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This article explores the implications of the Swampland Conjectures on inflation and dark energy in cosmology, and how recent work in quantum gravity can potentially provide measurable constraints. It discusses the concepts of the Swampland, the Distance Conjecture, the de Sitter Conjecture, and the Refined de Sitter Conjecture, and presents constraints on single-field and multi-field inflation models.

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## What does Inflation say about Dark Energy given the Swampland Conjectures?

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**What does Inflation say about Dark Energy given the**Swampland Conjectures? Berkeley Week @ IPMU 1/11/19 Chien-I Chiang, Jacob M. Leedom, Hitoshi Murayama arXiv: 1811.01987**Modern Cosmology:Inflation & Dark Energy**• Inflation • Early phase of rapid expansion. • Provides a solution to a number of problems – CMB uniformity, monopoles, flatness. • A number of potential QFT models. • Dark Energy • Causing the current accelerating expansion of the universe. • Typically taken to be a cosmological constant. • Origin and small scale are very mysterious. How does recent work in quantum gravity constrain models of these two phenomena in a potentially measurable way?**Outline**• Introduction to the Swampland • Cosmological Implications of the Swampland • Swampland Refinements • Results • Conclusion**Quantum Gravity & The Swampland**• The Landscape – set of all EFTs that can be obtained by some compactification of string theory • The Swampland – set of all EFTs coupled to gravity that are inconsistent with quantum gravity • In other words, the swampland is the set of all theories that are not in the string landscape**Swampland Conjectures**• The goal of the swampland program is to find a set of criterion by which one can determine if a given EFT is in the swampland or landscape. • These criterion are in the form of a set of conjectures – no continuous global symmetries, no free parameters, Weak Gravity Conjecture, ….. • Distance Conjecture – As one traverses a distance D in field space, a tower of light modes appears with mass Therefore, the effective theory breaks down at transplanckian field excursions. • de Sitter Conjecture (dSC) – The scalar potential of an EFT must satisfy**Cosmology and the Swampland:Quintessence**• From dSC - de Sitter vacua not permitted: • Observed cosmological constant should actually be a slow rolling quintessence field! • Consider an Exponential Potential: Experimental bounds [Agrawal et al] and dSC:**Cosmology and the Swampland:Inflation**• Slow Roll parameters • Constraint from e-folds, distance conjecture, and dSC: For 50 e-folds and α=1,**Cosmology and the Swampland:Observation**• Dark Energy Equation of State • The swampland parameter c should be universal in a given EFT, so we can use the upper bound on c from applying the distance conjecture to inflation and find that • Next generation of experiments (Euclid, LSST, DESI,.. ) will probe λ to 0.2-0.4) and it is unlikely that we can constrain λ < 0.1 in the near future [Heisenberg et al]. • Therefore, even if the swampland conjectures are correct, it is possible we would live in a universe with quintessence but be unable to distinguish it from a cosmological constant!**Refined de Sitter Conjecture**• Several refinements of the dSC have been proposed. We will focus on the one in [Ooguri et al] & [Garg & Krishnan] (see [Andriot & Roupec] for a related proposal) • Refined de Sitter Conjecture (RdSC): The scalar potential must satisfy either OR • Relaxes constraint on inflation:**Cosmology and the (refined) Swampland:Single-field Inflation**• We now use the freedom from the RdSC to re-examine the observability of quintessence • We consider a piecewise inflaton potential with Ntot e-folds that satisfies the first part of the RdSC during the first N1 e-folds and the second of the conjecture during the remaining N2 = Ntot - N1 e-folds • Then we require • RdSC: • RdSC + Spectral tilt: • Distance:**Cosmology and the (refined) Swampland: Constraints on**single-field inflation • The constraints on the swampland parameters can be packaged as which is valid so long as N1 < Ntot. • The running of the spectral tilt can be modelled using PLANCK 2018 data: to maximize the parameter space for the swampland parameters, we take the lower 1σ allowed lower end. • Also the experimental bound:**Solid Lines – running of the spectral index up to N1 10 .**• Dashed Lines – continued running of • spectral index to N150. • Dotted lines – Distance conjecture constraint when N1 = Ntot. • Grey region – excluded by bound on r0.002. • If N1 is substantial (>5) then the lower bound on λ is just below observability. • If N1 is small, then quintessence can easily be bounded from below such that it is observable. • Tension with the notion that both c and c’ are both O(1).**Cosmology and the (refined) Swampland:Multi-field Inflation**• In a multi-field inflation model, the Hubble and potential slow roll parameters are not identical: [Credit: Hetz & Palma]**Cosmology and the (refined) Swampland:Multi-field Inflation**• The expression for the spectral index is changed to incorporate a sound speed that varies with time: • Tensor to Scalar ratio expression: • Analysis proceeds as in the single field case. • One caveat – The Distance Conjecture • We take the conservative approach of applying the conjecture to the path length, not geodesic.**For lower sound speeds, one can have**• N110 and force quintessence to be • observable. • Both c and c’ can be O(0.1).**Conclusions**• The refined de Sitter conjecture allows for the possibility of measuring quintessence as dark energy! • Single-Field Inflation • Concave down inflaton potentials are favored but potentials that are concave up are borderline. • Tension with the notion that both c and c’ are both O(1). • Multi-Field Inflation • Reduced sound speed allows for observable quintessence. • Furthermore, O(1)-ish parameters can be accommodated. • Better understanding of c and c’ essential to further constraining observables • Exciting prospects for model building!**References**• [1] . Georges Obied, HirosiOoguri, Lev Spodyneiko ,CumrunVafa – arXiv: 1806.08362. Prateek Agrawal, Georges Obied, Paul J. Steinhardt, CumrunVafa – arXiv 1806.09718 • [2]. Lvinia Heisenberg, Matthias Bartelmann, Robert Brandenberger, Alexandre Refregier – arXiv: 1808.02877 • [3]. Sumit K. Garg, Chethan Krishnan – arXiv:1807.05193 • [4]. HirosiOoguri, Eran Palti, Gary Shiu, CumrunVafa – arXiv:1810.05506 • [5]. David Andriot, Christoph Roupec – arXiv: 1811.08889 • [6]. Alexander Hetz, Gonzalo Palma – arXiv: 1601.05457 • [7]. Daniel Baumann – arXiv: 0907.5424 • [8].Hajime Fukuda, Ryo Saito, Satohi Shirai, Masahito Yamazaki- arXiv:1810.06532

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