10 research outputs found
Decoherence and Dynamical Entropy Generation in Quantum Field Theory
We formulate a novel approach to decoherence based on neglecting
observationally inaccessible correlators. We apply our formalism to a
renormalised interacting quantum field theoretical model. Using
out-of-equilibrium field theory techniques we show that the Gaussian von
Neumann entropy for a pure quantum state increases to the interacting thermal
entropy. This quantifies decoherence and thus measures how classical our pure
state has become. The decoherence rate is equal to the single particle decay
rate in our model. We also compare our approach to existing approaches to
decoherence in a simple quantum mechanical model. We show that the entropy
following from the perturbative master equation suffers from physically
unacceptable secular growth
Effect of the Trace Anomaly on the Cosmological Constant
It has been argued that the quantum (conformal) trace anomaly could
potentially provide us with a dynamical explanation of the cosmological
constant problem. In this paper, however, we show by means of a semiclassical
analysis that the trace anomaly does not affect the cosmological constant. We
construct the effective action of the conformal anomaly for flat FLRW
spacetimes consisting of local quadratic geometric curvature invariants.
Counterterms are thus expected to influence the numerical value of the
coefficients in the trace anomaly and we must therefore allow these parameters
to vary. We calculate the evolution of the Hubble parameter in quasi de Sitter
spacetime, where we restrict our Hubble parameter to vary slowly in time, and
in FLRW spacetimes. We show dynamically that a Universe consisting of matter
with a constant equation of state, a cosmological constant and the quantum
trace anomaly evolves either to the classical de Sitter attractor or to a
quantum trace anomaly driven one. When considering the trace anomaly truncated
to quasi de Sitter spacetime, we find a region in parameter space where the
quantum attractor destabilises. When considering the exact expression of the
trace anomaly, a stability analysis shows that whenever the trace anomaly
driven attractor is stable, the classical de Sitter attractor is unstable, and
vice versa. Semiclassically, the trace anomaly does not affect the classical
late time de Sitter attractor and hence it does not solve the cosmological
constant problem.Comment: 18 pages, 11 figures, published versio
Decoherence in an Interacting Quantum Field Theory: The Vacuum Case
We apply the decoherence formalism to an interacting scalar field theory. In
the spirit of the decoherence literature, we consider a "system field" and an
"environment field" that interact via a cubic coupling. We solve for the
propagator of the system field, where we include the self-energy corrections
due to the interaction with the environment field. In this paper, we consider
an environment in the vacuum state (T=0). We show that neglecting inaccessible
non-Gaussian correlators increases the entropy of the system as perceived by
the observer. Moreover, we consider the effect of a changing mass of the system
field in the adiabatic regime, and we find that at late times no additional
entropy has been generated.Comment: 40 pages, published versio
The Scalar Field Kernel in Cosmological Spaces
We construct the quantum mechanical evolution operator in the Functional
Schrodinger picture - the kernel - for a scalar field in spatially homogeneous
FLRW spacetimes when the field is a) free and b) coupled to a spacetime
dependent source term. The essential element in the construction is the causal
propagator, linked to the commutator of two Heisenberg picture scalar fields.
We show that the kernels can be expressed solely in terms of the causal
propagator and derivatives of the causal propagator. Furthermore, we show that
our kernel reveals the standard light cone structure in FLRW spacetimes. We
finally apply the result to Minkowski spacetime, to de Sitter spacetime and
calculate the forward time evolution of the vacuum in a general FLRW spacetime.Comment: 13 pages, 1 figur
Entropy and Correlators in Quantum Field Theory
It is well known that loss of information about a system, for some observer,
leads to an increase in entropy as perceived by this observer. We use this to
propose an alternative approach to decoherence in quantum field theory in which
the machinery of renormalisation can systematically be implemented: neglecting
observationally inaccessible correlators will give rise to an increase in
entropy of the system. As an example we calculate the entropy of a general
Gaussian state and, assuming the observer's ability to probe this information
experimentally, we also calculate the correction to the Gaussian entropy for
two specific non-Gaussian states.Comment: 23 pages, 9 figure
Decoherence in an Interacting Quantum Field Theory: Thermal Case
We study the decoherence of a renormalised quantum field theoretical system.
We consider our novel correlator approach to decoherence where entropy is
generated by neglecting observationally inaccessible correlators. Using
out-of-equilibrium field theory techniques at finite temperatures, we show that
the Gaussian von Neumann entropy for a pure quantum state asymptotes to the
interacting thermal entropy. The decoherence rate can be well described by the
single particle decay rate in our model. Connecting to electroweak baryogenesis
scenarios, we moreover study the effects on the entropy of a changing mass of
the system field. Finally, we compare our correlator approach to existing
approaches to decoherence in the simple quantum mechanical analogue of our
field theoretical model. The entropy following from the perturbative master
equation suffers from physically unacceptable secular growth.Comment: 36 pages, 22 figure
Fermion Propagator in Cosmological Spaces with Constant Deceleration
We calculate the fermion propagator in FLRW spacetimes with constant
deceleration , for excited states. For
fermions whose mass is generated by a scalar field through a Yukawa coupling
, we assume . We
first solve for the mode functions by splitting the spinor into a direct
product of helicity and chirality spinors. We also allow for non-vacuum states.
We normalise the spinors using a consistent canonical quantisation and by
requiring orthogonality of particle and anti-particle spinors. We apply our
propagator to calculate the one loop effective action and renormalise using
dimensional regularisation. Since the Hubble parameter is now treated
dynamically, this paves the way to study the dynamical backreaction of fermions
on the background spacetime.Comment: 18 pages, 1 figure, published versio