arXiv subjects

David Andriot

Publications and source records attributed to David Andriot.

At least 19 recordsLinked to original sources

arXiv2026

Dark energy from string theory: an introductory review

Dark energy, the main constituent in our expanding universe, responsible for its acceleration, is currently being observed with unprecedented precision through various experiments. While several cosmological models can fit this latest data, deriving some of them from string theory would provide a valuable theoretical prior, with information on the nature of dark energy. This article reviews the efforts towards such a derivation, namely the options from string theory to get a cosmological constant (a de Sitter solution) or a dynamical dark energy (via a quintessence model). After providing a brief historical perspective, we first review proven or conjectured constraints on obtaining dark energy from string theory, in classical or asymptotic regimes. Circumventing such obstructions, by changing regime or ansatz, one can try to construct a de Sitter solution: we present a long list of such attempts, and the difficulties encountered. Among them, we discuss in detail efforts towards classical de Sitter solutions. Then, we review quintessence from string theory, focusing on single-field exponential models. Related topics are discussed, including the coupling to matter, the comparison to observational data, and the absence of a cosmological event horizon.

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arXiv2025

Phantom matters

Cosmological observations of the recent universe suggest that dark energy equation of state parameter $w$ is growing with time, departing from a cosmological constant for which $w=-1$. Standard quintessence models allow for a varying $w\geq-1$, but observations report that a phantom regime, $w<-1$, is quickly reached in the past. Often discarded because of uncertainties or parametrisation, we rather propose here to embrace the reality of this phantom regime. We revisit an elegant mechanism that accounts for it, thanks to a coupling of quintessence field(s) to matter (and possibly radiation). We show that this allows for steep scalar potentials, and illustrate this with string-inspired models, where $V=V_0\, e^{-\lambda\, \varphi}$ and $\lambda \geq \sqrt{2}$. Those provide solutions in very good agreement with observations, including the phantom regime. We then discuss poles that can appear in $w$, making it diverge at recent times ($z\leq 4$), and that could be detected by observations. We finally comment on an Early Dark Energy-like feature, that systematically appears for free from the models considered, and could be of interest for the Hubble tension.

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arXiv2025

Scale separation, rolling solutions and entropy bounds

We revisit scale separation for compactifications of ten- and eleven-dimensional supergravity. For cosmological solutions rolling down flux-generated potentials, we observe that scale separation is achieved as time flows, and is fairly generic. This is realized without the need of orientifolds nor corrections to the classical supergravity approximation. We then confront scale separation with the Covariant Entropy Bound (CEB) and the CKN bound. We show that a naive application of these bounds to vacua hints at the existence of at least two extra dimensions. For rolling solutions, we observe that the CEB is not always respected, but since these examples lack a cosmic horizon, the application of entropy bounds remains delicate.

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arXiv2025

Single versus multifield scalar potentials from string theory

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 are multifield, 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 potential $V$ is also more delicate in a multifield setting. We provide several examples of string effective theories with $V>0$, where the latter admits an asymptotically flat direction along an off-shell field trajectory: in other words, there exists a limit $\varphi \rightarrow \infty$ for which $\frac{|\partial_{\varphi} V|}{V} \rightarrow 0$. It is thus meaningless to look for a lower bound to this single field quantity in a multifield setting; the complete gradient $\nabla V$ is 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 for $V<0$.

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arXiv2024

Quintessence: an analytical study, with theoretical and observational applications

We focus on minimally coupled (multi)field quintessence models, of thawing type, and their realistic solutions. In a model-independent manner, we describe analytically these cosmological solutions throughout the universe history. Starting with a kination - radiation domination phase, we obtain an upper bound on the scalar potential to guarantee an early kination: $V(\varphi) \ll e^{-\sqrt{6} \varphi}$. Turning to the radiation - matter phase, we obtain analytic expressions for the scale factor $a(t)$ (not $t(a)$) and the scalar fields $\varphi^i(t)$ (usually neglected). These allow us to evaluate analytically the freezing of scalar fields, typically $\Delta \varphi \lesssim 10^{-2}$, as well as the transition moment of the dark energy equation of state parameter $w_{\varphi}$ from $+1$ to $-1$, with excellent agreement to the numerics. We comment on this freezing in view of string theory model building, and of some cosmological events. Turning to the latest phase of matter - dark energy domination, we show that the (multi)field displacement is sub-Planckian: $\Delta \varphi \leq 1$. We also provide for that phase analytic expressions for $\int (w_{\varphi}+1)\, d N$ in terms of matter evolution; we relate those to observational targets that we propose. Using finally the CPL parametrisation, while discussing a phantom behaviour, we derive analytic bounds on $w_0$ and $w_a$.

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arXiv2024

Exponential Quintessence: curved, steep and stringy?

We explore the possibility that our universe's current accelerated expansion is explained by a quintessence model with an exponential scalar potential, $V =V_0\, e^{-\lambda\, \phi}$, keeping an eye towards $\lambda \geq \sqrt{2}$ 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 all $\lambda>0$, 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 $\lambda \lesssim \sqrt{3}$, 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 for $\lambda>\sqrt{2}$. 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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arXiv2024

On classical de Sitter solutions and parametric control

Finding string backgrounds with de Sitter spacetime, where all approximations and corrections are controlled, is an open problem. We revisit the search for de Sitter solutions in the classical regime for specific type IIB supergravity compactifications on group manifolds, an under-explored corner of the landscape that offers an interesting testing ground for swampland conjectures. While the supergravity de Sitter solutions we obtain numerically are ambiguous in terms of their classicality, we find an analytic scaling that makes four out of six compactification radii, as well as the overall volume, arbitrarily large. This potentially provides parametric control over corrections. If we could show that these solutions, or others to be found, are fully classical, they would constitute a counterexample to conjectures stating that asymptotic de Sitter solutions do not exist. We discuss this point in great detail.

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arXiv2023

Accelerated expansion of an open universe, and string theory realizations

Recently, many works have tried to realize cosmological accelerated expansion in string theory models in the asymptotic regions of field space, with a typical scalar potential $V(φ)$ having an exponential fall-off $e^{-γ\, φ}$. Those attempts have been plagued by the fact that $V$ is too steep, namely $γ\geq 2/\sqrt{d-2}$ in a $d$-dimensional spacetime. We revisit the corresponding dynamical system for arbitrary $d$ and $γ$, and show that for an open universe ($k=-1$), there exists a new stable fixed point $P_1$ precisely if $γ> 2/\sqrt{d-2}$. Building on the recent work arXiv:2210.10813, we show in addition that cosmological solutions asymptoting to $P_1$ exhibit accelerated expansion in various fashions (semi-eternal, eternal, transient with parametrically controlled number of e-folds, or rollercoaster). We finally present realizations in string theory of these cosmological models with asymptotically accelerating solutions, for $d=4$ or $d=10$. We also show that these solutions do not admit a cosmological event horizon, and discuss the possibility of this being a generic feature of quantum gravity.

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arXiv2023

Extensions of a scale-separated AdS$_4$ solution and their mass spectrum

We consider two extensions of the so-called DGKT solution, a 4d scale-separated anti-de Sitter (AdS) solution obtained as a compactification on a 6d torus orbifold. Each extension consists in a specific large $n$ expansion beyond the DGKT solution, where $n$ is the unbounded $F_4$-flux parameter. One of the extensions considered generalizes the known warped, partially backreacted solution. We analyse the two extensions in 10d massive type IIA supergravity as well as in a 4d effective theory, using a general warped compactification formalism, including axions. On top of known corrections to DGKT, we mainly get new ones from $F_4$; other fluxes are very constrained by flux quantization. In each extension, one would expect corresponding corrections to the mass spectrum, before reaching contributions from $\alpha'$-corrections. But the mass spectrum turns out to be robust, and conformal dimensions remain unchanged.

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arXiv2023

Bumping into the species scale with the scalar potential

As a quantum gravity cut-off, the species scale $Λ_s$ gets naturally compared to the energy scale of a scalar potential $V$ in an EFT. In this note, we compare the species scale, its rate $|\nabla Λ_s|/Λ_s$ and their field dependence, to those of a scalar potential. To that end, we first identify a string compactification leading to a scalar potential with the same properties as the species scale, namely, being positive, starting at a maximum in the bulk of field space and going asymptotically to zero. The trajectory followed in our 14-fields scalar potential is the steepest descent. Evaluating the rate $|\nabla V|/V$ along this path, we then observe a local maximum, or bump, a feature noticed as well for the species scale. We investigate the origin of this bump for the scalar potential, and compare it to that of the species scale.

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arXiv2023

Negative scalar potentials and the swampland: an Anti-Trans-Planckian Censorship Conjecture

In this paper, we derive a characterisation of negative scalar potentials, $V<0$, in $d$-dimensional effective theories of quantum gravity. This is achieved thanks to an Anti-Trans-Planckian Censorship Conjecture (ATCC), inspired by a refined version of the TCC. The ATCC relies on the fact that in a contracting universe, modes that become sub-Planckian in length violate the validity of the effective theory. In the asymptotics of field space, we deduce that $-V'/V \geq c_0$ when $V' \geq 0$. The rate $c_0 = 2/\sqrt{(d-1)(d-2)}$ is successfully tested in several string compactifications for $d\geq 4$. In addition, a new asymptotic condition, $V''/V \geq c_0^2$, is derived. By extrapolation to anti-de Sitter solutions of radius $l$, we infer the existence of a scalar whose mass should obey $m^2 l^2 \lesssim -2$. This property is verified in many supersymmetric examples.

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arXiv2023

Automated consistent truncations and stability of flux compactifications

Classical flux compactifications contribute to a well-controlled corner of the string landscape, therefore providing an important testing ground for a variety of conjectures. We focus here on type II supergravity compactifications on 6d group manifolds towards 4d maximally symmetric spacetimes. We develop a code where the truncation to left-invariant scalars and the dimensional reduction to a 4d theory are automated, for any possible configuration of Op-planes and Dp-branes. We then prove that any such truncation is consistent. We further compute the mass spectrum and analyse the stability of many de Sitter, Minkowski or anti-de Sitter solutions, as well as their consistency with swampland conjectures.

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arXiv2023

(Quasi-) de Sitter solutions across dimensions and the TCC bound

In this work, we investigate the existence of string theory solutions with a $d$-dimensional (quasi-) de Sitter spacetime, for $3 \leq d \leq 10$. Considering classical compactifications, we derive no-go theorems valid for general $d$. We use them to exclude (quasi-) de Sitter solutions for $d \geq 7$. In addition, such solutions are found unlikely to exist in $d=6,5$. For each no-go theorem, we further compute the $d$-dependent parameter $c$ of the swampland de Sitter conjecture, $M_p \frac{|\nabla V|}{V} \geq c$. Remarkably, the TCC bound $c \geq \frac{2}{\sqrt{(d-1)(d-2)}}$ is then perfectly satisfied for $d \geq 4$, with several saturation cases. However, we observe a violation of this bound in $d=3$. We finally comment on related proposals in the literature, on the swampland distance conjecture and its decay rate, and on the so-called accelerated expansion bound.

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arXiv2022

Exploring the landscape of (anti-) de Sitter and Minkowski solutions: group manifolds, stability and scale separation

We classified in arXiv:2201.04152 certain 10d supergravity solutions with a 4d de Sitter, Minkowski or anti-de Sitter spacetime. We then found new solutions in previously unexplored classes. In this paper we study their properties, compare them to swampland conjectures, and make new observations. Using new numerical tools, we first identify all Lie algebras underlying the 6d group manifolds, allowing us to discuss their compactness. We then investigate scale separation, and prove related no-go theorems. Last but not least, we automatize and analyze the stability of all solutions. This leads us to propose the Massless Minkowski Conjecture, claiming the systematic presence of a 4d massless scalar field.

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arXiv2022

Looking for structure in the cobordism conjecture

The cobordism conjecture of the swampland program states that the bordism group of quantum gravity must be trivial. We investigate this statement in several directions, on both the mathematical and physical side. We consider the Whitehead tower construction as a possible organising principle for the topological structures entering the formulation of the conjecture. We discuss why and how to include geometric structures in bordism groups, such as higher U(1)-bundles with connection. The inclusion of magnetic defects is also addressed in some detail. We further elaborate on how the conjecture could predict Kaluza--Klein monopoles, and we study the gravity decoupling limit in the cobordism conjecture, with a few observations on NSNS string backgrounds. We end with comments in relation to T-duality, as well as the finiteness conjecture.

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arXiv2022

Charting the landscape of (anti-) de Sitter and Minkowski solutions of 10d supergravities

We classify solutions of 10d type IIA/B supergravities with orientifolds, on a 4d maximally symmetric spacetime times a 6d group manifold. We then look for new solutions in previously unexplored solution classes, and find some: (anti-) de Sitter solutions with intersecting O4, O6 and D6, or Minkowski solutions with 3 intersecting O5, among others. We provide the numerical code that we developed for this purpose. We also prove new no-go theorems against (anti-) de Sitter solutions. We finally conjecture the absence of de Sitter solution for 2 or less intersecting source sets, implying that a 4d effective theory with de Sitter is at most N=1 supersymmetric.

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arXiv2021

Warp factor and the gravitational wave spectrum

A distinct signature of compact extra dimensions would be a Kaluza-Klein tower of gravitational waves. Motivated by this prospect, we compute the corresponding spectrum on a warped toroidal background. We evaluate in particular the impact of the warp factor on the spectrum. To that end, we use the complete warp factor H of standard string compactifications, generated by D-branes and orientifolds, thus connecting to recent works on stringy de Sitter constructions. The problematic region close to an orientifold where H < 0 leads to unphysical tachyonic modes in the spectrum. We develop tools that overcome this difficulty and lead to a tachyon-free spectrum. We show, in particular, that the warp factor can lower the first Kaluza-Klein mass by at least 69%.

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arXiv2021

Tachyonic de Sitter solutions of 10d type II supergravities

Cosmological models of the early or late universe exhibit (quasi) de Sitter space-times with different stability properties. Considering models derived from string theory, the swampland program does not provide for now a definite characterisation of this stability. In this work we focus on de Sitter solutions of 10d type II supergravities, candidates for classical de Sitter string backgrounds: surprisingly, all known examples are unstable with $η_V < -1$. We aim at proving the existence of such a systematic tachyon, and getting formally a bound on the value of $η_V$. To that end, we develop three methods, giving us various sufficient conditions for having a tachyon upon assumptions, in analogy with de Sitter no-go theorems. Our analysis eventually indicates the existence of variety of different tachyons, and related bounds on $η_V$. We use this knowledge to find 10 new de Sitter solutions of type IIB supergravity, that have tachyons of a different kind, higher $η_V$ values and new 6d geometries. One solution even appears to be stable, with however non-compact extra dimensions.

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