Sommerfeld Theory Colloquium (ASC)

Sommerfeld Theory Colloquium, Several new techniques are currently being employed to probe the strong gravitational fi�eld in the vicinity of black holes. Long baselineinterferometry at sub-millimeter wavelengths sets constraints on the

silhouette of the black holes in the Galactic center (SgrA*) and M87.

Stars which get tidally disrupted as they orbit too close to a single

black hole are being discovered at cosmological distances. Electromagnetic counterparts of black hole binaries in galaxy mergers are being identifi�ed, and can be used to calibrate the rate of gravitational wave sources. Most interestingly, the recoil induced by the

anisotropic emission of gravitational waves in the �nal plunge of binaries leaves unusual imprints on their host galaxies. Finally, the

lecture will describe new constraints on the contribution of primordial black holes to the dark matter.

Sommerfeld Theory Colloquium, Three-dimensional Einstein gravity has no local dynamical degree of

freedom. Yet, it is far from being trivial when the cosmological constant is negative. (i) It admits black hole solutions. (ii) It easily allows for consistently interacting and tractable higher-spin extensions.

(iii) It possesses remarkable asymptotic properties at infi�nity where

an infi�nite-dimensional symmetry algebra emerges. These features

make three-dimensional gravity a perfect "theoretical laboratory" in which to explore the conceptual issues related to (i), (ii) and (iii) in

a simpler context. The talk will not only discuss three-dimensional

gravity assuming no previous knowledge on the subject, but will also

provide access to recent work where new results on points (i), (ii)

and (iii) are developed.

Sommerfeld Theory Colloquium, In this seminar I will describe some of the hidden structures recently

discovered in the scattering amplitudes of elementary particles, such

as those measured at the Large Hadron Collider. These structures are

responsible for the mysterious simplicity of these quantities, which is

completely obscured by a calculation based on textbook techniques

such as Feynman diagrams. I will then move on to discuss form factors. These are slightly off-shell quantities and, similarly to amplitudes,

are also much simpler than what expected based on conventional

approaches. In particular I will focus on form factors of particular (half-BPS) operators in a special theory, known as N=4 super Yang-Mills, and briefly discuss some unexpected connections

to scattering amplitudes in phenomenologically relevant theories.

Sommerfeld Theory Colloquium, In the standard picture of a quantum phase transition, a single quantum critical point separates the phases at zero temperature. Here

we show that the two-dimensional case is considerably more complex. Instead of the single point separating the antiferromagnet from

the normal metal, we have discovered a broad region between these two phases where the magnetic order is destroyed but certain areas of the Fermi surface are closed by a large gap. This gap reflects the formation of a novel quantum state characterized by a superposition of d-wave superconductivity and a quadrupole density wave

(QDW), which builds a checkerboard pattern with a period incommensurate with that of the original spin density wave. At moderate temperatures both orders co-exist over comparatively large distances but thermal fluctuations destroy the long-range order. Below

a critical temperature the

fluctuations are less essential and super-

conductivity becomes stable. Applying a magnetic field destroys the superconductivity but establishes QDW. In addition to these phases

we obtain also a charge density wave (CDW) arising as a result of interaction of electrons with superconducting fluctuations. This phase

is possible when the superconductivity is destroyed by either thermal fluctuations or a magnetic field. The results of our theory can serve as explanation of recent experiments on cuprates performed with the help of STM, NMR, hard and resonant soft X-ray scattering, sound

propagation, and other techniques.

Sommerfeld Theory Colloquium, Einstein is well known for his rejection of quantum mechanics in the form it emerged from the work of Heisenberg, Born and Schrodinger in 1926. Much less appreciated are the many seminal contributions

he made to quantum theory prior to his �final scientifi�c verdict, that

the theory was at best incomplete. In this talk I present an overview

of Einsteins many conceptual breakthroughs and place them in historical context. I argue that Einstein, much more than Planck, introduced the concept of quantization of energy in atomic mechanics.

Einstein proposed the photon, the fi�rst force-carrying particle discovered for a fundamental interaction, and put forward the notion of wave-particle duality, based on sound statistical arguments 14 years before De Broglies work. He was the fi�rst to recognize the intrinsic

randomness in atomic processes, and introduced the notion of transition probabilities, embodied in the A and B coeffi�cients for atomic emission and absorption. He also preceded Born in suggesting the interpretation of wave fi�elds as probability densities for particles, photons, in the case of the electromagnetic �field. Finally, stimulated by

Bose, he introduced the notion of indistinguishable particles in the

quantum sense and derived the condensed phase of bosons, which is

one of the fundamental states of matter at low temperatures. His

work on quantum statistics in turn directly stimulated Schrodinger

towards his discovery of the wave equation of quantum mechanics. It

was only due to his rejection of the �final theory that he is not generally recognized as the most central �figure in this historic achievement

of human civilization.

Sommerfeld Theory Colloquium, The problem of understanding how gravity fits together with other fundamental interactions of matter has been at the forefront of the- oretical research for many decades, leading to the rich framework of string theory and M-theory. In this framework, many fundamen- tal questions are being resolved, but many remain quite mysterious, suggesting that search for novel concepts may be well justified. I review the recent concept of multicritical gravity with Lifshitz-type anisotropic scaling, and its applications in areas ranging from par- ticle phenomenology beyond the standard model to non-relativistic

versions of the holographic AdS/CFT correspondence.

Sommerfeld Theory Colloquium, Since the emergence of information theory in the middle of the 20th century, this new scientific field has been deeply entangled with statistical physics, as attested from the beginning by the use of entropy to quantify information content. The talk will explore the major

impact of these transdisciplinary exchanges, with a particular focus on the important new research topic called "compressed sensing". Starting from the observation that interesting signals can be

compressed, and thus are sparse in some representation, compressed sensing aims at acquiring data directly in a compressed way, using

then computational methods to reconstruct the original signal. It

opens the way to faster, less destructive, and more e�ective signal acquisition, with possible applications in many branches of science,

from magnetic resonance imaging to astronomy, tomography, or gene

interaction network reconstruction. The talk will describe the spectacular progress that can be made using various statistical physics

ideas, from spin glass theory to crystal nucleation.

Sommerfeld Theory Colloquium, With the discovery of the Higgs boson and the determination of its mass, the Standard Model is complete and its parameters are now

known and over-constrained. I will review the status and future directions in precision electroweak physics both at high and low en- ergies. There is strong evidence that the Standard Model is correct at the level of quantum corrections, and that in the absence of major conspiracies, any new physics beyond it is either significantly heavier than the electroweak scale or very weakly coupled.

Sommerfeld Theory Colloquium, In this seminar, I shall describe recent work by our group on iden- tifying pockets of gas at high redshift that have undergone mini- mum processing through stars. The chemical composition of such gas still bears the imprints of the first few generations of stars that formed only a few hundred million years after the Big Bang, and thereby gives us clues to the physical properties of these still mys- terious objects which heralded the so-called ‘epoch of reionisation’. Near-pristine gas at high redshift is also the astrophysical environ- ment where the primordial abundance of deuterium can be measured most precisely. I will show how determinations of the cosmic den- sity of baryons from Big-Bang Nucleosynthesis and from the Cosmic Microwave Background have now reached comparable precision, in both cases of order of a few percent. The excellent agreement be- tween these two measures at widely different cosmic epochs places interesting limits on the existence of relativistic particles beyond the standard model of physics.

Sommerfeld Theory Colloquium, Remarkably, a strong candidate for Dark Matter exists within the Standard Model. Theoretical arguments suggest that QCD forces in the flavor-singlet sector may be strong enough that the H-dibaryon is a deeply-bound, compact state which is absolutely stable with a mass < 2mp. This possibility has gotten recent support from lattice QCD studies, which – although not yet at high enough resolution and sensitivity to confront the hypothesis – show a deeper binding than other states. As I will show, reasonable assumptions about the H’s mass and wave function lead to the observed dark matter density and DM-to- (3-quark) baryon ratio. I will discuss why H-DM is not excluded by the various experimental and observational limits, and discuss what its observational signatures would be.