Quantum Typicality and Initial Conditions

If the state of a quantum system is sampled out of a suitable ensemble, the measurement of some observables will yield (almost) always the same result. This leads us to the notion of quantum typicality: for some quantities the initial conditions are immaterial. We discuss this problem in the framework of Bose-Einstein condensates.Comment: 8 page

Initial Conditions in String Cosmology

We take a critical look at a recent conjecture concerning the past attractor in the pre-big-bang scenario. We argue that the Milne universe is unlikely to be a general past attractor for such models and support this with a number of examples.Comment: 10 pages standard Latex format, no figures. Submitted to Phys. Rev.

Initial conditions for hybrid inflation

In hybrid inflation models, typically only a tiny fraction of possible initial conditions give rise to successful inflation, even if one assumes spatial homogeneity. We analyze some possible solutions to this initial conditions problem, namely assisted hybrid inflation and hybrid inflation on the brane. While the former is successful in achieving the onset of inflation for a wide range of initial conditions, it lacks sound physical motivation at present. On the other hand, in the context of the presently much discussed brane cosmology, extra friction terms appear in the Friedmann equation which solve this initial conditions problem in a natural way.Comment: 6 pages RevTeX file with four figures incorporated (uses RevTeX and epsf). Updates to match accepted versio

Naturalness in Cosmological Initial Conditions

We propose a novel approach to the problem of constraining cosmological initial conditions. Within the framework of effective field theory, we classify initial conditions in terms of boundary terms added to the effective action describing the cosmological evolution below Planckian energies. These boundary terms can be thought of as spacelike branes which may support extra instantaneous degrees of freedom and extra operators. Interactions and renormalization of these boundary terms allow us to apply to the boundary terms the field-theoretical requirement of naturalness, i.e. stability under radiative corrections. We apply this requirement to slow-roll inflation with non-adiabatic initial conditions, and to cyclic cosmology. This allows us to define in a precise sense when some of these models are fine-tuned. We also describe how to parametrize in a model-independent way non-Gaussian initial conditions; we show that in some cases they are both potentially observable and pass our naturalness requirement.Comment: 35 pages, 8 figure

Initial Conditions for a Universe

In physical theories, boundary or initial conditions play the role of selecting special situations which can be described by a theory with its general laws. Cosmology has long been suspected to be different in that its fundamental theory should explain the fact that we can observe only one particular realization. This is not realized, however, in the classical formulation and in its conventional quantization; the situation is even worse due to the singularity problem. In recent years, a new formulation of quantum cosmology has been developed which is based on quantum geometry, a candidate for a theory of quantum gravity. Here, the dynamical law and initial conditions turn out to be linked intimately, in combination with a solution of the singularity problem.Comment: 7 pages, this essay was awarded First Prize in the Gravity Research Foundation Essay Contest 200

Tubular initial conditions and ridge formation

The 2D azimuth & rapidity structure of the two-particle correlations in relativistic A+A collisions is altered significantly by the presence of sharp inhomogeneities in superdense matter formed in such processes. The causality constraints enforce one to associate the long-range longitudinal correlations observed in a narrow angular interval, the so-called (soft) ridge, with peculiarities of the initial conditions of collision process. This study's objective is to analyze whether multiform initial tubular structures, undergoing the subsequent hydrodynamic evolution and gradual decoupling, can form the soft ridges. Motivated by the flux-tube scenarios, the initial energy density distribution contains the different numbers of high density tube-like boost-invariant inclusions that form a bumpy structure in the transverse plane. The influence of various structures of such initial conditions in the most central A+A events on the collective evolution of matter, resulting spectra, angular particle correlations and v_n-coefficients is studied in the framework of the HydroKinetic Model (HKM).Comment: 18 pages, 6 figures, the paper to be published in Advances of High Energy Physics (2013, in press

Initial Conditions for Large Cosmological Simulations

This technical paper describes a software package that was designed to produce initial conditions for large cosmological simulations in the context of the Horizon collaboration. These tools generalize E. Bertschinger's Grafic1 software to distributed parallel architectures and offer a flexible alternative to the Grafic2 software for ``zoom'' initial conditions, at the price of large cumulated cpu and memory usage. The codes have been validated up to resolutions of 4096^3 and were used to generate the initial conditions of large hydrodynamical and dark matter simulations. They also provide means to generate constrained realisations for the purpose of generating initial conditions compatible with, e.g. the local group, or the SDSS catalog.Comment: 12 pages, 11 figures, submitted to ApJ