Sommerfeld Theory Colloquium (ASC)

Testing fundamental physics with CMB, The Planck mission was launched successfully in May last year. I will give a summary of the scientific aims of the Planck mission and a brief overview of its current status. I will also place the Planck mission in context with ground and suborbital CMB experiments and other probes of early universe cosmology.

Gedanken World without Higgs Fields, To illuminate how electroweak symmetry breaking shapes the physical world, we investigate what the world would be like in the absence of electroweak symmetry breaking at the usual scale, whether by the conventional Higgs mechanism or by any of its alternatives, including dynamical symmetry breaking and higher-dimensional formulations. Many interesting charac- teristics of the models stem from the fact that the effective strength of the weak interactions is much closer to that of the residual strong interactions than in the real world. The Higgs- free models not only provide informative contrasts to the real world, but also lead us to consider intriguing issues in the application of field theory to the real world.

The knowledge of all correlation functions of a system is equivalent to solving the corresponding quantum many-body

problem. If one can identify the relevant degrees of freedom, the knowledge of a finite set of correlation functions is in

many cases sufficient to determine a sufficiently accurate solution of the corresponding field theory. Complete

factorization is equivalent to identifying the relevant degrees of freedom where the Hamiltonian becomes diagonal. I

will give examples how one can apply this powerful theoretical concept in experiment.

A detailed study of non-translation invariant correlation functions reveals that the pre-thermalized state a system of

two 1-dimensional quantum gas relaxes to after a splitting quench [1], is described by a generalized Gibbs ensemble

[2]. This is verified through phase correlations up to 10th order.

Interference in a pair of tunnel-coupled one-dimensional atomic super-fluids, which realize the quantum Sine-Gordon /

massive Thirring models, allows us to study if, and under which conditions the higher correlation functions factorize

[3]. This allowed us to characterize the essential features of the model solely from our experimental measurements:

detecting the relevant quasi-particles, their interactions and the different topologically distinct vacuum-states the

quasi-particles live in. The experiment thus provides a comprehensive insight into the components needed to solve a

non-trivial quantum field theory.

Our examples establish a general method to analyse quantum systems through experiments. It thus represents a

crucial ingredient towards the implementation and verification of quantum simulators.

Work performed in collaboration with E.Demler (Harvard), Th. Gasenzer und J. Berges (Heidelberg).

Supported by the Wittgenstein Prize, the Austrian Science Foundation (FWF): SFB FoQuS: F40-P10 and

the EU: ERC-AdG QuantumRelax

[1] M. Gring et al., Science, 337, 1318 (2012);

[2] T. Langen et al., Science 348 207-211 (2015).

[3] T. Schweigler et al., Nature 545, 323 (2017), arXiv:1505.03126

We shall review the attempts to extend the quantum

mechanical property of non-commutativity from phase space to

ordinary space. These attempts took a more precise form in the

case of gauge theories for which some concrete results have

been obtained. In flat space they amount to a reformulation of

the theory which looks interesting but they have not given so

far any novel physical results. However, the introduction of

gravity gives a richer structure and may offer some new

insights.

The construction of a quantum theory of gravity

remains an open problem despite decades of efforts.

In time, the very perspective on this problem evolved.

From quantising General Relativity, the goal is now

mostly understood to be unraveling a more

fundamental microstructure of spacetime, based on

non-geometric building blocks, and to show how

spacetime and matter emerge as effective, approximate

notions. Given some candidate building blocks, the task

becomes analogous to that of extracting the

macroscopic, collective behaviour of the atoms of a

condensed matter system, but even more challenging

since we cannot use the usual spacetime intuition and

no direct observational input is available to guide

theory construction.

Lacking a fundamental theory of quantum gravity,

existing cosmological models which have proven

extremely successful in accounting for the observed

features of the very early universe (via CMB data)

remain without a solid foundation, having to make a

number of assumptions about a physical regime (close

to the big bang), where the quantum nature of gravity

and spacetime is expected to be relevant. This is all the

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more unfortunate, since the very early universe is also

where any proposed quantum theory of gravity has the

highest chance of finding its observational test-bed.

The gap needs to be bridged.

In this talk I will first of all review the basic aspects of

the problem of quantum gravity, and of some current

approaches. I will then focus on one specific formalism

for quantum gravity, so-called group field theories

(strictly related to a number of other modern

approaches). I will introduce its main features, trying to

clarify the nature of the suggested building blocks of

spacetime and their mathematical description. Next, I

will outline a general strategy to extract an effective

cosmological dynamics from quantum gravity, within

this formalism. In this setting, the universe emerges as

a quantum condensate of the fundamental “atoms of

spacetime”, and cosmology is its corresponding

hydrodynamics. Finally, I will summarize the recent

results obtained along this research direction.

Metagenomics has revealed hundreds to thousands of microbial species

coexisting in almost all microbiota. It is increasingly appreciated

that microbial communities condition their own environments.

To better understand the role of this environmental conditioning in

promoting diversity, we physically model the population dynamics of

microbes that compete for steadily supplied resources. In a model

where cells require multiple nutrients, we find that population dynamics

generally leads to the coexistence of different metabolic types,

which satisfy an extended competitive exclusion principle. Moreover,

we establish that these consortia of metabolic types act as cartels,

whereby population dynamics pins down resource concentrations at

values for which no other strategy can invade. Strikingly, these cartels

also yield maximum biomass, constituting a microbial example

of Adam Smith’s “invisible hand” leading to collective optimal usage

of resources. Curiously, in a model where only total resource acquisition

is considered, diversity can arbitrarily exceed that predicted by

the competitive exclusion principle.

The observed deviations from the laws of gravity of Newton and Einstein

in galaxies and clusters can logically speaking be either due to

the presence of unseen dark matter particles or due to a change in

the way gravity works in these situations. Until recently there was

little reason to doubt that general relativity correctly describes gravity

in all circumstances. In the past few year insights from black hole

physics and string theory have lead to a new theoretical framework in

which the gravitational laws are derived from the quantum entanglement

of the microscopic information that is underlying space-time.

An essential ingredient in the derivation is of the Einstein equations

is that the vacuum entanglement obeys an area law, a condition that

is known to hold in Anti-de Sitter space due to the work of Ryu

and Takayanagi. We will argue that in de Sitter space due to the

positive dark energy, that the microscopic entanglement entropy also

contains also a volume law contribution in addition to the area law.

This volume law contribution is related to the thermal properties of

de Sitter space and leads to a total entropy that precisely matches the

Bekenstein-Hawking formula for the cosmological horizon. We study

the effect of this extra contribution on the emergent laws of gravity,

and argue that it leads to a modification compared to Einstein gravity.

We provide evidence for the fact this modification explains the

observed phenomena in galaxies and clusters currently attributed to

dark matter.

In February 2016 the LIGO team announced the detection of gravitational

waves (GW) created by the merger of two black holes. In addition

to confirming a major prediction of general relativity, successful

GW detection would provide a powerful new tool for astrophysics.

Given their evident importance, the LIGO results and the methods

which led to them deserve independent critical analysis. This talk

will present the results of one such study in a manner suitable for

non-specialists.

Over the past ten years, there has been a growing interest among physicists

for ‘active matter’, a codename that encompasses systems in which energy is

taken from the environment to generate self-propulsion at the single particle

level. Active particles, such as run-and-tumble bacteria, self-diffusiophoretic

colloids or actin filaments in motility assays, are strongly out-of-equilibrium

and exhibit much richer behaviours than their passive counterpart.

In this talk I will review recent progresses regarding the physics of active

particles. I will show how simple concepts like pressure, the force density

exerted by assemblies of particles on their container, play a new role for

active systems because of the lack of equation of state. I will also show how

new collective phenomena emerge, from the transition to collective motion

to the existence of cohesive matter without cohesive forces, that have no

counterpart in thermal equilibrium.

I will review for a general audience some recent developments in our

understanding of the mathematical structure of scattering amplitudes

in quantum field theory. Many of these developments involve properties

that have been discovered ”experimentally”: not in actual experiments,

but by carrying out a tedious calculation and then observing

that the result has some remarkable hidden simplicity. I will give

examples of this phenomenon, and in particular I will discuss some

aspects of the geometry of the ”amplituhedron”, a geometric object

that is believed to completely encode certain scattering amplitudes.