Schedule for: 19w5094 - Geometrical Tools for String Cosmology

Cosmological inflation predicts the existence of a stochastic background of gravitational waves, whose features depend on the model of inflation under consideration. There exist well motivated frameworks predicting an enhancement of the primordial gravity wave spectrum at frequency scales testable with gravitational wave experiments, with distinctive features as parity violation and non-Gaussianity. I will explain the properties of such scenarios, and their distinctive predictions for what respect the gravity wave power spectrum and bispectrum. I will then discuss how to quantitatively test these predictions with current and future interferometers.

It was recently shown that axions can be a prominent source of gravitational waves. We first show that an axion which was located at a shallower region of the scalar potential than the quadratic one generically undergoes parametric resonance instability. We also studied non-linear dynamics of the axion caused by the resonance instability based on lattice simulation. We show that string axions in various mass ranges generate gravitational waves (GWs) with peaks at various frequencies determined by the mass scales, dubbed the GW forest. This may allow us to explore string axiverse through future multi-frequency GW observations.

Recent years have seen the introduction of various multi-field inflationary scenarios, in which the scalar curvature plays a crucial role. We outline a simple description that unifies these different proposals. Moreover, one can easily see which dynamical attractors will dominate the late-time behaviour of the system. We demonstrate which bifurcations govern the underlying dynamics.

In general relativity producing a regular bounce entails violation of Null Energy Condition for a dynamical source in the model. That generically indicates existence of ghosts or other instabilities. However, in cuscuton modification of gravity, the correspondence between a background bounce and violation of Null Energy Condition for dynamical sources is broken. Cuscuton action, modifies equations of motion in Infra Red limit allowing the background to go through a regular bounce phase. At the same time, since it does not contain any dynamical degrees of freedom, it does not lead to ghosts or other instabilities. Here, I present a toy scenario of a regular bouncing cosmology and prove this claim

We argue that the presence of an inflationary epoch is a natural, almost unavoidable, consequence of the existence of a sensible effective action involving an infinite tower of higher-curvature corrections to the Einstein-Hilbert action. No additional fields besides the metric are required. This correction to General Relativity has the following features: (i) its vacuum spectrum solely consists of a graviton and is ghost free, (ii) it possesses well-behaved black hole solutions, (iii) its cosmology is well-posed as an initial value problem and, most importantly, (iv) it has the potential to provide a late-time cosmology arbitrarily close to LCDM while, at the same time, giving an inflationary period in the early universe with a graceful exit

Effective field theory methods underpin quantum calculations with gravity, and thereby ensure the reliability of predictions for Hawking radiation or inflationary primordial fluctuations. Yet persistent concerns (like the phenomenon of secular growth in primordial fluctuations or information loss for Hawking radiation) appear to indicate a breakdown of these techniques in what ought to be a regime under good control. This talk argues that gravity indeed introduces new boundaries to the domain of validity of EFT methods, particularly at late times (where most of the puzzles seem to lie). Evidence for this picture is provided, together with remedies that come from other, more familiar, areas of physics where similar issues also arise.

Non-perturbative effects were long ago proposed to be useful for supersymmetric moduli stabilization in Type II string theory compactifications. Their use has been widely studied from the 4D perspective but from the 10D perspective this mechanism is far from being understood. By performing a 10D analysis I will show several features of the mechanism, confirming some old 4D results as well as providing new properties of the compactification.

We study how the presence of hierarchy on the moduli fields plays a determinant role for satisfying the recent Swampland criteria for some concrete examples.

Inflation is a phase of accelerated expansion, taking place at very high energy in the early Universe. During this epoch, inhomogeneities are generated on cosmological scales from the amplification of quantum fluctuations of the gravitational and matter fields, that are stretched to distances of astrophysical interest today. The energy scales involved during this early epoch are many orders of magnitude greater than those accessible in particle physics experiments. The early Universe is thus one of the most promising probes to test far beyond standard model physics. When inflation is embedded in high-energy theories (supersymmetry, super-gravity, string theory, etc), different inflationary scenarios emerge that lead to different observational predictions. Those can be tested against high-precision measurements of the cosmic microwave background temperature and polarisation anisotropies. I will review the current status of the numerous proposed models, and highlight those for which the data show the strongest preference.

We explore the connection between mirror symmetry in non-complete intersection Calabi-Yau varieties and newly developed non- abelian dualities in their gauged linear sigma model descriptions.

I will focus on the Swampland Distance Conjecture for which infinite distances in field space imply an infinite tower of states becoming exponentially light. We present new evidence for this conjecture in Calabi-Yau manifolds, by studying the monodromy transformations associated to the infinite distance singularities in the moduli space. This monodromy generates an infinite orbit within the spectrum of BPS states and allow us to classify the different types of infinite distance loci. We apply the results to the complex structure and Kahler moduli spaces of Type II and M/F-theory Calabi-Yau compactifications. I will also discuss the implications for Inflation, including alpha-attractor models.

I will focus on type IIB Fibre Inflation and discuss first the interplay between reheating and the production of axionic dark radiation after the end of inflation. I will then analyse the possibility to have a potentially dangerous geometrical destabilisation of ultra-light isocurvature modes

Inflation can be thought of as a low energy process of an ultraviolet theory. It is therefore natural to expect that several degrees of freedom might constitute the inflationary dynamics. We will discuss how such multifield set-ups can leave signatures in the statistics of temperature and matter inhomogeneities, giving us observational access to the UV framework.

I will briefly review the issues of de Sitter vacua in string theory in relation to the various swampland conjectures which suggest a tension between an effective description of universe in accelerated expansion and its possible unitary UV completion. Subsequently I will address this UV/IR puzzle within the specific context of massive type IIA compactifications and discuss the status of the search for classical de Sitter solutions arising from reductions on SU(3) structure manifolds. I will then conclude by discussing the controversial issue of defining a good weakly coupled regime for such backgrounds.

We consider the geometry and physics of F-theory on smooth torus fibered quotient Calabi-Yau threefolds. The free quotient produces orbifold fixed points on the base with multiple fibers and destroys the section of the fibration. We proof full anomaly cancellation for a generic quotient and discuss the remarkable features of these theories: The multiple fiber introduces a monodromy, that breaks the gauge group of the covering theory to a discrete remnant and in addition gauges the strongly coupled sector at the fixed points. These theories have a non-trivial tensor branch, which are resolutions of Lens spaces in the threefold geometry. Finally we use this procedure to comment on realizations and bounds of higher order discrete symmetries within the context of the swampland program.

We construct models with U(1) gauge group and matter with charges up to 6, in the context of type IIB compactifications. We show explicitly that models with charges up to 4 can be derived from corresponding models in F-theory by applying the Sen weak coupling limit. We derive which type IIB models should be the limit of charge 5 and 6 F-theory models. Explicit six dimensional type IIB models with maximal charge 5 and 6 are constructed on an algebraic K3 surface that is the double cover of $\mathbb{CP}^2$. By using type IIB results we are also able to rediscover the F-theory charge 4 model in a straightforward way

Theories with several hundred axion fields have enormous numbers of distinct meta-stable minima. A small fraction of these local minima have vacuum energy compatible with current measurements of dark energy. The potential also contains regions suitable for inflation, and gives rise to a natural type of dark matter. First-order phase transitions from one minimum to the vicinity of another play the role of big bangs and produce many bubbles containing evolving Friedmann-Lemaitre-Robertson-Walker universes. The great majority either collapse in a tiny fraction of a second, or expand exponentially forever as empty, structureless universes. However, restricting to those bubble universes that form non-linear structure at some time in their history we find cosmologies that look remarkably similar to ours. They undergo about 60 efolds of inflation, making them flat, homogeneous and isotropic, and endowing them with a nearly scale-invariant spectrum of primordial density perturbations with roughly the observed magnitude and tilt. They reheat after inflation to a period of radiation domination, followed by matter domination with roughly the observed abundance, followed by vacuum energy domination at roughly the observed density. None of these features require any model building or small parameters. Instead, all dimensionful parameters in the theory can be set equal to the grand unified scale 0.01 M_p, and the dimensionless parameters are order one and can be chosen randomly. The small value of dark energy ultimately comes from non-perturbative gravitational effects, giving an exponentially small vacuum energy density. Therefore, random axion landscapes can account for many of the apparently tuned features of our universe, including its current enormous size, age, and tiny energy densities compared to the scales of fundamental physics.

In string theory, we face a gigantic number of backgrounds, each of which comes with different implications for particle physics and cosmology. On top of this, every backgrounds has a huge number of possible vacua or near-vacua. We describe the computational complexity of the challenges associated with both finding a viable background and finding vacua for this background, and apply machine learning to study a small subset of them.

Compactifications of string theory with geometric and non-geometric fluxes give rise to effective field theories with non-trivial scalar potentials, which can be used for moduli stabilization and cosmology. In this talk I briefly review such flux compactifications, emphasize various consistency conditions, and report on preliminary new results on the distribution of flux vacua.

We show how the Wess-Zumino terms of the different branes in string theory can be embedded within Double Field Theory. Crucial ingredients in our construction are the identification of the correct brane charge tensors and using the Double Field Theory potentials that arise from dualizing the standard Double Field Theory fields. This leads to a picture where under T-duality the brane does not change its worldvolume directions but where instead it shows different faces depending on whether some of the worldvolume and/or transverse directions invade the winding space.