arXiv subjects

Alexandros Papageorgiou

Publications and source records attributed to Alexandros Papageorgiou.

At least 19 recordsLinked to original sources

arXiv2026

Axion Inflation: Perturbative control in the strong backreaction regime

We study the strong backreaction regime in axion inflation, in which the friction from Abelian gauge fields generated via the pseudo-scalar interaction $\fracβ{4 M_p} ϕF {\tilde F}$ is comparable to Hubble friction. The non-linear dynamics associated with the gauge fields render this regime phenomenologically particularly interesting, but also notoriously difficult to study with perturbative methods. Quantifying the perturbative control through the direct impact on the spectral backreaction using (i) a first-order gradient-expansion formalism including axion gradients, and (ii) a one-loop in-in calculation, we discover an extended mild backreaction regime at large couplings $β$, with a relatively large, nearly constant particle production parameter $ξ$. In passing, we point out that CMB non-Gaussianity bounds impose a general upper limit on the product of axion gauge field coupling and Hubble parameter during inflation, $βH/M_p < 1.4\cdot 10^{-3}$, independently of the choice of the axion potential.

astro-ph.CO↗

arXiv2026

Pure Natural Inflation Passes the ACT

Pure natural inflation is a compelling effectively single-field model of inflation stemming from a top-down approach to the acceleration mechanism. In this short letter we show that such model is compatible with the latest CMB constraints obtained from the Atacama Cosmology Telescope combined with baryon acoustic oscillation data from the Dark Energy Spectroscopic Instrument. Under both the instantaneous reheating hypothesis and standard assumptions for reheating, we rule in a non-trivial fraction of the parameter space. We apply our analysis also to a phenomenological extension of the model and chart its viable parameter space.

astro-ph.CO↗

arXiv2026

Primordial Gravitational Waves from Scalar Backreaction in Axion-SU(2) Inflation

In this work, we perform the first numerical study of strong scalar backreaction in spectator chromo-natural inflation (SCNI) in the case where the spectator sector decays during inflation. The tachyonic instability in scalar fluctuations, activated as the system crosses the $m_Q = \sqrt{2}$ threshold, amplifies perturbations and may significantly alter the background dynamics. The strong scalar backreaction regime introduces an effective quartic term in the potential for the gauge field background that rapidly drives it to zero, accelerating the axion-gauge system decay. We describe the dynamics of such decay and derive the gravitational wave spectrum for a set of benchmark parameters. Interestingly, the signal may peak at interferometer scales and lie within LISA's projected sensitivity.

astro-ph.CO↗

arXiv2025

On the prospects of thermalization of axion-SU(2) inflation

Axion inflation models coupled to a gauge sector via a Chern-Simons term exhibit an array of interesting phenomenology including a chiral gravitational wave spectrum and primordial black hole production. They may also provide a useful mechanism for generating lepton asymmetry. The possibility to embed this class of models in UV-finite theories and their intriguing, testable, signatures make for a compelling candidate for early acceleration. Due to the Chern-Simons coupling, gauge modes may undergo a finite tachyonic growth during which non-linearities become important. Naturally, this raises the question of whether such (self) interactions can lead to thermalization during inflation. We provide a set of useful criteria for sustained thermalization in an axion-$SU(2)$ model and chart the parameter space of the model accordingly. We find that the cold inflation regime constitutes a very significant fraction of the parameter space. Our analysis accounts for a initially vanishing as well as non-zero gauge field vacuum expectation value (VEV). We also consider the possibility of a dynamically generated VEV.

astro-ph.CO↗

arXiv2025

Kinetic Fragmentation of the QCD Axion on the Lattice

Kinetic misalignment, one of the most compelling scenarios for the non-thermal generation of axion dark matter, is generally accompanied by axion fragmentation, a process in which the energy of the axion condensate is transferred to its perturbations. The dynamics of fragmentation, at least in the context of dark matter production, have so far been studied semi-analytically using perturbation theory. In this work, we present the first classical lattice simulation of kinetic axion fragmentation in the context of dark matter production, focusing on parameters relevant to the QCD axion. Our findings indicate that the non-perturbative dynamics captured by the lattice lead to a significantly broader spectrum of axion fluctuations, with a sustained transfer of energy to mildly relativistic modes and with smaller occupation numbers compared to the linear approximation. As a consequence, the final dark matter abundance is typically O(1) lower than in the linear approximation, which is itself O(1) lower than the zero-mode-only prediction. This broadening and suppression of the spectrum could have a significant impact on axion mini-halo formation, one of the main experimental handles on kinetic fragmentation.

astro-ph.CO↗

arXiv2025

Pure Chromo-Natural Inflation: Signatures of Particle Production from Weak to Strong Backreaction

We consider, in the context of axion-inflation, the \textit{Pure Natural Inflation} (PNI) model coupled with an SU(2) gauge sector via a Chern-Simons term. As the axion rolls down its potential, it dissipates energy in the gauge sector thus sourcing fluctuations of scalar and tensor degrees of freedom therein. Gauge field fluctuations will, in turn, feed primordial gravitational waves as well as curvature perturbations. Remarkably, we can use upcoming cosmological probes to test this mechanism across a vast range of scales, from the CMB to laser interferometers. Due to their flat plateau at large field values, we find that PNI potentials fare better vis-á-vis CMB observations than the conventional sinusoidal potential of chromo-natural inflation (CNI). We show that, even when the dynamics begin in the weak backreaction regime, the rolling of the axion leads to a build-up of the gauge-quanta production, invariably triggering the strong backreaction of the gauge sector tensors on the background dynamics. This transition results in the copious production of both scalar and tensor perturbations, which we study in detail. The gravitational wave signatures include a rich peak structure with a characteristic scale-dependent chirality, a compelling target for future gravitational wave detectors. Additionally, the peak in scalar perturbations may lead to the formation of primordial black holes, potentially accounting for a significant fraction of the observed dark matter abundance.

astro-ph.CO↗

arXiv2024

Scale-dependent chirality as a smoking gun for Abelian gauge fields during inflation

Axion-inflation models are a compelling candidate as a mechanism behind the accelerated expansion in the early universe in light of the possibility to embed them in higher dimensional UV complete theories and the exciting prospect of testing them with next-generation cosmological probes. Adding an Abelian gauge sector to axion-inflation models makes for a rich, interesting, phenomenology spanning from primordial black holes to gravitational waves (GWs). Several recent studies employ an approximate analytic (Gaussian) template to characterize the effect of gauge field production on cosmological perturbations. In this work we go beyond such approximation and numerically study particle production and the ensuing scalar and tensor spectra. We find a significant deviation from results based on log-normally distributed vector field excitations. As an important phenomenological application of the improved method, we study the expected chirality and spectral index of the sourced GW background at scales relevant for current and next-generation GW detectors. One striking feature is that of a scale-dependent chirality. We derive a consistency relation between these two observables that can serve as an important tool in identifying key signatures of multi-field dynamics in axion inflation.

astro-ph.CO↗

arXiv2024

Axion-Gauge Dynamics During Inflation as the Origin of Pulsar Timing Array Signals and Primordial Black Holes

We demonstrate that the recently announced signal for a stochastic gravitational wave background (SGWB) from pulsar timing array (PTA) observations, if attributed to new physics, is compatible with primordial GW production due to axion-gauge dynamics during inflation. More specifically we find that axion-$U(1)$ models may lead to sufficient particle production to explain the signal while simultaneously source some fraction of sub-solar mass primordial black holes (PBHs) as a signature. Moreover there is a parity violation in GW sector, hence the model suggests chiral GW search as a concrete target for future. We further analyze the axion-$SU(2)$ coupling signatures and find that in the low/mild backreaction regime, it is incapable of producing PTA evidence and the tensor-to-scalar ratio is low at the peak, hence it overproduces scalar perturbations and PBHs.

astro-ph.CO↗

arXiv2024

Dissipative Emergence of Inflation from Quasi-Cyclic Universe

Inflationary models, especially those with plateau-type potentials, are consistent with the cosmological data, but inflation itself does not resolve the initial singularity. This singularity is resolved, for example, by the idea of the quantum creation of the Universe from nothing such as the tunneling and no-boundary proposals. The simplest one predicts a closed Universe. Motivated by these facts, we investigate the classical dynamics of a closed universe with a plateau-type potential. Depending on the initial inflaton field value, the universe can undergo a variety of events: an immediate big crunch, a bounce or cyclic phase, and inflation. Although the non-inflationary solutions may appear irrelevant to our Universe, they can be turned into single or multiple bounces followed by inflation, taking into account the interactions necessary for the reheating of the Universe after inflation. Thus, the dissipative mechanism in our setup explains both the graceful entry to and exit from inflation and gives us an indirect observational handle on the Universe just after creation. We also comment on the implications of these solutions for probabilistic interpretations of the wave function of the Universe.

gr-qc↗

arXiv2024

Dissipative Genesis of the Inflationary Universe

We study an inflation model with a flat scalar potential supported by observations and find that slow-roll inflation can emerge after a quasi-cyclic phase of the Universe, where it undergoes repeated expansions and contractions for a finite time period. The initial conditions and the positive spatial curvature required for such nontrivial dynamics align with the quantum creation of the Universe. The key ingredients that trigger inflation are dissipative interactions of the inflaton, which are necessary to reheat the Universe after inflation and thus give us an observational handle on pre-inflationary physics. Our discovery implies that inflation occurs more robustly after the creation.

gr-qc↗

arXiv2024

A novel PBH production mechanism from non-Abelian gauge fields during inflation

We consider the case of axion-like particles (ALPs) during inflation. When coupled to a non-Abelian gauge sector via a Chern-Simons term, ALPs support an intriguing, testable, phenomenology with very distinctive features including chiral primordial gravitational waves. For sufficiently small values of the gauge vev and coupling, scalar perturbations in the gauge sector exhibit a known instability. We harness the power of such instability for primordial black hole (PBH) generation. In the case of an axion-inflaton, one is dynamically driven into a strong-backreaction regime that crosses the instability band thereby sourcing a peaked scalar spectrum leading to PBH production and the related scalar-induced gravitational waves. Remarkably, this dynamics is largely insensitive to the initial conditions and the shape of the potential, highlighting the universal nature of the sourcing mechanism. In the case of spectator ALPs one can identify the parameter space that sets off the strong backreaction regime and the ensuing features. We show that spectator ALP models may also access the scalar instability region without triggering strong backreaction.

astro-ph.CO↗

arXiv2024

Memory of a Random Walk: Astrometric deflections from gravitational wave memory accumulation over cosmological scales

We study the impact of gravitational wave memory on the distribution of far away light sources in the sky. For the first time we compute the built up of small, but permanent tensor distortions of the metric over cosmological time-scales using realistic models of compact binary coalescences (CBCs) whose rate of occurrence is extrapolated at $z\sim {\cal O}(1)$. This allows for a consistent computation of the random-walk like evolution of gravitational wave memory which, in turn, is used to estimate the overall shape and magnitude of astrometric deflections of far away sources of light. We find that for pulsar or quasar proper motions, the near-Earth contribution to the astrometric deflections dominates the result and the deflection is analogous to a stochastic gravitational wave memory background that is generally subdominant to the primary stochastic gravitational wave background. We find that this contribution can be within the reach of future surveys such as Theia. Finally, we also study the deviation of the presently observed angular distribution of quasars from perfect isotropy, which arises from the slow build-up of gravitational wave memory over the entire history of the universe. In this case, we find that astrometric deflections depend on the entire light trajectory from the source to the Earth, yielding a quadruple pattern whose magnitude is unlikely to be within reach of the next generation of astrometric surveys due to shot noise and cosmic variance limitations.

astro-ph.CO↗

arXiv2023

The Cluster Mass Function and the $σ_8$-tension

We use a large set of halo mass function (HMF) models in order to investigate their ability to represent the observational Cluster Mass Function (CMF), derived from the $\mathtt{GalWCat19}$ cluster catalogue, within the $Λ$CDM cosmology. We apply the $χ^2$ minimization procedure to constrain the free parameters of the models, namely $Ω_m$ and $σ_8$. We find that all HMF models fit well the observational CMF, while the Bocquet et. al. model provides the best fit, with the lowest $χ^2$ value. Utilizing the {\em Index of Inconsistency} (IOI) measure, we further test the possible inconsistency of the models with respect to a variety of {\em Planck 2018} $Λ$CDM cosmologies, resulting from the combination of different probes (CMB - BAO or CMB - DES). We find that the HMF models that fitted well the observed CMF provide consistent cosmological parameters with those of the {\em Planck} CMB analysis, except for the Press $\&$ Schechter, Yahagi et. al., and Despali et. al. models which return large IOI values. The inverse $χ_{\rm min}^2$-weighted average values of $Ω_m$ and $σ_8$, over all 23 theoretical HMF models are: ${\bar Ω_{m,0}}=0.313\pm 0.022$ and ${\bar σ_8}=0.798\pm0.040$, which are clearly consistent with the results of {\em Planck}-CMB, providing $S_8=σ_8\left(Ω_m/0.3\right)^{1/2}= 0.815\pm 0.05$. Within the $Λ$CDM paradigm and independently of the selected HMF model in the analysis, we find that the current CMF shows no $σ_8$-tension with the corresponding {\em Planck}-CMB results.

astro-ph.CO↗

arXiv2023

A flashing beacon in axion inflation: recurring bursts of gravitational waves in the strong backreaction regime

The coupling between a pseudo-scalar inflaton and a gauge field leads to an amount of additional density perturbations and gravitational waves (GWs) that is strongly sensitive to the inflaton speed. This naturally results in enhanced GWs at (relatively) small scales that exited the horizon well after the CMB ones, and that can be probed by a variety of GW observatories (from pulsar timing arrays, to astrometry, to space-borne and ground-based interferometers). This production occurs in a regime in which the gauge field significantly backreacts on the inflaton motion. Contrary to earlier assumptions, it has been recently shown that this regime is characterized by an oscillatory behavior of the inflaton speed, with a period of~${\rm O } \left( 5 \right)$ e-folds. Bursts of GWs are produced at the maxima of the speed, imprinting nearly periodic bumps in the frequency-dependent spectrum of GWs produced during inflation. This can potentially generate correlated peaks appearing in the same or in different GWs experiments.

astro-ph.CO↗

arXiv2023

Axion dark matter from frictional misalignment

We study the impact of sphaleron-induced thermal friction on the axion dark-matter abundance due to the interaction of an axion-like particle (ALP) with a dark non-abelian gauge sector in a secluded thermal bath. Thermal friction can either enhance the axion relic density by delaying the onset of oscillations or suppress it by damping them. We derive an analytical formula for the \emph{frictional adiabatic invariant}, which remains constant along the axion evolution and which allows us to compute the axion relic density in a general set-up. Even in the most minimal scenario, in which a single gauge group is responsible for both the generation of the ALP mass and the friction force, we find that the resulting dark-matter abundance from the misalignment mechanism deviates from the standard scenario for axion masses $m_a\gtrsim 100 \; {\rm eV}$. We also generalize our analysis to the case where the gauge field that induces friction and the gauge sector responsible for the ALP mass are distinct and their couplings to the axion have a large hierarchy as can be justified by means of alignment or clockwork scenarios. We find that it is easy to open up the ALP parameter space where the resulting axion abundance matches the observed dark-matter relic density both in the traditionally over- and underabundant regimes. This conclusion also holds for the QCD axion.

hep-ph↗

arXiv2022

Constraining modified gravity theories with scalar fields using black-hole images

We study a number of well-motivated theories of modified gravity with the common overarching theme that they predict the existence of compact objects such as black holes and wormholes endowed with scalar hair. We compute the shadow radius of the resulting compact objects and demonstrate that black hole images such as that of M87$^*$ or the more recent SgrA$^*$ by the Einstein Horizon Telescope (EHT) collaboration may provide a powerful way to constrain deviations of the metric functions from what is expected from general relativity (GR) solutions. We focus our attention on Einstein-scalar-Gauss-Bonnet (EsGB) theory with three well motivated couplings, including the dilatonic and $Z_2$ symmetric cases. We then analyze the shadow radius of black holes in the contest of the spontaneous scalarization scenario within EsGB theory with an additional coupling to the Ricci scalar (EsRGB). Finally, we turn our attention to spontaneous scalarization in the Einstein-Maxwell-Scalar (EMS) theory and demonstrate the impact of the parameters on the black hole shadow. Our results show that black hole imaging is an important tool for constraining black holes with scalar hair and for some part of the parameter space, black holes solutions with scalar hair may be marginally favoured compared to solutions of GR.

gr-qc↗

arXiv2022

Rectifying No-Hair Theorems in Gauss-Bonnet theory

We revisit the no-hair theorems in Einstein-Scalar-Gauss-Bonnet theory with a general coupling function between the scalar and the Gauss-Bonnet term in four dimensional spacetime. In the case of the old no-hair theorem the surface term has so far been ignored, but this plays a crucial role when the coupling function does not vanish at infinity and the scalar field admits a power expansion with respect to the inverse of the radial coordinate in that regime. We also clarify that the novel no-hair theorem is always evaded for regular black hole solutions without any restrictions as long as the regularity conditions are satisfied.

hep-th↗

arXiv2020

Gravitational Wave Signatures from Warm Dark Energy

Motivated by some of the recent swampland conjectures, we study a model of dark energy, in which a quintessence axion slowly rolls in a steep potential due to its interactions with a U(1) or an SU(2) gauge field. The gauge fields produced by this interaction can generate a stochastic gravitational wave background with frequencies in the range $f \sim \left(10^{-16} - 10^{-14}\right)\; {\rm Hz}$. Gravitational waves in this range can in principle be probed by CMB spectral distortions. We show that the amplitude of the signal produced in this model is above the level for detectability only under very favourable choices of parameters.

astro-ph.CO↗