arXiv subjects

Nikolaos Tetradis

Publications and source records attributed to Nikolaos Tetradis.

At least 19 recordsLinked to original sources

arXiv2026

Boundary Conditions and Entanglement in Anti-de Sitter Space

We study the entanglement entropy of a conformally coupled scalar field at its ground state in $(3+1)$-dimensional AdS space in global coordinates. We consider spherical entangling surfaces centered at the origin of AdS and allow for general boundary conditions at the conformal boundary. Through numerical and analytical means, we show that the UV-divergent part of the entropy has a universal form, while the UV-finite part is sensitive to the boundary conditions. We determine the dependence of the latter part on mixed boundary conditions that interpolate between Dirichlet and Neumann.

hep-th↗

arXiv2025

Computing $c$- and $a$-functions from entanglement

We confirm the direct connection between entanglement entropy and the notion of irreversibility in the renormalization-group flow in the context of a simple theory for which a calculation from first principles is feasible. The change of the entanglement entropy for a spherical entangling surface as its radius grows from zero to infinity corresponds to the flow from the UV to the IR. Through analytical and numerical means, we compute the entanglement entropy for a free massive scalar theory, making use of the method of correlation functions. We deduce a $c$-function in $1+1$ dimensions and an $a$-function in $3+1$ dimensions. Both functions are monotonic and vary continuously between one and zero, as expected for this simple theory.

hep-th↗

arXiv2025

A Numerical Calculation of Entanglement Entropy in de Sitter Space

The entanglement entropy of a massless scalar field in de Sitter space depends on multiple scales, such as the radius of the entangling surface, the Hubble constant and the UV cutoff. We perform a high-precision numerical calculation using a lattice model in order to determine the dependence on these scales in the Bunch-Davies vacuum. We derive the leading de Sitter corrections to the flat-space entanglement entropy for subhorizon entangling radii. We analyze the structure of the finite-size effects and we show that the contribution to the entanglement entropy of the sector of the theory with vanishing angular momentum depends logarithmically on the size of the overall system, which extends beyond the horizon.

hep-th↗

arXiv2025

Entanglement on a Sphere

We study the entanglement entropy of a massive scalar field in the background of the Einstein universe. We determine numerically the structure of the UV-divergent terms. We study analytically the IR term that originates in the long-range correlations arising from the field zero mode on the sphere. We compare with previous results for a scalar field in a dS or AdS background.

hep-th↗

arXiv2025

Entanglement Entropy of a Scalar Field in Anti-de Sitter Space

We study the entanglement entropy of a free massive scalar field at its ground state in (3+1)-dimensional AdS space in global coordinates. We consider spherical entangling surfaces centered at the origin of AdS. We determine the structure of the UV-divergent terms in the entanglement entropy and compute the numerical values of the respective coefficients. We confirm the connection between the coefficient of the logarithmic term and the conformal anomaly.

hep-th↗

arXiv2024

Entanglement Entropy of a Scalar Field in a Squeezed State

We study the entanglement entropy within a spherical region for a free scalar field in a squeezed state in 3+1 dimensions. We show that, even for small squeezing, a volume term appears, whose coefficient is essentially independent of the field mass. This is in line with Page's argument that the entanglement entropy in an arbitrary quantum state is proportional to the number of degrees of freedom of the smaller subsystem. It follows that squeezed states can be considered as arbitrary quantum states, in contrast to the ground or coherent states that give rise to entanglement entropy that is dominated by a term proportional to the area of the entangling surface.

hep-th↗

arXiv2024

Entanglement Entropy as a Probe Beyond the Horizon

The entanglement entropy of a free field in de Sitter space is enhanced by the squeezing of its modes. We show analytically that the expansion induces a term in the entanglement entropy that depends logarithmically on the size of the overall system, which may extend beyond the horizon. In cosmology the size of the system can be identified with the size of a spatially finite universe, or with the wavelength of the first mode that exited the horizon in the beginning of inflation.

hep-th↗

arXiv2024

Entanglement in Cosmology

We compute the evolution of the entanglement entropy for a massless field within a spherical region throughout the inflationary period and the subsequent era of radiation domination, starting from the Bunch-Davies vacuum. In order to focus on the entanglement of modes that are directly accessible to observations, we impose an ultraviolet cutoff set by the wavelength of the last mode that exited the horizon at the end of inflation. The transition of each mode towards a squeezed state upon horizon exit during inflation and the additional squeezing when radiation domination sets in enhance the entanglement entropy. Shortly after the transition to the radiation-dominated era, a volume term develops and becomes the leading contribution to the entropy at late times, as is common for systems lying in squeezed states. We estimate the magnitude of the entropy and discuss its interpretation in the light of the quantum to classical transition for modes exiting the horizon during inflation. Our results raise the possibility that the quantum nature of weakly interacting fields, such as gravitational waves resulting from tensor modes during inflation, may be detectable in today's universe. On the other hand, an observer with no knowledge of the degrees of freedom beyond the horizon would interpret the entropy as thermal. From this point of view, the reheating after inflation would be a result of quantum entanglement.

gr-qc↗

arXiv2023

Entanglement of Harmonic Systems in Squeezed States

The entanglement entropy of a free scalar field in its ground state is dominated by an area law term. It is noteworthy, however, that the study of entanglement in scalar field theory has not advanced far beyond the ground state. In this paper, we extend the study of entanglement of harmonic systems, which include free scalar field theory as a continuum limit, to the case of the most general Gaussian states, namely the squeezed states. We find the eigenstates and the spectrum of the reduced density matrix and we calculate the entanglement entropy. Finally, we apply our method to free scalar field theory in 1+1 dimensions and show that, for very squeezed states, the entanglement entropy is dominated by a volume term, unlike the ground-state case. Even though the state of the system is time-dependent in a non-trivial manner, this volume term is time-independent. We expect this behaviour to hold in higher dimensions as well, as it emerges in a large-squeezing expansion of the entanglement entropy for a general harmonic system.

hep-th↗

arXiv2023

Exact solutions for Vacuum Decay in Unbounded Potentials

The Standard Model Higgs potential may become unbounded from below at large field values, with important cosmological implications. For a potential of this form, the commonly assumed scenario of a nucleated thin-wall bubble driving the transition from the electroweak vacuum to the unstable region does not apply. We present exact analytical solutions for potentials that have the same qualitative form as the Higgs potential. They show that the transition is driven by a thick-wall spherical bubble of true vacuum, with a surface that expands at asymptotically the speed of light. A `crunch' singularity appears in the quasi-AdS interior, with the collapsed region also expanding at asymptotically the speed of light. The singularity is surrounded by a region of trapped surfaces whose boundary forms an apparent horizon. An event horizon separates the singularity from the bubble exterior, so that the expansion of the bubble surface is not affected by the collapse of the interior. The solutions provide exact descriptions of the geometry for thick-wall bubbles and are consistent with the analysis of [1,2] for the Higgs potential.

hep-ph↗

arXiv2023

Entanglement and Expansion

We study the entanglement entropy resulting from tracing out local degrees of freedom of a quantum scalar field in an expanding universe. It is known that when field modes become superhorizon during inflation they evolve to increasingly squeezed states. We argue that this causes the entanglement entropy to grow continuously as successive modes cross the horizon. The resulting entropy is proportional to the total duration of inflation. It is preserved during a subsequent radiation or matter dominated era, and thus it may be relevant for today's universe. We demonstrate explicitly these features in a toy model of a scalar field in 1+1 dimensions.

hep-th↗

arXiv2022

Higgstory repeats itself

We consider a scalar potential with two minima, one of which is arbitrarily deep, such as could be the case for the Higgs potential in the Standard Model. A recent calculation within the thin-wall approximation [arXiv:2205.10240] concludes that regions in which the scalar field takes values beyond the top of the potential barrier are forced by gravity to collapse, while they remain hidden behind a black hole horizon. We show that the thin-wall approximation is not applicable to this problem. We clarify the issue through numerical and analytical solutions to the field equations of the gravity-scalar system. We find that regions around the deeper minimum expand, and would thereby engulf the Universe in post-inflationary cosmology. We also show that black holes with Higgs hair are unstable. Even though the physics of the true vacuum is different, our final conclusion replicates the earlier `Higgstory' paper [arXiv:1505.04825].

hep-ph↗

arXiv2022

Entanglement entropy in a four-dimensional cosmological background

We compute the holographic entanglement entropy of a thermalized CFT on a time-dependent background in four dimensions. We consider a slab configuration extending beyond the cosmological horizon of a Friedmann-Lemaitre-Robertson-Walker metric. We identify a volume term that corresponds to the thermal entropy of the CFT, as well as terms proportional to the proper area of the entangling surface which are associated with strongly entangled degrees of freedom in the vicinity of this surface or with the expansion.

hep-th↗

arXiv2021

De Sitter entropy as holographic entanglement entropy

We review the results of refs. [1,2], in which the entanglement entropy in spaces with horizons, such as Rindler or de Sitter space, is computed using holography. This is achieved through an appropriate slicing of anti-de Sitter space and the implementation of a UV cutoff. When the entangling surface coincides with the horizon of the boundary metric, the entanglement entropy can be identified with the standard gravitational entropy of the space. For this to hold, the effective Newton's constant must be defined appropriately by absorbing the UV cutoff. Conversely, the UV cutoff can be expressed in terms of the effective Planck mass and the number of degrees of freedom of the dual theory. For de Sitter space, the entropy is equal to the Wald entropy for an effective action that includes the higher-curvature terms associated with the conformal anomaly. The entanglement entropy takes the expected form of the de Sitter entropy, including logarithmic corrections.

hep-th↗

arXiv2021

Backreaction from inhomogeneous matter fields during large-scale structure formation

We study how inhomogeneities of the cosmological fluid fields backreact on the homogeneous part of energy density and how they modify the Friedmann equations. In general, backreaction requires to go beyond the pressureless ideal fluid approximation, and this can lead to a reduced growth of cosmological large scale structure. Since observational evidence favours evolution close to the standard growing mode in the linear regime, we focus on two-component fluids in which the non-ideal fluid is gravitationally coupled to cold dark matter and in which a standard growing mode persists. This is realized, e.g. for a baryonic fluid coupled to cold dark matter. We calculate the backreaction for this case and for a wide range of other two-fluid models. Here the effect is either suppressed because the non-ideal matter properties are numerically too small, or because they lead to a too stringent UV cut-off of the integral over the power spectrum that determines backreaction. We discuss then matter field backreaction from a broader perspective and generalize the formalism such that also far-from-equilibrium scenarios relevant to late cosmological times and non-linear scales can be addressed in the future.

gr-qc↗

arXiv2020

Entropy of Thermal CFTs on Curved Backgrounds

We use holography in order to study the entropy of thermal CFTs on (1+1)-dimensional curved backgrounds that contain horizons. Starting from the metric of the BTZ black hole, we perform explicit coordinate transformations that set the boundary metric in de Sitter or black-hole form. For a de Sitter boundary, the dual picture describes a CFT at a temperature different from that of the cosmological horizon. We determine minimal surfaces that allow us to compute the entanglement entropy of a boundary region, as well as the temperature affecting the energy associated with a probe quark on the boundary. For an entangling surface that coincides with the horizon, we study the relation between entanglement and gravitational entropy through an appropriate definition of the effective Newton's constant. We find that the leading contribution to the entropy is proportional to the horizon area, with a coefficient that accounts for the degrees of freedom of a CFT thermalized above the horizon temperature. We demonstrate the universality of our findings by considering the most general metric in a (2+1)-dimensional AdS bulk containing a non-rotating black hole and a static boundary with horizons.

hep-th↗

arXiv2020

Corrections to de Sitter entropy through holography

The holographic entanglement entropy is computed for an entangling surface that coincides with the horizon of a boundary de Sitter metric. This is achieved through an appropriate slicing of anti-de Sitter space and the implementation of a UV cutoff. The entropy is equal to the Wald entropy for an effective action that includes the higher-curvature terms associated with the conformal anomaly. The UV cutoff can be expressed in terms of the effective Planck mass and the number of degrees of freedom of the dual theory. The entanglement entropy takes the expected form of the de Sitter entropy, including logarithmic corrections.

hep-th↗

arXiv2019

The dark matter bispectrum from effective viscosity and one-particle irreducible vertices

Dark matter evolution during the process of cosmological structure formation can be described in terms of a one-particle irreducible effective action at a characteristic scale $k_m$ and a loop expansion below this scale, based on the effective propagators and vertices. We calculate the form of the effective vertices and compute the bispectrum of density perturbations within a one-loop approximation. We find that the effective vertices play a subdominant role as compared to the effective viscosity and sound velocity that modify the (inverse) propagators. For the bispectrum we reproduce the results of standard perturbation theory in the range where it is applicable, and find a slightly improved agreement with N-body simulations at larger wavenumbers.

astro-ph.CO↗