## Phenomenological aspects of string theory:

from cosmology to particle physics and quantum matter

Focus session with DIEP support | 26th January 2022| 14:35 - 16:15 | hybrid | Physics@Veldhoven 2022

## Eva Silverstein

## Alex Cole

## Umut Gursoy

## Organizers and chairs: Alejandra Castro and Jácome Armas

## Koenraad Schalm

## An invitation to string theory

###### Combining gravity with quantum theory is one of the most challenging problems that theoretical physics faces today. On the one hand, this inherent difficulty rests on the fact that general relativity and quantum field theory, two of the pillars of theoretical physics, are very tight frameworks while, on the other hand, there is yet no direct experimental evidence of quantum gravitational effects that can guide theoreticians in a specific direction. This session focuses on phenomenological aspects of string theory - the most promising candidate for a theory of quantum gravity. During the past few years string theory has contributed to the constraining of early universe physics, visible via the PLANCK collaboration reports; it has taken holography to the labs in the Netherlands via collaborations with quantum matter experimentalists and theoreticians; it has made crucial contributions to the modeling of heavy-ion collisions; and it began making use of machine learning techniques in order to determine which solution of string theory describes our universe. This focus session features a talk by Eva Silverstein, a world expert in cosmological applications of string theory, and Netherlands-based scientists Alex Cole, Koenraad Schalm, and Umut Gursoy, whose talks will cover developments on string vacua and the connections of string theory with quantum matter and heavy-ion collisions.

## Programme:

######

###### The accelerated expansion of the universe demands a more complete framework for quantum gravity, and offers new observational handles on high energy physics. After summarizing the basic observations and theoretical implications, I will explain some recent developments. In string theory, metastable accelerated expansion plausibly results from an interplay of highly structured energy sources. The resulting de Sitter geometry contains horizons, which play a key role in seeding structure, while raising basic conceptual questions. The anti-de Sitter/conformal field theory duality, which formulates quantum gravity in terms of quantum field theory, does not directly apply. We show how it can be upgraded in a way that preserves some essential features, leading to a statistical interpretation of the Gibbons-Hawking horizon entropy while raising new questions. Moving from thought experiments back to empirical observations, we provide some new examples of observational probes of high energy physics in the early universe, taking into account the strong nonlinearities that can build up in cosmology.

###### Cosmological observables and string theory

Eva Silverstein, Stanford University | 14:35-15:00

###### Do androids dream of Calabi-Yau?

Progress in machine learning the string landscape

Alex Cole, UvA | 15:00-15:25

String theory appears to have a vast number of vacua, making up what is called the string landscape. One of the primary goals of string phenomenology is to understand the consequences of this data set for low-energy physics. The landscape’s size (e.g. 10^272,000 states in a small corner) makes a brute-force scan infeasible. Moreover, constructing consistent string vacua is generally subject to the wrath of computational complexity. On the other hand, data science and machine learning techniques have recently proven able to tame large data sets (e.g. in computer vision) and solve complex tasks (e.g. the game of Go). In this talk, I will review recent progress in using data science and machine learning techniques to understand the string landscape. Topics to be covered include (1) evolutionary and reinforcement learning-based search algorithms for constrained vacua (2) explicit construction of Calabi-Yau metrics (3) sampling algorithms and the statistics of vacua.

######

The plasma of quarks of gluons produced in heavy ion collisions is believed to be a strongly correlated quantum fluid. A standard approach to QGP, therefore, has involved relativistic hydrodynamic simulations which show remarkable agreement with flow correlators observed in these collisions. Being an effective theory, however, hydrodynamics is incomplete without specification of microscopic properties such as the transport coefficients. Gauge-gravity duality has emerged as a supplementary technique that determines the latter in a consistent fashion. After reviewing applications of hydrodynamics and holography to heavy-ion collisions, emphasising their universal aspects and limitations, I will focus on the latest developments in this field, in particular effects of the intense magnetic fields and the strong vortical structure in the plasma recently observed in off-central collisions.

###### Hydrodynamics and holography for heavy ion collisions

Umut Gursoy, U. Utrecht | 15:25-15:50

###### Explaining Quantum Matter with Black Holes

Koenraad Schalm, U. Leiden | 15:50-16:15

25 years ago Maldacena uncovered a remarkable mathematical equivalence between strongly coupled quantum systems and quantum gravity/string theory in one extra dimension: 15 years ago we pioneered the application of this holographic theoretical tool to strongly correlated electron systems in condensed matter. Comparing its theoretical predictions to experiments on so called strange metals --- a novel state of quantum critical matter where no long-lived quasiparticles exist --- the resemblance is remarkable. There is now an energetic quest for a "smoking gun" prediction in strongly correlated systems that is singularly provided by a holographic black hole computation. We will show a number of recent candidates and explain why the predicted experimental signatures are directly tied to black hole physics.