arXiv subjects

Dražen Glavan

Publications and source records attributed to Dražen Glavan.

At least 19 recordsLinked to original sources

arXiv2026

On the Bondi accretion of a self-interacting complex scalar field

Scalar fields with a global U(1) symmetry often appear in cosmology and astrophysics. We study the spherically-symmetric, stationary accretion of such a classical field onto a Schwarzschild black hole in the test-field approximation. Thus, we consider the relativistic Bondi accretion beyond a simplified perfect-fluid setup. We focus on the complex scalar field with canonical kinetic term and with a generic quartic potential which either preserves the U(1) symmetry or exhibits spontaneous symmetry breaking. It is well known that in the lowest order in gradient expansion the dynamics of such a scalar field is well approximated by a perfect superfluid; we demonstrate that going beyond this approximation systematically reduces the accretion rate with respect to the perfect fluid case. Hence, black holes can provide a way to distinguish a perfect fluid from its ultraviolet completion in form of the complex scalar field.

gr-qc↗

arXiv2026

Massive scalar's self-mass from gravitons in de Sitter

We compute the fully renormalized one-graviton-loop contribution to the self-mass of a massive scalar field in de Sitter space. The computation is performed using dimensional regularization and the divergences are absorbed by BPHZ counterterms. This self-mass can be used to study one-loop quantum-gravitational corrections to the dynamics of massive spectator scalars in inflation.

gr-qc↗

arXiv2026

Cosmological properties of black-hole hair in the linearly coupled scalar-Gauss-Bonnet theory

We investigate the superhorizon behavior of scalar hair sourced by black holes in de Sitter spacetime in the linearly coupled shift-symmetric scalar-Gauss-Bonnet theory. Working in the test-field regime, we show that this hair exhibits both temporal and spatial growth on superhorizon scales. This growth is not a special consequence of the black hole, but instead follows from the dynamics of a minimally coupled massless scalar field in expanding de Sitter spacetime. Moreover, it is not even specific to black holes, but also arises for a point scalar charge in de Sitter, indicating that a scalarized black hole acts effectively as a localized subhorizon source of scalar perturbations. Backreaction, when important, first arises on subhorizon scales and does not by itself eliminate the superhorizon profile. The time-dependent scalar hair also carries a steady outward energy flux, which frames the test-field regime as a transient, and helps explain the difficulties encountered in attempts to construct self-consistent static solutions.

gr-qc↗

arXiv2026

Removing spurious degrees of freedom from EFT of gravity

In the effective field theory approach to gravity, the Lagrangian density for general relativity is supplemented by generally covariant terms of higher order in the Riemann tensor and its derivatives. At face value, these terms will result in higher derivative equations of motion and additional degrees of freedom beyond those of general relativity. This is seemingly at odds with the goal of effective field theory which is to encode ultraviolet corrections in terms of existing low energy degrees of freedom. Here we apply an action-based procedure for the removal of spurious degrees of freedom for the case of general relativity supplemented by a term cubic in the Riemann tensor. To the order we work in, the resulting reduced theory takes the form of minimally modified gravity, which is a class of modified gravity theories that propagate just the massless spin-2 graviton, but exhibit a preferred frame at short distances due to a deformation of time diffeomorphisms. Our work suggests that this class of theories can be understood as an effective theory of gravity, capturing effects of unknown ultraviolet physics.

gr-qc↗

arXiv2026

On phase-space singular surfaces in $f(R)$ gravity

We perform a Hamiltonian constraint analysis of metric $f(R)$ gravity in the Jordan frame and show that the regular constraint classification degenerates on singular phase-space surfaces located at $f'(R)\!=\!0$ and $f''(R)\!=\!0$. We then study the perturbative implications of these surfaces. For exact backgrounds satisfying $f(R)\!=\!0$ and $f'(R)\!=\!0$, the linearized spectrum is empty; the known pure $R^2$ result is therefore a special case of a more general degeneracy in $f(R)$ gravity. We also show that FLRW trajectories in the Starobinsky model can cross the surface $f'(R)=0$, but that inhomogeneous perturbations develop a degenerate constraint structure at the crossing. The resulting crossing condition is better interpreted as a regularity condition for perturbative evolution than as an ordinary constraint within the Dirac--Bergmann algorithm. Together, these results distinguish backgrounds that lie entirely on a singular surface from backgrounds that cross one dynamically, and show that the two situations lead to different perturbative degeneracies.

gr-qc↗

arXiv2026

Spectrum of pure $R^2$ gravity: full Hamiltonian analysis

We perform a full Hamiltonian constraint analysis of pure Ricci-scalar-squared ($R^2$) gravity to clarify recent controversies regarding its particle spectrum. While it is well established that the full theory consistently propagates three degrees of freedom, we confirm that its linearised spectrum around Minkowski spacetime is empty. Moreover, we show that this is not a feature unique to Minkowski spacetime, but a generic property of all traceless-Ricci spacetimes that have a vanishing Ricci scalar, such as the Schwarzschild and Kerr black hole spacetimes. The mechanism for this phenomenon is a change in the nature of the constraints upon linearisation: ten second-class constraints of the full theory become first-class, while the three momentum constraints degenerate into a single constraint. Furthermore, we show that higher order perturbation theory around these singular backgrounds reveals no degrees of freedom at any order. This is in conflict with the general analysis and points to the fact that such backgrounds are surfaces of strong coupling in field space, where the dynamics of perturbations becomes nonperturbative. We further show via a cosmological phase-space analysis that the evolving universe is able to penetrate through the singular $R=0$ surface.

gr-qc↗

arXiv2026

Graviton propagator in de Sitter space in a simple one-parameter gauge

We construct the graviton propagator in de Sitter space in a one-parameter family of non-covariant gauges. This family generalizes the simple gauge in which most graviton loop computations in de Sitter space have been performed. The resulting propagator has a relatively simple form and will facilitate checks of the gauge dependence of one-loop computations and proposed observables.

gr-qc↗

arXiv2026

On the importance of radiation-era initial conditions for tensor perturbations

Conservation of super-horizon tensor fluctuations is crucial for connecting inflation to observations. Starting from first principles, recent works have found violations of this conservation if free-streaming radiation is produced during reheating. We show that the non-conservation is sensitive to the radiation initial state, and argue that the physical state should be affected by tensor perturbations that are already present during reheating. The deviation from super-horizon conservation is then negligible, recovering the standard result from kinetic theory. In contrast, a globally homogeneous and isotropic plasma state leads to a large suppression of tensor amplitudes. This difference between the local (physical) and global thermal equilibrium settles the discrepancy between the older and recent literature.

hep-th↗

arXiv2026

Cancellation of one-parameter graviton gauge dependence in the effective scalar field equation in de Sitter

We investigate gauge dependence of one-graviton-loop corrections to the effective field equation of the massless, minimally coupled scalar in de Sitter, obtained by including source and observer corrections to the effective self-mass correcting the equation. Using the $Δα$ variation of the de Sitter-breaking graviton propagator in a one-parameter family of gauges, we compute the gauge-dependent contributions to the effective self-mass of a massless minimally coupled scalar mediating interactions between heavy scalars. We show that gauge dependence cancels provided the contributions from all diagram classes are collected, including one-loop corrections to external mode functions, which play a qualitatively new role relative to flat space. The resulting cancellation supports the construction of graviton gauge-independent cosmological quantum-gravitational observables from quantum-corrected effective equations.

hep-th↗

arXiv2025

One-loop correction to primordial tensor modes during radiation era

The ability to infer properties of primordial inflation relies on the conservation of the superhorizon perturbations between their exit during inflation, and their re-entry during radiation era. Any considerable departure from this property would require reinterpreting the data. This is why it is important to understand how superhorizon perturbations interact with the thermal plasma driving the radiation dominated Universe. We model the plasma by free photons in a thermal state and compute the one-loop correction to the power spectrum of primordial tensor perturbations. This correction grows in time and is not suppressed by any small parameter. While one-loop result is not reliable because it invalidates perturbation theory, it signals potentially interesting effects that should be investigated further.

astro-ph.CO↗

arXiv2025

Measurements in stochastic gravity and thermal variance

We analyze the thermal fluctuations of a free, conformally invariant, Maxwell quantum field (photon) interacting with a cosmological background spacetime, in the framework of quantum field theory in curved spacetimes and semiclassical and stochastic gravity. The thermal fluctuations give rise to backreaction effects upon the spacetime geometry, which are incorporated in the semiclassical Einstein-Langevin equation, evaluated in the cosmological Friedmann-Lemaître-Robertson-Walker spacetime. We first evaluate the semiclassical Einstein equation for the background geometry sourced by the thermal quantum stress-energy tensor. For large enough temperature, the solution is approximated by a radiation-dominated expanding universe driven by the thermal bath of photons. We then evaluate the thermal noise kernel associated to the quantum fluctuations of the photon field using point-splitting regularization methods, and give its explicit analytic form in the limits of large and small temperature, as well as a local approximation. Finally, we prove that this thermal noise kernel corresponds exactly to the thermal variance of the induced fluctuations of the linearized metric perturbation in the local and covariant measurement scheme defined by Fewster and Verch. Our analysis allows to quantify the extent to which quantum fluctuations may give rise to non-classical effects, and thus become relevant in inflationary cosmology.

gr-qc↗

arXiv2025

Two simple photon gauges in inflation

Photon propagators for power-law inflation are constructed in two one-parameter families of noncovariant gauges, in an arbitrary number of spacetime dimensions. In both gauges photon propagators take relatively simple forms expressed in terms of scalar propagators and their derivatives. These are considerably simpler compared to their general covariant gauge counterpart. This makes feasible performing dimensionally regulated loop computations involving massless vector fields in inflation.

gr-qc↗

arXiv2024

Critical reassessment of the restricted Weyl symmetry

A class of globally scale-invariant scalar-tensor theories have been proposed to be invariant under a larger class of transformations that take the form of local Weyl transformations supplemented by a restriction that the conformal factor satisfies a covariant Klein-Gordon equation. The action of these theories indeed seems to be invariant under such transformations up to boundary terms, this property being referred to as ``restricted Weyl symmetry''. However, we find that corresponding equations of motion are not invariant under these transformations. This is a paradox, that is explained by realizing that the restriction condition on the conformal factor forces the restricted Weyl transformation to be a nonlocal transformation. For nonlocal transformations would-be boundary terms cannot in general be discarded from the action. Moreover, variations of trajectories cannot be assumed to vanish at boundaries of the action when deriving equations of motion. We illustrate both of these less known properties by considering a series of simple examples. Finally, we apply these observations to the case of globally scale-invariant scalar-tensor theories to demonstrate that restricted Weyl transformations are, in fact, not symmetries of the full system.

hep-th↗

arXiv2024

Photon propagator for inflation in the general covariant gauge

Photon propagator for power-law inflation is considered in the general covariant gauges within the canonical quantization formalism. Photon mode functions in covariant gauges are considerably more complicated than their scalar counterparts, except for the special choice of the gauge-fixing parameter we call the simple covariant gauge. We explicitly construct the position space photon propagator in the simple covariant gauge, and find the result considerably more complicated than its scalar counterpart. This is because of the need for explicitly inverting the Laplace operator acting on the scalar propagator, which results in Appell's fourth function. Our propagator correctly reproduces the de Sitter and flat space limits. We use this propagator to compute two simple observables: the off-coincident field strength-field strength correlator and the energy-momentum tensor, both of which yield consistent results. As a spinoff of our computation we also give the exact expression for the Coulomb gauge propagator in power-law inflation in arbitrary dimensions.

hep-th↗

arXiv2024

Hamiltonian analysis of metric-affine-$R^2$ theory

Determining the number of propagating degrees of freedom in metric-affine theories of gravity requires the use of Hamiltonian constraint analysis, except in some subclasses of theories. We develop the technicalities necessary for such analyses and apply them to the Weyl-invariant and projective-invariant case of metric-affine-$R^2$ theory that is known to propagate just the graviton. This serves as a check of the formalism and a case study where we introduce appropriate ADM variables for the distortion 3-tensor tensor and its time derivatives, that will be useful when analyzing more general metric-affine theories where the physical spectrum is not known.

gr-qc↗

arXiv2024

Photon quantization in cosmological spaces

Canonical quantization of the photon -- a free massless vector field -- is considered in cosmological spacetimes in a two-parameter family of linear gauges that treat all the vector potential components on equal footing. The goal is setting up a framework for computing photon two-point functions appropriate for loop computations in realistic inflationary spacetimes. The quantization is implemented without relying on spacetime symmetries, but rather it is based on the classical canonical structure. Special attention is paid to the quantization of the canonical first-class constraint structure that is implemented as the condition on the physical states. This condition gives rise to subsidiary conditions that the photon two-point functions must satisfy. Some of the de Sitter space photon propagators from the literature are found not to satisfy these subsidiary conditions, bringing into question their consistency.

hep-th↗

arXiv2023

When tadpoles matter: One-loop corrections for spectator Higgs in inflation

We consider the classical attractor regime of the spectator Abelian Higgs model in power-law inflation, and compute the one-loop corrections to its evolution. For computations we utilize dimensional regularization and the propagators in the unitary gauge. The corrections to both the scalar condensate and the energy-momentum tensor exhibit secular ultraviolet contributions, that tend to slow down the rolling of the scalar down its potential, and drive it away from the classical attractor. These corrections need not be suppressed if the U(1) charge is much larger than the scalar self-coupling, which is seen already in flat space. In addition, at late times the secular corrections necessarily invalidate the perturbative loop expansion. We find the late time secular corrections to be captured by the renormalization group, which opens up the possibility to resum them past the breakdown of perturbativity.

hep-ph↗

arXiv2022

Photon propagator in de Sitter space in the general covariant gauge

We consider a free photon field in $D$-dimensional de Sitter space, and construct its propagator in the general covariant gauge. Canonical quantization is employed to define the system starting from the classical theory. This guarantees that the propagator satisfies both the equation of motion and subsidiary conditions descending from gauge invariance and gauge fixing. We first construct the propagator as a sum-over-modes in momentum space, carefully accounting for symmetry properties of the state. We then derive the position space propagator in a covariant representation, that is our main result. Our conclusions disagree with previous results as we find that the position space photon propagator necessarily breaks de Sitter symmetry, except in the exact transverse gauge limit.

gr-qc↗