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Alexandros Karam

Publications and source records attributed to Alexandros Karam.

At least 19 recordsLinked to original sources
arXiv2026

Post-Newtonian Constraints on Scalar-Tensor Gravity

Solar-System constraints on a general scalar-tensor theory with generic non-minimal coupling function, non-canonical kinetic function, and scalar potential, are investigated in both the metric and Palatini formalisms. A unified post-Newtonian treatment is developed, yielding analytical expressions for the effective scalar mass, the effective gravitational coupling, and the parametrised post-Newtonian parameters $γ$ and $β$. The results show explicitly how the choice of variational principle affects the weak-field phenomenology. Comparison with Solar-System observations, primarily the Cassini bound on $γ$, indicates that the observational impact of the formalism is strongly model dependent. Generic non-minimally coupled scalar fields may satisfy significantly weaker local bounds in the Palatini case because of stronger Yukawa suppression, whereas in Brans-Dicke gravity the differences are typically small and become appreciable only in restricted regions of parameter space. For the point-particle source considered here, Palatini $f(\hat{R})$ gravity reproduces the general-relativistic exterior post-Newtonian limit, unlike metric $f(R)$ gravity.

gr-qc↗
arXiv2025

Has ACT measured radiative corrections to the tree-level Higgs-like inflation?

Starobinsky inflation and nonminimally coupled (NMC) Higgs inflation have been among the most favored models of the early Universe, as their predictions for the scalar spectral index $n_s$ and tensor-to-scalar ratio $r$ fall comfortably within the constraints set by Planck and BICEP/Keck. However, new results from the Atacama Cosmology Telescope (ACT) suggest a preference for higher values of $n_s$, introducing tension with the simplest realizations of these models. In this work, being agnostic about the nature of the inflaton, we show that incorporating one-loop corrections to a quartic NMC inflationary scenario leads to a shift in the predicted value of $n_s$, which brings NMC inflation into better agreement with ACT observations. The effect is even more significant when the model is formulated in the Palatini approach, where the modified field-space structure naturally enhances deviations from the metric case. These findings highlight the importance of quantum corrections and gravitational degrees of freedom in refining inflationary predictions in light of new data.

astro-ph.CO↗
arXiv2025

Non-Minimally Coupled Quintessence in Light of DESI

We analyze a model of quintessence governed by an exponential potential and non-minimally coupled to gravity, in light of recent datasets, including cosmic microwave background, baryon acoustic oscillations, and supernovae distance moduli observations. Mainly focusing on the Palatini formulation of gravity, a phase space analysis reveals the existence of a late-time stable de Sitter attractor as long as the non-minimal coupling constant is negative, regardless of the value of the slope of the exponential. Fitting to CMB+DESI+DESY5 data, we find strong evidence for our model over $Λ$CDM, with a Bayes factor $\log B = 5.52$. Furthermore, the data seem to prefer dynamical dark energy at $>3σ$ C.L. and a phantom crossing in the barotropic parameter of dark energy at $2-3σ$ C.L.. We find that the scalar field dynamics in the Palatini formalism provides marginally better agreement to the data compared to the metric formalism.

astro-ph.CO↗
arXiv2025

How deep is the dip and how tall are the wiggles in inflationary power spectra?

We study linear scalar perturbations in single-field models of inflation featuring a non-attractor phase. These models lead to a peak in the curvature power spectrum that may result in the formation of primordial black holes. We develop a transfer-matrix formalism, analogous to the S-matrix program in quantum-field theory, that maps perturbations throughout the transitory phase. At scales smaller than the peak, the power spectrum features damped oscillations, and the duration of the transition sets the scale at which power-law damping switches to exponential damping. At scales larger than the peak, we demonstrate that a dip appears in the power spectrum if and only if the inflaton's velocity does not flip sign. We show that the amplitude at the dip always scales as the inverse square-rooted amplitude of the peak, and comment on the physical consequences of this universal relationship. We also test the robustness of our results with a few toy models and interpret them with an intuitive mechanical analogy.

astro-ph.CO↗
arXiv2025

Palatini Linear Attractors Are Back in ACTion

Recent results from the Atacama Cosmology Telescope (ACT) indicate a scalar spectral index $n_s \simeq 0.9743$, in excellent agreement with the prediction of linear inflation. However, the corresponding tensor-to-scalar ratio $r \simeq 0.0667$ is in tension with current observational bounds. In this work, we investigate how this tension can be alleviated in the Palatini formulation of gravity. We consider two classes of models based on simple monomial potentials: (i) models with a non-minimal coupling between the inflaton and gravity, and (ii) models including an $αR^2$ term. In the first case, we find that a quadratic potential with a linear non-minimal coupling leads to the linear inflation attractor, with $r$ suppressed as $ξ$ increases. In the second case, we show that a linear potential can yield values of $r$ consistent with observations for sufficiently large $α$. Our results demonstrate that simple monomial models can remain compatible with current observational constraints when embedded in the Palatini framework.

gr-qc↗
arXiv2023

Observable Gravitational Waves from Hyperkination in Palatini Gravity and Beyond

We consider cosmology with an inflaton scalar field with an additional quartic kinetic term. Such a theory can be motivated by Palatini $R+R^2$ modified gravity. Assuming a runaway inflaton potential, we take the Universe to become dominated by the kinetic energy density of the scalar field after inflation. Initially, the leading kinetic term is quartic and we call the corresponding period hyperkination. Subsequently, the usual quadratic kinetic term takes over and we have regular kination, until reheating. We study, both analytically and numerically, the spectrum of primordial gravitational waves generated during inflation and re-entering the horizon during the subsequent eras. We demonstrate that the spectrum is flat for modes re-entering during radiation domination and hyperkination and linear in frequency for modes re-entering during kination: kinetic domination boosts the spectrum, but hyperkination truncates its peak. As a result, the effects of the kinetic period can be extended to observable frequencies without generating excessive gravitational waves, which could otherwise destabilise the process of Big Bang Nucleosynthesis. We show that there is ample parameter space for the primordial gravitational waves to be observable in the near future. If observed, the amplitude and `knee' of the spectrum will provide valuable insights into the background theory.

gr-qc↗
arXiv2023

Implications of Palatini gravity for inflation and beyond

We present an introduction to cosmic inflation in the framework of Palatini gravity, which provides an intriguing alternative to the conventional metric formulation of gravity. In the latter, only the metric specifies the spacetime geometry, whereas in the former, the metric and the spacetime connection are independent variables-an option that can result in a gravity theory distinct from the metric one. In scenarios where the field(s) responsible for cosmic inflation are non-minimally coupled to gravity or the gravitational sector is extended, assumptions about the underlying gravitational degrees of freedom can have substantial implications for the observational effects of inflation. We examine this explicitly by discussing various compelling scenarios, such as Higgs inflation with non-minimal coupling to gravity, Higgs inflation with non-minimal derivative coupling, $\mathcal{R}^2$ inflation, and beyond. We also comment on reheating in these models. Finally, as an application of the general results of Palatini $\mathcal{R}^2$ inflation, we review a model of successful quintessential inflation, where a single scalar field acts initially as the inflaton and then becomes dynamical dark energy, in agreement will all experimental constraints.

gr-qc↗
arXiv2023

Primordial black holes and inflation from double-well potentials

We investigate the formation of large peaks in the inflationary curvature power spectrum from double-well potentials. In such scenarios, the initial CMB spectrum is created at large field values. Subsequently, the inflaton will cross one of the minima and will decelerate rapidly as it reaches the local maximum at the origin, either falling back or crossing it. During this final phase, a significant peak in the curvature power spectrum can be generated. Our analysis reveals that this class of models produces more pronounced peaks than the more commonly studied quasi-inflection point scenarios with less tuning for the model parameters. Finally, we construct an explicit theoretically motivated inflationary scenario that is consistent with the latest CMB observations and capable of generating sufficiently large curvature perturbations for primordial black holes.

astro-ph.CO↗
arXiv2023

Gravitational corrections to electroweak vacuum decay: metric vs. Palatini

We consider the standard Einstein-Hilbert-Higgs action where the Higgs field couples nonminimally with gravity via the term $ξh^2 \mathcal{R}$, and investigate the stability of the electroweak vacuum in the presence of gravitational corrections in both the metric and Palatini formulations of gravity. In order to identify the differences between the two formalisms analytically, we follow a perturbative approach in which the gravitational corrections are taken into consideration via a leading order expansion in the gravitational coupling constant. Our analysis shows that in the Palatini formalism, the well-known effect of gravity suppressing the vacuum decay probability, becomes milder in comparison with the metric case for any value of the nonminimal coupling $ξ$. Furthermore, we have found that in the Palatini formalism, the positivity of the gravitational corrections, which is a necessary requirement for the unitarity of the theory, entails the lower bound $ξ>-1/12$.

hep-ph↗
arXiv2023

Scale-Invariant Model for Gravitational Waves and Dark Matter

We have conducted a revised analysis of the first-order phase transition that is associated with symmetry breaking in a classically scale-invariant model that has been extended with a new $SU(2)$ gauge group. By incorporating recent developments in the understanding of supercooled phase transitions, we were able to calculate all of its features and significantly limit the parameter space. We were also able to predict the gravitational wave spectra generated during this phase transition and found that this model is well-testable with LISA. Additionally, we have made predictions regarding the relic dark matter abundance. Our predictions are consistent with observations but only within a narrow part of the parameter space. We have placed significant constraints on the supercool dark matter scenario by improving the description of percolation and reheating after the phase transition, as well as including the running of couplings. Finally, we have also analyzed the renormalization-scale dependence of our results.

astro-ph.CO↗
arXiv2023

Conformal model for gravitational waves and dark matter: A status update

We present an updated analysis of the first-order phase transition associated with symmetry breaking in the early Universe in a classically scale-invariant model extended with a new SU(2) gauge group. Including recent developments in understanding supercooled phase transitions, we compute all of its characteristics and significantly constrain the parameter space. We then predict gravitational wave spectra generated during this phase transition and by computing the signal-to-noise ratio we conclude that this model is well testable (and falsifiable) with LISA. We also provide predictions for the relic dark matter abundance. It is consistent with observations in a rather narrow part of the parameter space, since we exclude the so-called supercool dark matter scenario based on an improved description of percolation and reheating after the phase transition as well as inclusion of the running of couplings. Finally, we devote attention to renormalisation-scale dependence of the results. Even though our main results are obtained with the use of renormalisation-group improved effective potential, we also perform a fixed-scale analysis which proves that the dependence on the scale is not only qualitative but also quantitative.

astro-ph.CO↗
arXiv2022

Palatini $R^2$ Quintessential Inflation

We construct a model of quintessential inflation in Palatini $R^2$ gravity employing a scalar field with a simple exponential potential and coupled to gravity with a running non-minimal coupling. At early times, the field acts as the inflaton, while later on it becomes the current dark energy. Combining the scalar sector with an ideal fluid, we study the cosmological evolution of the model from inflation all the way to dark energy domination. We interpret the results in the Einstein frame, where a coupling emerges between the fluid and the field, feeding energy from the former to the latter during the matter-dominated era. We perform a numerical scan over the parameter space and find points that align with observations for both the inflationary CMB data and the late-time behaviour. The final dark energy density emerges from an interplay between the model parameters, without requiring the extreme fine-tuning of the cosmological constant in $Λ$CDM.

gr-qc↗
arXiv2022

Anatomy of single-field inflationary models for primordial black holes

We construct an analytically solvable simplified model that captures the essential features for primordial black hole (PBH) production in most models of single-field inflation. The construction makes use of the Wands duality between the constant-roll (or slow-roll) and the preceding ultra-slow-roll phases and can be realized by a simple inflaton potential of two joined parabolas. Within this framework, it is possible to formulate explicit inflationary scenarios consistent with the CMB observations and copious production of PBHs of arbitrary mass. We quantify the variability of the shape of the peak in the curvature power spectrum in different inflationary scenarios and discuss its implications for probing PBHs with scalar-induced gravitational wave backgrounds. We find that the COBE/Firas $μ$-distortion constraints exclude the production of PBHs heavier than $10^4 M_\odot$ in single-field inflation.

astro-ph.CO↗
arXiv2022

Modelling Quintessential Inflation in Palatini Modified Gravity

We study a model of quintessential inflation constructed in $R^2$ modified gravity with a non-minimally coupled scalar field, in the Palatini formalism. Our non-minimal inflaton field is characterised by a simple exponential potential. We find that successful quintessential inflation can be achieved with no fine-tuning on the model parameters. Predictions on the characteristics of dark energy will be tested by observations in the near future, while contrast with existing observations provides insights on the modified gravity background, such as the value of the non-minimal coupling and its running.

gr-qc↗
arXiv2021

Gravitational dark matter production in Palatini preheating

We study preheating in plateau inflation in the Palatini formulation of general relativity, in a special case that resembles Higgs inflation. It was previously shown that the oscillating inflaton field returns to the plateau repeatedly in this model, and this leads to tachyonic production of inflaton particles. We show that a minimally coupled spectator scalar field can be produced even more efficiently by a similar mechanism. The mechanism is purely gravitational, and the scalar field mass can be of order $10^{13}$ GeV, larger than the Hubble scale by many orders of magnitude, making this a candidate for superheavy dark matter.

astro-ph.CO↗
arXiv2021

Tachyonic Preheating in Palatini $R^2$ Inflation

We study preheating in the Palatini formalism with a quadratic inflaton potential and an added $αR^2$ term. In such models, the oscillating inflaton field repeatedly returns to the plateau of the Einstein frame potential, on which the tachyonic instability fragments the inflaton condensate within less than an e-fold. We find that tachyonic preheating takes place when $α\gtrsim 10^{13}$ and that the energy density of the fragmented field grows with the rate $Γ/H \approx 0.011 \times α^{0.31}$. The model extends the family of plateau models with similar preheating behaviour. Although it contains non-canonical quartic kinetic terms in the Einstein frame, we show that, in the first approximation, these can be neglected during both preheating and inflation.

astro-ph.CO↗
arXiv2021

Higgs-like spectator field as the origin of structure

We show that the observed primordial perturbations can be entirely sourced by a light spectator scalar field with a quartic potential, akin to the Higgs boson, provided that the field is sufficiently displaced from vacuum during inflation. The framework relies on the indirect modulation of reheating, which is implemented without any direct coupling between the spectator field and the inflaton and does not require non-renormalisable interactions. The scenario gives rise to local non-Gaussianity with $f_{\rm NL}\simeq 5$ as the typical signal. As an example model where the indirect modulation mechanism is realised for the Higgs boson, we study the Standard Model extended with right-handed neutrinos. For the Standard Model running we find, however, that the scenario analysed does not seem to produce the observed perturbation.

hep-ph↗