arXiv subjects

George Zahariade

Publications and source records attributed to George Zahariade.

At least 19 recordsLinked to original sources

arXiv2025

Evolution of a kink-antikink ensemble in a quantum vacuum

We study the $1+1$ flat spacetime dynamics of a classical field configuration corresponding to an ensemble of sine-Gordon kinks and antikinks, semi-classically coupled to a quantum field. This coupling breaks the integrability of the sine-Gordon model resulting in the background's decay into quantum radiation as kink-antikink pairs annihilate. We find evidence that, on average, the energy of the ensemble scales as $t^{-α}$ with $α<1$ and independent of the coupling strength or the mass of the quantum field. The generalization of this result to domain wall networks in higher spacetime dimensions could be relevant to particle production in the early universe.

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arXiv2025

Cosmological scaling of precursor domain walls

Domain wall (DW) networks have a large impact on cosmology and present interesting dynamics that can be controlled by various scaling regimes. In the first stage after spontaneous breaking of the discrete symmetry, the network is seeded with `DW precursors', the zeros of a tachyonic field. At sufficiently weak coupling, this stage can be quite long. The network is then driven to a non-relativistic scaling regime: in flat spacetime the correlation length grows like $L\sim t^κ$ with $κ=1/2$. We focus on the precursor regime in cosmology, assuming a power-law scale factor $a\propto t^α$. We obtain the scaling exponent as a function of the external parameter, $κ(α)$, by explicit computation in $1+1$ and $2+1$ dimensions, and find a smooth transition from nonrelativistic scaling with $κ\simeq 1/2$ for $α\lesssim1/2$ to DW gas regime $κ\simeq α$ for $α\gtrsim1/2$, confirming previous arguments. The precise form of the transition $κ(α)$ is surprisingly independent of dimension, suggesting that similar results should also be valid in $3+1$ dimensions.

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arXiv2025

Semiclassical Backreaction: A Qualitative Assessment

The backreaction of quantum degrees of freedom on classical backgrounds is a poorly understood topic in theoretical physics. Most often it is treated within the semiclassical approximation with the help of various ad hoc prescriptions accounting for the effect of quantum excitations on the dynamics of the background. We focus on two popular ones: (i) the mean-field approximation whereby quantum degrees of freedom couple to the classical background via their quantum expectation values; (ii) the (stochastic) Truncated Wigner method whereby the fully coupled system is evolved using classical equations of motion for various randomly sampled initial conditions of the quantum degree of freedom, and a statistical average is performed a posteriori. We evaluate the performance of each method in a simple toy model against a fully quantum mechanical treatment, and identify its regime of validity. We interpret the results in terms of quantum entanglement and loss of classicality of the background.

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arXiv2023

Domain wall annihilation -- a QFT perspective

Domain wall networks in the early universe, formed upon spontaneous breaking of a discrete symmetry, have a rich impact on cosmology. Yet, they remain somewhat unexplored. We introduce a new analytic strategy to understand better the domain wall epoch, from formation to annihilation. Our method includes a quantum field theoretical treatment of the initial state at domain wall formation, as well as of the time evolution. We find that the domain wall area density for a network with biased initial condition in $d+1$ dimensional flat spacetime evolves as $t^{-1/2}\,\exp\big(- (t/t_{ann})^{d/2}\big)$. We comment on the relation between this and previous results obtained in condensed matter and in cosmology. The extrapolation of this law to an expanding universe applies to networks that are close to the domain wall `gas' limit.

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arXiv2022

The Harmonic Quantum Szilárd Engine

The Szilárd engine is a mechanism (akin to Maxwell's demon) for converting information into energy, which seemingly violates the second law of thermodynamics. Originally a classical thought experiment, it was extended to a quantized treatment by Zurek. Here, we examine a new, elegant model of a quantum Szilárd engine by replacing the traditional rigid box with a harmonic potential, extending the scope of the model. Remarkably, almost all calculations are exact. This article is suitable to students, researchers and educators interested in the conceptual links between information, entropy, and quantum measurement.

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arXiv2022

Kink-antikink scattering in a quantum vacuum

We study kink-antikink scattering in the sine-Gordon model in the presence of interactions with an additional scalar field, $ψ$, that is in its quantum vacuum. In contrast to the classical scattering, now there is quantum radiation of $ψ$ quanta and the kink-antikink may form bound states that resemble breathers of the sine-Gordon model. We quantify the rate of radiation and map the parameters for which bound states are formed. Even these bound states radiate and decay, and eventually there is a transition into long-lived oscillons.

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arXiv2021

A Heat Shield for de Sitter space

We investigate a quantum vacuum state for $1 + 1$ dimensional de Sitter space, corresponding to a region within a perfectly reflecting symmetric box of fixed physical size. We find that a particle detector inside the box registers zero response, implying that the walls of the box screen out the thermal effects of the de Sitter horizon. The box thus creates a `quiescent oasis' with a temperature below the de Sitter horizon temperature, an unexpected feature that opens the way to an analysis of heat and entropy exchange between the box and the horizon in the context of the generalized second law of thermodynamics. We also calculate the stress-energy-momentum tensor of the region within the box, showing that the total energy in the box is less than the same volume of de Sitter space in the presence of thermal fluctuations.

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arXiv2020

Quantum Formation of Topological Defects

We consider quantum phase transitions with global symmetry breakings that result in the formation of topological defects. We evaluate the number densities of kinks, vortices, and monopoles that are produced in $d=1,2,3$ spatial dimensions respectively and find that they scale as $t^{-d/2}$ and evolve towards attractor solutions that are independent of the quench timescale. For $d=1$ our results apply in the region of parameters $λτ/m \ll 1$ where $λ$ is the quartic self-interaction of the order parameter, $τ$ is the quench timescale, and $m$ the mass parameter.

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arXiv2020

The Noise of Gravitons

We show that when the gravitational field is treated quantum-mechanically, it induces fluctuations -- noise -- in the lengths of the arms of gravitational wave detectors. The characteristics of the noise depend on the quantum state of the gravitational field, and can be calculated exactly in several interesting cases. For coherent states the noise is very small, but it can be greatly enhanced in thermal and (especially) squeezed states. Detection of this fundamental noise would constitute direct evidence for the quantization of gravity and the existence of gravitons.

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arXiv2020

Signatures of the Quantization of Gravity at Gravitational Wave Detectors

We develop a formalism to calculate the response of a model gravitational wave detector to a quantized gravitational field. Coupling a detector to a quantum field induces stochastic fluctuations ("noise") in the length of the detector arm. The statistical properties of this noise depend on the choice of quantum state of the gravitational field. We characterize the noise for vacuum, coherent, thermal, and squeezed states. For coherent states, corresponding to classical gravitational configurations, we find that the effect of gravitational field quantization is small. However, the standard deviation in the arm length can be enhanced -- possibly significantly -- when the gravitational field is in a non-coherent state. The detection of this fundamental noise could provide direct evidence for the quantization of gravity and for the existence of gravitons.

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arXiv2020

Quantum Mechanics of Gravitational Waves

For the purpose of analyzing observed phenomena, it has been convenient, and thus far sufficient, to regard gravity as subject to the deterministic principles of classical physics, with the gravitational field obeying Newton's law or Einstein's equations. Here we treat the gravitational field as a quantum field and determine the implications of such treatment for experimental observables. We find that falling bodies in gravity are subject to random fluctuations ("noise") whose characteristics depend on the quantum state of the gravitational field. We derive a stochastic equation for the separation of two falling particles. Detection of this fundamental noise, which may be measurable at gravitational wave detectors, would vindicate the quantization of gravity, and reveal important properties of its sources.

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arXiv2020

Emergence of classical structures from the quantum vacuum

After a quantum phase transition the quantum vacuum can break up to form classical topological defects. We examine this process for scalar field models with $Z_2$ symmetry for different quench rates for the phase transition. We find that the number density of kinks at late times universally scales as $C m^{1/2} t^{-1/2}$ where $m$ is a mass scale in the model and $C\approx 0.22$; it does not depend on the quench timescale in contrast to the Kibble-Zurek scaling for thermal phase transitions. A subleading correction $\propto t^{-3/2}$ to the kink density depends on the details of the phase transition.

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arXiv2020

On the Constraint Structure of Vacuum Energy Sequestering

We carry out the Hamiltonian analysis of the local vacuum energy sequestering model - a manifestly local and diffeomorphism invariant extension of general relativity which has been shown to remove the radiatively unstable contribution to the vacuum energy generated by matter loops. We find that the degravitation of this UV sensitive quantity is enforced via global relations that are a consequence of the model's peculiar constraint structure. We also show that the model propagates the proper number of degrees of freedom and thus locally reduces to general relativity on-shell.

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arXiv2019

Classical-Quantum Correspondence for Fields

We map the quantum problem of a free bosonic field in a space-time dependent background into a classical problem. $N$ degrees of freedom of a real field in the quantum theory are mapped into $2N^2$ classical simple harmonic oscillators with specific initial conditions. We discuss how this classical-quantum correspondence (CQC) may be used to evaluate quantum radiation and fully treat the backreaction of quantum fields on classical backgrounds. The technique has widespread application, including to the quantum evaporation of classical breathers ("oscillons").

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arXiv2019

Quantum Evaporation of Classical Breathers

We apply the recently discovered classical-quantum correspondence (CQC) to study the quantum evaporation of breathers in an extended sine-Gordon model. We present numerical results for the decay rate of the breather as a function of the coupling strength in the model. This is a complete treatment of the backreaction of quantum radiation on the classical dynamics of oscillons.

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arXiv2019

Classical-Quantum Correspondence and Hawking Radiation

A recently discovered classical-quantum correspondence (CQC) maps certain quantum problems to corresponding classical problems. We illustrate the CQC for a quantum scalar field in the gravitational background of a collapsing spherical shell. By truncating the problem to a small set of variables, we show how the CQC can yield Hawking radiation as well as the slow down of the collapse due to radiation backreaction.

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arXiv2018

A Classical-Quantum Correspondence and Backreaction

We work in the Heisenberg picture to demonstrate the classical-quantum correspondence (CQC) in which the dynamics of a quantum variable is equivalent to that of a complexified classical variable. The correspondence provides a tool for analyzing quantum backreaction problems which we illustrate by a toy model in which a rolling particle slows down due to quantum radiation. The dynamics found using the CQC is in excellent agreement with that found using the much more laborious full quantum analysis.

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arXiv2017

An Etude on Global Vacuum Energy Sequester

Recently two of the authors proposed a mechanism of vacuum energy sequester as a means of protecting the observable cosmological constant from quantum radiative corrections. The original proposal was based on using global Lagrange multipliers, but later a local formulation was provided. Subsequently other interesting claims of a different non-local approach to the cosmological constant problem were made, based again on global Lagrange multipliers. We examine some of these proposals and find their mutual relationship. We explain that the proposals which do not treat the cosmological constant counterterm as a dynamical variable require fine tunings to have acceptable solutions. Furthermore, the counterterm often needs to be retuned at every order in the loop expansion to cancel the radiative corrections to the cosmological constant, just like in standard GR. These observations are an important reminder of just how the proposal of vacuum energy sequester avoids such problems.

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