Scientific Series

In this talk I will discuss several recent advances in loop quantum cosmology and its extension to inhomogeneous models. I will focus on spherically symmetric spacetimes and Gowdy cosmologies with local rotational symmetry in vacuum. I will discuss how to implement a quantum Hamiltonian evolution on these quantizations. Then, I will focus on how we can extract predictions from those quantum geometries, and finally analyze a concrete example: cosmological perturbations on Bianchi I spacetimes in LQC.

The power spectrum for fluctuations in the number density of galaxies can be very different in shape from the power spectrum for fluctuations in the mass density at very small wave vectors (i.e., large length scales) if the primordial density fluctuations are non-Gaussian. I review this phenomena. (It  is fairly well known in the more astrophysical part of the cosmology community but less so in the particle physics part of the field.) Then I show that primordial non-Gaussianities that arise from quantum loop diagrams in de-Sitter space can give rise to this phenomena. I also discuss if it is observable given constraints on primordial non-Gaussianity that arise from the Cosmic Microwave Background

When a particle is accelerated, as in a scattering event, it will radiate gravitons and, if electrically charged, photons. The infrared tail of the spectrum of this radiation has a divergence: an arbitrarily small amount of total energy is divided into an arbitrarily large number of radiated bosons. Each of these in turn carries a momentum and a helicity degree of freedom, and thus this radiation carries significant amounts of quantum information. I will demonstrate that the infrared bosonic radiation causes nearly complete decoherence of the final state of the hard particles into the momentum basis. When applied to radiation coming from the incoming state, it appears that the entire dynamical history of the process will be distinguishable by the radiation, thus causing a loss of any interference effects between branches of an incoming momentum superposition, such as in a wavepacket. I will explain how infrared dressed states, in the framework of Fadeev and Kulish, evade this issue. Finally, I'll conclude with some remarks on the potential relevance of these issues to black hole information loss

We demonstrate that extremely long range correlations may develop in systems that start from equilibrium and are then rapidly cooled (or driven in other ways). Amongst other things, these correlations suggest a collapse of the viscosity data of glass formers. This collapse is found to be obeyed over 16 decades of relaxation times in experimental data on all known types of supercooled fluids. 

We present fundamental limits of axion and hidden-photon dark matter searches probing the electromagnetic coupling.  These limits are informed by constraints on noise in phase-insensitive amplifiers, as well as constraints on impedance matching. We motivate the use of quantum-limited amplifiers for dark matter searches, in particular at low masses/frequencies, where they provide a substantial enhancement due to sensitivity outside of the detector bandwidth. We discuss the role of priors, e.g. direct detection and astrophysical constraints, in optimizing scan strategies and comparing receiver architectures. We show that the figure of merit for wideband dark matter searches is the integrated sensitivity of the receiver circuit, and that the integrated sensitivity is constrained by the Bode-Fano criterion. The optimized single-pole resonator read out by a quantum-limited amplifier is close to the Bode-Fano limit, establishing such a search technique as a fundamentally near-ideal setup for dark matter measurement. We discuss the implications of these broad optimization statements for DM Radio, a lumped-element search for axion and hidden-photon dark matter operating in the 100 Hz (~0.5 peV)- 300 MHz (~1 ueV) range. Our results strongly motivate the use of quantum measurement techniques (e.g. squeezing, entanglement, photon counting, backaction evasion), which evade the limits, in future dark matter searches.

I suggest a minimal practical formal structure for a more fundamental theory than the Standard Model + GR and review a mechanism that produces such a structure.  The proposed mechanism has possibilities of producing non-canonical phenomena in SU(2) and SU(3) gauge theories which might allow conditional predictions that can be tested.

The slides and other writings are posted on my web page

     http://www.physics.rutgers.edu/~friedan/#Perimeter

During my visit to PI, I hope also to discuss informally a separate project in pure QFT, a scheme to construct a new kind of QFT of extended objects (also described on my web page).

[joint work with: Victor Albert, John Preskill (Caltech), Sepehr Nezami, Grant Salton, Patrick Hayden (Stanford University), and Fernando Pastawski (Freie Universität Berlin)]

Quantum error correction and symmetries are concepts that are relevant in many physical systems, such as in condensed matter physics and in holographic quantum gravity, and play a central role in error correction of reference frames [Hayden et al., arXiv:1709.04471]. I will show that codes that are covariant with respect to a continuous local symmetry necessarily have a limit to their ability to serve as approximate error-correcting codes against erasures at known locations. This is because the environment necessarily gets information about the global logical charge of the encoded state due to the covariance of the code. Our bound vanishes either in the limit of large individual subsystems, or in the limit of a large number of subsystems; in either case there exist codes which approximately achieve the scaling of our bound and become good covariant error-correcting codes. Our results can be interpreted as an approximate version of the Eastin-Knill theorem, quantifying to which extent it is not possible to carry out universal computation approximately on encoded states. In the context of holographic AdS/CFT, our approach provides some insight on time evolution in the bulk versus time evolution on the boundary. We expect further implications for symmetries in holography and in condensed matter physics.

I will present a brief introduction to non-Lorentzian geometries, an important example of such geometries being Newton-Cartan geometry and its torsionful generalization, which is the natural geometry to which non-relativistic field theories couple to. The talk will subsequently review how such geometries have in recent years appeared in gravity, string theory and holography. In particular, torsional Newton-Cartan geometry has been shown to appear as the boundary geometry for Lifshitz spacetimes. Furthermore, dynamical Newton-Cartan geometry is related to Horava-Lifsthiz gravity theories and appears in novel Chern-Simons theories of gravity in three dimensions. The latter can be obtained from a well-defined limit of the AdS3/CFT2 correspondence. Finally, I will briefly comment on how Newton-Cartan geometry appears in non-relativistic string theory.

The Bondi mass loss formula has been central in the context of early research on gravitational waves. We show how it can be understood as a particular case of BMS current algebra and discuss the associated central extension. We then move down to three dimensions where a more complete picture emerges.

Suppose the eigenvalue distributions of two matrices $M_1$ and $M_2$ are known. What is the eigenvalue distribution of the sum $M_1+M_2$? This problem has a rich pure mathematics history dating back to H. Weyl (1912) with many applications in various fields. Free probability theory (FPT) answers this question under certain conditions, which often involves some degree of randomness (disorder). We will describe FPT and show examples of its powers for the qualitative understanding (often approximations) of physical quantities such as density of states, and gapped vs. gapless phases of some Floquet systems. These physical quantities are often hard to compute exactly. Nevertheless, using FPT and other ideas from random matrix theory excellent approximations can be obtained. Besides the applications presented, we believe the techniques will find new applications in new contexts.