leading to so-called permissive boundary conditions for the 10D supergravity fields. We argue that such sources do not arise in classical supergravity but ask whether including the 4-derivative corrections leads to sources that have a similar effect. However, we find that the 4-derivative corrections do not allow meta-stable dS in a class of models with O8-planes and/or D8-branes we consider. We also study related models which in addition contain O6-planes/D6-branes and find that again no meta-stable dS is allowed, both classically and including the 4-derivative corrections. While some of the arguments in this work require the backreaction of the O-plane/D-brane sources to be small, others are valid including the full backreaction.

On (A)dS solutions from Scherk-Schwarz orbifolds

Parameswaran,

Serra

2024

We investigate the existence of dS vacua in supersymmetry-breaking Scherk-Schwarz toroidal compactifications of type II string theory, using the well-understood ingredients of curvature, fluxes and 1-loop Casimir energy. Starting from the 10d equations, we derive a series of no-go theorems and existence conditions for dS, and present two explicit, fully-backreacted, solutions: a dS one, which turns out to be not under control, and an AdS one, which can be chosen at arbitrarily weak coupling and large volume by dialling the unbounded fluxes. We then use a lower-dimensional EFT description to show that any dS solution has a universal tachyon and no parametric control. The simplest AdS solutions are also perturbatively unstable. We extend the no-go theorems to slow-roll acceleration and test various swampland conjectures in our non-supersymmetric string setup. The question of numerically controlled, unstable dS is left open.