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Georgios K. Karananas

Publications and source records attributed to Georgios K. Karananas.

35 records · Page 2Linked to original sources
arXiv2021

Islands in Linear Dilaton Black Holes

We derive a novel four-dimensional black hole with planar horizon that asymptotes to the linear dilaton background. The usual growth of its entanglement entropy before Page's time is established. After that, emergent islands modify to a large extent the entropy, which becomes finite and is saturated by its Bekenstein-Hawking value in accordance with the finiteness of the von Neumann entropy of eternal black holes. We demonstrate that viewed from the string frame, our solution is the two-dimensional Witten black hole with two additional free bosons. We generalize our findings by considering a general class of linear dilaton black hole solutions at a generic point along the $σ$-model renormalization group (RG) equations. For those, we observe that the entanglement entropy is "running" i.e. it is changing along the RG flow with respect to the two-dimensional worldsheet length scale. At any fixed moment before Page's time the aforementioned entropy increases towards the infrared (IR) domain, whereas the presence of islands leads the running entropy to decrease towards the IR at later times. Finally, we present a four-dimensional charged black hole that asymptotes to the linear dilaton background as well. We compute the associated entanglement entropy for the extremal case and we find that an island is needed in order for it to follow the Page curve.

hep-th↗
arXiv2020

Spontaneous Conformal Symmetry Breaking in Fishnet CFT

Quantum field theories with exact but spontaneously broken conformal invariance have an intriguing feature: their vacuum energy (cosmological constant) is equal to zero. Up to now, the only known ultraviolet complete theories where conformal symmetry can be spontaneously broken were associated with supersymmetry (SUSY), with the most prominent example being the $\mathcal N$=4 SUSY Yang-Mills. In this Letter we show that the recently proposed conformal "fishnet" theory supports at the classical level a rich set of flat directions (moduli) along which conformal symmetry is spontaneously broken. We demonstrate that, at least perturbatively, some of these vacua survive in the full quantum theory (in the planar limit, at the leading order of $1/N_c$ expansion) without any fine tuning. The vacuum energy is equal to zero along these flat directions, providing the first non-SUSY example of a four-dimensional quantum field theory with "natural" breaking of conformal symmetry.

hep-th↗
arXiv2020

One residue to rule them all: Electroweak symmetry breaking, inflation and field-space geometry

We point out that the successful generation of the electroweak scale via gravitational instanton configurations in certain scalar-tensor theories can be viewed as the aftermath of a simple requirement: the existence of a quadratic pole with a sufficiently small residue in the Einstein-frame kinetic term for the Higgs field. In some cases, the inflationary dynamics may also be controlled by this residue and therefore related to the Fermi-to-Planck mass ratio, up to possible uncertainties associated with the instanton regularization. We present here a unified framework for this hierarchy generation mechanism, showing that the aforementioned residue can be associated with the curvature of the Einstein-frame target manifold in models displaying spontaneous breaking of dilatations. Our findings are illustrated through examples previously considered in the literature.

hep-th↗
arXiv2019

Scale-invariant alternatives to general relativity. III. The inflation--dark-energy connection

We discuss the cosmological phenomenology of biscalar-tensor models displaying a maximally symmetric Einstein-frame kinetic sector and constructed on the basis of scale symmetry and volume-preserving diffeomorphisms. These theories contain a single dimensionful parameter $Λ_0$-associated with the invariance under the aforementioned restricted coordinate transformations-and a massless dilaton field. At large field values these scenarios lead to inflation with no generation of isocurvature perturbations. The corresponding predictions depend only on two dimensionless parameters, which characterize the curvature of the field--manifold and the leading order behavior of the inflationary potential. For $Λ_0=0$ the scale symmetry is unbroken and the dilaton admits only derivative couplings to matter, evading all fifth force constraints. For $Λ_0\neq 0$ the field acquires a run-away potential that can support a dark energy dominated era at late times. We confront a minimalistic realization of this appealing framework with present data sets. The impact of possible consistency relations among the early and late Universe dynamics that could appear within this setting is also discussed.

astro-ph.CO↗
arXiv2018

Small-scale structure from neutron dark decay

It was recently proposed that the disagreement in the experimental measurements of the lifetime of the neutron might be eradicated if the neutron decays to particles responsible for the dark matter in the Universe. In this paper we construct a proto- type self-interacting dark matter model which, apart from reproducing the correct relic abundance, resolves all small-scale problems of the $Λ$CDM paradigm. The theory is compatible with all present cosmological observations and astrophysical bounds.

hep-ph↗
arXiv2017

Gauge coupling unification without leptoquarks

We propose an interpretation of the gauge coupling unification scale which is not related to any new particle threshold. We revisit Grand Unified Theories and show that it is possible to completely eliminate the scalar as well as vector leptoquarks from the particle physics spectrum. As a consequence, in our approach the gauge hierarchy problem is put on different grounds, and the proton may be absolutely stable. In order to achieve that, we employ a number of nonlinear gauge-invariant constraints which only affect the superheavy degrees of freedom. We illustrate our considerations in a model based on the SU(5) group, with the generalization to other groups being straightforward. We discuss how scale or conformal invariance may be added to our proposal.

hep-ph↗
arXiv2016

Poincaré, Scale and Conformal Symmetries: Gauge Perspective and Cosmological Ramifications

In the first part of the thesis we focus on local symmetries. We review a self-consistent framework that we employed in order to discuss the dynamics of the theories of interest. Its merit lies in that we can make the symmetry group act internally and thus be effectively separated from coordinate transformations. We investigate under which conditions it is not needed to introduce extra compensating fields to make relativistic as well as nonrelativistic theories invariant under local symmetries and more precisely under scale transformations. We clarify the role that torsion plays in this context. We highlight the difference between Weyl and conformal invariance and we demonstrate that not all conformal theories can be coupled to gravity in a Weyl invariant way. Once this minimalistic treatment for gauging symmetries is left aside, new possibilities appear. Namely, if we consider the Poincaré group, the presence of the extra modes leads to nontrivial particle dynamics. We derive constraints such that the theory is free from pathologies. In the second part we make clear that even when not gauged, the presence of scale invariance is appealing. First, it makes possible for the dimensionful parameters that appear in a theory to be generated dynamically and be sourced by the vacuum expectation value of the dilaton. If the Standard Model is embedded into a scale-invariant framework, a number of interesting implications for cosmology arise. The inflationary stage of our Universe and its present-day acceleration become linked, a connection that might give us insight into the dark energy dynamics. We show that in the context of gravitational theories which are invariant under restricted coordinate transformations, the dilaton instead of being introduced ad hoc, can emerge from the gravitational part of a theory. We discuss the consequences of the nontrivial way this field emerges in the action.

hep-th↗
arXiv2016

On the geometrical interpretation of scale-invariant models of inflation

We study the geometrical properties of scale-invariant two-field models of inflation. In particular, we show that when the field-derivative space in the Einstein frame is maximally symmetric during inflation, the inflationary predictions can be universal and independent of the details of the theory.

hep-ph↗
arXiv2016

Scale invariant alternatives to general relativity. II. Dilaton properties

In the present paper, we revisit gravitational theories which are invariant under TDiffs -- transverse (volume preserving) diffeomorphisms and global scale transformations. It is known that these theories can be rewritten in an equivalent diffeomorphism-invariant form with an action including an integration constant (cosmological constant for the particular case of non-scale-invariant unimodular gravity). The presence of this integration constant, in general, breaks explicitly scale invariance and induces a runaway potential for the (otherwise massless) dilaton, associated with the determinant of the metric tensor. We show, however, that if the metric carries mass dimension $\left[\text{GeV}\right]^{-2}$, the scale invariance of the system is preserved, unlike the situation in theories in which the metric has mass dimension different from $-2$. The dilaton remains massless and couples to other fields only through derivatives, without any conflict with observations. We observe that one can define a specific limit for fields and their derivatives (in particular, the dilaton goes to zero, potentially related to the small distance domain of the theory) in which the only singular terms in the action correspond to the Higgs mass and the cosmological constant. We speculate that the self-consistency of the theory may require the regularity of the action, leading to the absence of the bare Higgs mass and cosmological constant, whereas their small finite values may be generated by nonperturbative effects.

hep-th↗
arXiv2016

Weyl vs. Conformal

In this note we show that given a conformally invariant theory in flat space-time, it is not always possible to couple it to gravity in a Weyl invariant way.

hep-th↗
arXiv2016

Gauging nonrelativistic field theories using the coset construction

We discuss how nonrelativistic spacetime symmetries can be gauged in the context of the coset construction. We consider theories invariant under the centrally extended Galilei algebra as well as the Lifshitz one, and we investigate under what conditions they can be supplemented by scale transformations. We also clarify the role of torsion in these theories.

hep-th↗
arXiv2016

Weyl and Ricci gauging from the coset construction

In this paper we demonstrate how, using the coset construction, a theory can be systematically made Weyl invariant by gauging the scale symmetry. We show that an analog of the inverse Higgs constraint allows the elimination of the Weyl vector (gauge) field in favor of curvatures. We extend the procedure -- previously coined Ricci gauging -- and discuss its subtlety for the case of theories with higher derivatives of conformally variant fields.

hep-th↗
arXiv2015

The particle spectrum of parity-violating Poincaré gravitational theory

In this paper we investigate the physical spectrum of the gravitational theory based on the Poincaré group with terms which are at most quadratic in tetrad and spin connection, allowing for the presence of parity-even as well as parity-odd invariants. We determine restrictions on the parameters of the action so that all degrees of freedom propagate and are neither ghosts nor tachyons. We show that the addition of parity non-conserving invariants extends the healthy parameter space of the theory. To accomplish our goal, we apply the weak field approximation around flat spacetime and in order to facilitate the analysis, we separate the bilinear action for the excitations into completely independent spin sectors. For this purpose, we employ the spin-projection operator formalism and extend the original basis built previously, to be able to handle the parity-odd pieces.

gr-qc↗
arXiv2013

Higgs-Dilaton Cosmology: an effective field theory approach

The Higgs-Dilaton cosmological model is able to describe simultaneously an inflationary expansion in the early Universe and a dark energy dominated stage responsible for the present day acceleration. It also leads to a non-trivial relation between the spectral tilt of scalar perturbations n_s and the dark energy equation of state ω. We study the self-consistency of this model from an effective field theory point of view. Taking into account the influence of the dynamical background fields, we determine the effective cut-off of the theory, which turns out to be parametrically larger than all the relevant energy scales from inflation to the present epoch. We finally formulate the set of assumptions needed to estimate the amplitude of the quantum corrections in a systematic way and show that the connection between n_s and ωremains unaltered if these assumptions are satisfied.

hep-ph↗