Classical Inhomogeneities Research Articles

Quantum Coherences and Classical Inhomogeneities as Equivalent Thermodynamics Resources.

Quantum energy coherences represent a thermodynamic resource, which can be exploited to extract energy from a thermal reservoir and deliver that energy as work. We argue that there exists a closely analogous classical thermodynamic resource, namely, energy-shell inhomogeneities in the phase space distribution of a system’s initial state. We compare the amount of work that can be obtained from quantum coherences with the amount that can be obtained from classical inhomogeneities, and find them to be equal in the semiclassical limit. We thus conclude that coherences do not provide a unique thermodynamic advantage of quantum systems over classical systems, in situations where a well-defined semiclassical correspondence exists.

Planckian hypersurfaces, inflation and bounces

When the different wavelengths of the scalar and tensor modes of the geometry are all assigned on the same space-like hypersurface the maximally amplified frequencies of the spectrum remain smaller than the Planck mass only if the duration of a stage of accelerated expansion and the corresponding tensor to scalar ratio are severely constrained. All the different wavelengths can be initialized on the same space-like hypersurface at the onset of inflation but this strategy and the related conclusions are plausible only for classical inhomogeneities. We argue that a whole class of potential constraints is easily evaded provided the different wavelengths of the quantum fields are assigned as soon as they cross the corresponding Planckian hypersurfaces. In this case the Cauchy data for the mode functions depend on the wavenumber so that larger wavelengths start evolving earlier while shorter wavelengths are assigned later. Within this strategy the duration of a conventional inflationary phase and the corresponding tensor to scalar ratio are not constrained but the large-scale power spectra inherit specific large-scale corrections that remain however unobservable. We also address the problem of the bouncing dynamics and show that similar constraints do no not appear at the beginning of the bouncing phase but rather towards its end.

Effective Theory for the Cosmological Generation of Structure

The current understanding of structure formation in the early universe is mainly built on a magnification of quantum fluctuations in an initial vacuum state during an early phase of accelerated universe expansion. One usually describes this process by solving equations for a quantum state of matter on a given expanding background space-time, followed by decoherence arguments for the emergence of classical inhomogeneities from the quantum fluctuations. Here, we formulate the coupling of quantum matter fields to a dynamical gravitational background in an effective framework which allows the inclusion of back-reaction effects. It is shown how quantum fluctuations couple to classical inhomogeneities and can thus manage to generate cosmic structure in an evolving background. Several specific effects follow from a qualitative analysis of the back-reaction, including a likely reduction of the overall amplitude of power in the cosmic microwave background, the occurrence of small non-Gaussianities, and a possible suppression of power for odd modes on large scales without parity violation.

Decoherence during inflation: The generation of classical inhomogeneities

We show how the quantum to classical transition of the cosmological fluctuations produced during inflation can be described by means of the influence functional and the master equation. We split the inflaton field into the system-field (long-wavelength modes), and the environment, represented by its own short-wavelength modes. We compute the decoherence times for the system-field modes and compare them with the other time scales of the model. We present the renormalized stochastic Langevin equation for an homogeneous system-field and then we analyze the influence of the environment on the power spectrum for some modes in the system.

Singularity free dilaton-driven cosmologies and pre-little-bangs

There are no reasons why the singularity in the growth of the dilaton coupling should not be regularised, in a string cosmological context, by the presence of classical inhomogeneities. We discuss a class of inhomogeneous dilaton-driven models whose curvature invariants are all bounded and regular in time and space. We prove that the non-space-like geodesics of these models are all complete in the sense that none of them reaches infinity for a finite value of the affine parameter. We conclude that our examples represent truly singularity-free solutions of the low energy beta functions. We discuss some symmetries of the obtained solutions and we clarify their physical interpretation. We also give examples of solutions with spherical symmetry. In our scenario each physical quantity is everywhere defined in time and space, the big-bang singularity is replaced by a maximal curvature phase where the dilaton kinetic energy reaches its maximum. The maximal curvature is always smaller than one (in string units) and the coupling constant is also smaller than one and it grows between two regimes of constant dilaton, implying, together with the symmetries of the solutions, that higher genus and higher curvature corrections are negligible. We argue that our examples describe, in a string cosmological context, the occurrence of ``little bangs''(i.e. high curvature phases which never develop physical singularities). They also suggest the possibility of an unexplored ``pre-little-bang'' phase.

QUANTUM-TO-CLASSICAL TRANSITION FOR FLUCTUATIONS IN THE EARLY UNIVERSE

According to the inflationary scenario for the very early Universe, all inhomogeneities in the Universe are of genuine quantum origin. On the other hand, looking at these inhomogeneities and measuring them, clearly no specific quantum mechanical properties are observed. We show how the transition from their inherent quantum gravitational nature to classical behavior comes about — a transition whereby none of the successful quantitative predictions of the inflationary scenario for the present-day universe is changed. This is made possible by two properties. First, the quantum state for the spacetime metric perturbations produced by quantum gravitational effects in the early Universe becomes very special (highly squeezed) as a result of the expansion of the Universe (as long as the wavelength of the perturbations exceeds the Hubble radius). Second, decoherence through the environment distinguishes the field amplitude basis as being the pointer basis. This renders the perturbations presently indistinguishable from stochastic classical inhomogeneities.

Entropy generation in the very early universe

We investigate entropy generation in the early universe from the dissipation of classical inhomogeneities and anisotropies, both before and after they enter the particle horizon. General constraints on the specific entropy produced in various cases are obtained. Numerical estimates are given for two specific models of superdense cold matter: asymptotically free quarks and vector-meson-dominated interacting nucleons.