arXiv subjects

Theodoros Papanikolaou

Publications and source records attributed to Theodoros Papanikolaou.

At least 19 recordsLinked to original sources

arXiv2026

Induced Gravitational Waves as Cosmic Tracers of Leptogenesis

We demonstrate that induced gravitational waves (IGWs) can naturally emerge within well-motivated realizations of thermal leptogenesis, thereby providing a possible observational handle on this framework at remarkably high energy scales. To illustrate this principle, we put forth a simple leptogenesis model in which an early matter-dominated phase, connected to the leptogenesis scale, enhances the generation of gravitational waves induced by early structure formation. Leveraging recent N-body and lattice simulation results for IGW computations in the non-linear regime, we show that, within the assumptions of the model, the frequency and amplitude of these IGWs can be correlated with the thermal leptogenesis scale.

hep-ph↗

arXiv2026

Primordial black hole induced gravitational waves in $f(R)$ gravity

Ultra-light primordial black holes (PBHs) with masses $M_\mathrm{PBH}<5\times 10^{8}\mathrm{g}$ can trigger an early matter-dominated (eMD) era before Big Bang Nucleosynthesis (BBN) and reheat the Universe through their evaporation. Notably, the initial isocurvature in nature PBH energy density fluctuations can induce abundantly gravitational waves (GWs) due to second-order gravitational effects. In this work, we study this induced GW signal within the context of $f(R)$ gravity investigating the effect of $f(R)$ gravity on the behaviour of scalar perturbations during the PBH-driven eMD era as well as on the source of the second-order induced tensor modes. In particular, we focus on two minimal $f(R)$ models, that is $R^2$ and $R^{1+ε}$, with $ε\ll 1$, gravity theories as illustrative examples, finding at the end that $R^2$ gravity presents very small deviations from general relativity (GR) at the level of both the scalar and tensor perturbations. However, $R^{1+ε}$ gravity features a different behaviour, exhibiting in particular an exponential growth of scalar perturbations during a matter-dominated era. This unique feature leads ultimately to an enhanced induced GW signal even for very small initial PBH abundances, being characterized by a linear frequency scaling on large infrared scales away from the peak frequency, namely $Ω_\mathrm{GW}(k)\propto f$ for $f\ll f_\mathrm{peak}$ and a very peaky behaviour with a $f^{35/2}$ frequency scaling close to the non-linear cut-off scale below which perturbation theory breaks down. Interestingly, one finds as well that the induced GW signal can be well within the sensitivity curves of GW detectors, namely that of LISA, ET, BBO and SKA.

astro-ph.CO↗

arXiv2026

Primordial black hole dark matter from ultra-slow-roll inflation in Horndeski gravity

Primordial black holes (PBHs) provide a well-motivated non-particle candidate for dark matter, requiring an enhancement of curvature perturbations on small inflationary scales consistent with observational constraints. In this work we study PBH production within Horndeski gravity, accounting for compatibility with the GW170817 constraint on the gravitational-wave (GW) speed and imposing a constant coupling to the Ricci scalar. Under these conditions, and assuming an inflaton field characterised by a canonical kinetic term and a smooth potential, the inflationary dynamics is controlled by the cubic Horndeski interaction. By investigating standard functional forms of the latter we identify the specific kinetic structure that allows enhancement of the effective friction on the inflaton, thereby inducing a transient ultra-slow-roll phase embedded within a standard slow-roll evolution. For representative parameter choices we find that pronounced amplifications in the scalar power spectrum are generated, leading to the formation of asteroid-mass PBHs with masses of order $\mathcal{O}(10^{-16})\,M_\odot$, which can account for a substantial fraction of the dark matter abundance, reaching $f_{\rm PBH}\simeq 0.9$, while satisfying current observational constraints. The resulting characteristic features in the scalar power spectrum also imply potentially observable scalar-induced gravitational-wave (SIGW) signatures.

gr-qc↗

arXiv2026

Magnetically assisted primordial scalar perturbations: Scalar-Induced Gravitational Waves

Primordial magnetic fields (PMFs) provide a well-motivated source of cosmological perturbations through their anisotropic stress and may leave observable imprints in both the scalar and tensor sectors. In this work, we study scalar metric perturbations sourced by a PMF evolving through a finite post-inflationary epoch characterized by a constant equation-of-state (EoS) parameter $w$. Working in a gauge-invariant framework, we derive the sourced evolution equation for the Bardeen gravitational potential in a general constant-$w$ background. Our analysis indicates faster (slower) growth of PMF-sourced scalar perturbations on superhorizon scales for a stiffer $w>1/3$ (softer $w<1/3$) background compared to the marginal radiation-dominated scenario with $w=1/3$. We then derive the scalar-induced gravitational wave (SIGW) background induced at second order by the magnetically generated scalar perturbations. Our analysis indicates a narrow viable parametric region for a kination-like post-inflationary era and a nearly scale-invariant PMF that may give rise to a PMF-sourced SIGW signal dominating over the direct PMF-generated tensor background. Simultaneously, the induced tensor modes are found to be perturbatively small compared to the magnetic sector, which, in turn, exerts negligible backreaction on the background. Interestingly enough, for an inflationary scale $H_{\rm inf}\sim10^3-10^9$ GeV and duration of reheating in $e$-folds $N_{\rm reh}\sim8-10$, the dominant PMF-sourced SIGW signal may be detectable by next-generation terrestrial and space-based interferometric GW detectors spanning the mHz$-$kHz frequency range.

astro-ph.CO↗

arXiv2026

Inflation driven by repulsive-like primordial black holes

We review a new natural inflationary mechanism operated by repulsive-like primordial black holes (PBHs). In particular, working within the ``Swiss - Cheese" cosmological framework, we find that a Universe filled with PBHs, whose spacetime metric presents a repulsive-like behaviour, is characterised by an early quasi-de-Sitter cosmic expansion phase. Notably, for light PBHs with $m < 5 \times 10^8 \mathrm{g}$, evaporating before Big Bang Nucleosynthesis (BBN), one is met with an exponential inflationary phase with graceful exit and reheating proceeding through PBH evaporation. Furthermore, one finds as well that PBHs with $m \sim 10^{12}\mathrm{g}$ and abundances $0.107 < Ω^\mathrm{eq}_\mathrm{PBH} < 0.5$ near matter-radiation equality can act as an early dark energy component, easing in this way naturally the Hubble tension.

gr-qc↗

arXiv2026

Primordial black hole formation from transient $f(T)$ cosmology

We study primordial black hole (PBH) formation in a minimally coupled $f(T)$ teleparallel cosmology that generates a transient departure from standard radiation domination. The model is constructed so that modified-gravity effects are negligible at early and late times, but become dynamically relevant over a finite epoch, during which an effective torsion component reduces the total equation-of-state parameter below 1/3.We show that this transient softening lowers the collapse threshold for overdensities at horizon re-entry, leading to an exponential enhancement of PBH formation. In addition, the modified background alters the relation between temperature and horizon mass, producing a localized feature in the PBH mass function. For representative parameters, PBHs with asteroid-scale masses can account for a significant fraction, or even the entirety, of dark matter for perturbation amplitudes $σ^2 \sim \mathcal{O}(10^{-3})$, while remaining consistent with current constraints. Our results demonstrate that modified gravity alone can efficiently generate PBHs, without requiring ad hoc modifications of the radiation sector.

gr-qc↗

arXiv2026

Ultra-fast growth of primordial black holes through radiative absorption

We show that Schwarzschild primordial black holes (PBHs) formed in the radiation-dominated era can grow extremely rapidly through $\textit{radiative absorption}$ governed by the full Stefan-Boltzmann law. By introducing a principle of isonomy - ensuring identical particle species dependence for Hawking emission and absorption - we find that, whenever the temperature of the PBH environment is larger than the PBH horizon temperature, PBHs generically gain mass. In particular, for PBH masses following the critical collapse mass-scaling law with critical exponent $γ_\mathrm{crit}$, with $γ_\mathrm{crit} \in (0.33, 0.49)$, the aforementioned radiative absorption mass growth mechanism produces a striking effect: PBHs forming with a mass $10^6M_\odot$ during BBN can reach $\mathcal{O}(10^{10} M_\odot)$ within $\mathcal{O}(10^{6} \mathrm{s})$ ($\sim $ 58 days). Interestingly enough, small deviations from $γ_\mathrm{crit}$, depending itself on the number of relativistic species present in the primordial plasma, yield a continuous PBH mass spectrum providing us ultimately with a single, Standard-Model-based explanation for the origin of stellar-mass, intermediate-mass, and supermassive black holes (SMBHs), and naturally accounting for the early appearance of SMBHs. The Schwarzschild treatment presented here can be extended to spherically symmetric cosmological black holes, indicating that radiative absorption is a dominant and previously overlooked PBH growth channel in the early Universe.

astro-ph.CO↗

arXiv2025

Primordial black holes as cosmic expansion accelerators

We propose a novel and natural mechanism for cosmic acceleration driven by primordial black holes (PBHs) exhibiting repulsive behavior. Using a new ``Swiss Cheese'' cosmological approach, we demonstrate that this cosmic acceleration mechanism is a general phenomenon by examining three regular black hole spacetimes - namely the Hayward, the Bardeen and the Dymnikova spacetimes - as well as the singular de Sitter-Schwarzschild spacetime. Interestingly, by matching these black hole spacetimes with an isotropic and homogeneous expanding Universe, we obtain a phase of cosmic acceleration that ends at an energy scale characteristic to the black hole parameters or due to black hole evaporation. This cosmic acceleration mechanism can be relevant either to an inflationary phase with a graceful exit and reheating or to an early dark energy type of contribution pertinent to the Hubble tension. Remarkably, we find that ultra-light PBHs with masses $m<5\times 10^8\mathrm{g}$ dominating the energy content of the Univese before Big Bang Nucleosynthesis, can drive a successful inflationary expansion era without the use of an inflaton field. Additionally, PBHs with masses $m \sim 10^{12}\mathrm{g}$ and abundances $0.107 < Ω^\mathrm{eq}_\mathrm{PBH}< 0.5$, slightly before matter-radiation equality, can produce a substantial amount of early dark energy, helping to alleviate the $H_0$ tension.

gr-qc↗

arXiv2025

Primordial black hole driven cosmic acceleration

We propose a natural mechanism for cosmic acceleration driven by primordial black holes (PBHs) with repulsive behavior, within a ''Swiss Cheese'' cosmological framework. Considering regular black hole spacetimes such as Hayward, Bardeen, and Dymnikova-as well as the singular Schwarzschild-de Sitter case-we consistently find a robust PBH-driven cosmic acceleration phase. This phase ends either at an energy scale set by the PBH parameters or through black hole evaporation. Notably, one finds that ultra-light PBHs with $m < 5 \times 10^8 \, {\rm g}$ can trigger exponential inflation with graceful exit and reheating. Additionally, PBHs with $m \sim 10^{12} \, {\rm g}$ and abundances $0.107 < Ω_{\rm PBH}^{\rm eq} < 0.5$ near matter-radiation equality can act as an early dark energy component, offering a potential resolution to the Hubble tension.

gr-qc↗

arXiv2025

Is inflationary magnetogenesis sensitive to the post-inflationary history ?

Considering inflationary magnetogenesis induced by time-dependent kinetic and axial couplings of a massless Abelian vector boson field breaking the conformal invariance we show in this article that, surprisingly, the spectral shape of the large-scale primordial magnetic field power spectrum is insensitive to the post-inflationary history, namely the barotropic parameter ($w$) and the gauge coupling functions of the post-inflationary era.

astro-ph.CO↗

arXiv2025

Vector induced Gravitational Waves sourced by Primordial Magnetic Fields

In this work, we develop a generic formalism for the study of tensor perturbations induced at second order by first-order vector metric perturbations, dubbing these induced tensor modes $\textit{vector-induced gravitational waves}$ (VIGWs). Notably, considering an inflation-inspired power-law type magnetic field power spectrum of the form $P_B(k)\propto k^{n_\mathrm{B}}$ (where $n_{\rm B}$ is the magnetic spectral index), we show that the VIGW signal is enhanced for stiff post-inflationary EoS, with the maximum enhancement happening for $w=1$. We explicitly demonstrate this contribution is dominant over the first-order magnetically-sourced GWs. The VIGW spectrum exhibits a maximum at around the scale crossing the cosmological horizon at the end of reheating, $k_\mathrm{reh}$, with its present day peak amplitude scaling as $Ω_{\rm GW}(k_{\rm reh},η_0)\propto ΔN_{\rm reh}\times(H_{\rm inf}/M_{\rm Pl})^{8}$, where $H_{\rm inf}$ is the Hubble parameter at the end of inflation and $ΔN_{\rm reh}$ the duration of the post-inflationary era in $e$-folds. For $w=1$ (kination) and $n_{\rm B}>-3/2$, one further obtains a nearly $n_{\rm B}$-independent frequency scaling of the GW spectrum of the form $Ω_{\rm GW}(f,η_0)\propto \left(\frac{f}{f_{\rm reh}}\right)^{-2.8}$ for $f>f_\mathrm{reh}\equiv k_\mathrm{reh}/(2π)$. Finally, we highlight that the VIGW signal can be well within the detection bands of several next-generation interferometric GW missions at small scales. Indicatively, for $H_{\rm inf} \sim O(10^{7})\:\mathrm{GeV}$ and $O(10^{14})\:\mathrm{GeV}$, and $ΔN_{\rm reh} \sim 15$ and $10$, the VIGW signal is expected to be detectable by LISA and ET respectively.

astro-ph.CO↗

arXiv2025

Reconstructing Primordial Curvature Perturbations via Scalar-Induced Gravitational Waves with LISA

Many early universe scenarios predict an enhancement of scalar perturbations at scales currently unconstrained by cosmological probes. These perturbations source gravitational waves (GWs) at second order in perturbation theory, leading to a scalar-induced gravitational wave (SIGW) background. The LISA detector, sensitive to mHz GWs, will be able to constrain curvature perturbations in a new window corresponding to scales $k \in [10^{10}, 10^{14}] \,{\rm Mpc}^{-1}$, difficult to probe otherwise. In this work, we forecast the capabilities of LISA to constrain the source of SIGWs using different approaches: i) agnostic, where the spectrum of curvature perturbations is binned in frequency space; ii) template-based, modeling the curvature power spectrum based on motivated classes of models; iii) ab initio, starting from first-principles model of inflation featuring an ultra-slow roll phase. We compare the strengths and weaknesses of each approach. We also discuss the impact on the SIGW spectrum of non-standard thermal histories affecting the kernels of SIGW emission and non-Gaussianity in the statistics of the curvature perturbations. Finally, we propose simple tests to assess whether the signal is compatible with the SIGW hypothesis. The pipeline used is built into the SIGWAY code.

astro-ph.CO↗

arXiv2025

Gravitational wave signatures of non-singular matter bouncing cosmology in NANOGrav and beyond

Non-singular matter bouncing cosmological setups are of particular interest since apart from adressing the initial singularity problem they can give rise as well to a nearly scale-invariant curvature power spectrum on scales $k<10^{4}\mathrm{Mpc}^{-1}$, favored by Cosmic Microwave Background (CMB) experiments. Interestingly enough, one can find that within such non-singular bouncing cosmological setups, curvature perturbations grow on super-horizon scales during the matter contracting phase. In this work, we account for the evolution of cosmological perturbations during the transition to the Hot Big Bang expanding Universe, finding at the end naturally enhanced curvature perturbations on very small scales at horizon-crossing time during the expanding phase. These enhanced cosmological perturbations can induce gravitational waves (GWs) due to second order gravitational interactions and collapse as well to form primordial black holes (PBHs), with the latter acting as one of the most viable dark matter candidates. Remarkably, we find an induced GW background with a universal infrared (IR) frequency scaling of $f^2$, in excellent agreement with the recently released $\mathrm{nHz}$ GW data by the NANOGrav collaboration, being potentially detectable as well by other GW observatories such as LISA and ET, depending on the values of the bouncing cosmological parameters at hand.

gr-qc↗

arXiv2024

Gravitational waves from primordial black hole isocurvature: the effect of non-Gaussianities

Ultra-light primordial black holes (PBHs) with masses $M_{\rm PBH}<5\times 10^8\mathrm{g}$ can dominate transiently the energy budget of the Universe and reheat the Universe through their evaporation taking place before Big Bang Nucleosynthesis. The isocurvature energy density fluctuations associated to the inhomogeneous distribution of a population of such PBHs can induce an abundant production of GWs due to second-order gravitational effects. In this work, we discuss the effect of primordial non-Gaussianity on the clustering properties of PBHs and study the effect of a clustered PBH population on the spectral shape of the aforementioned induced GW signal. In particular, focusing on local-type non-Gaussianity we find a double-peaked GW signal with the amplitude of the low-frequency peak being proportional to the square of the non-Gaussian parameter $τ_\mathrm{NL}$. Remarkably, depending on the PBH mass $M_{\rm PBH}$ and the initial abundance of PBHs at formation time, i.e. $Ω_\mathrm{PBH,f}$, this double-peaked GW signal can lie well within the frequency bands of forthcoming GW detectors, namely LISA, ET, SKA and BBO, hence rendering this signal falsifiable by GW experiments and promoting it as a novel portal probing the primordial non-Gaussianity.

astro-ph.CO↗

arXiv2024

Gravitational wave signatures from reheating in Chern-Simons running-vacuum cosmology

Within the context of a Chern-Simons running-vacuum-model (RVM) cosmology, one expects an early-matter dominated (eMD) reheating period after RVM inflation driven by the axion field. Treating thus in this work Chern-Simons RVM cosmology as an effective $f(R)$ gravity theory characterized by logarithmic corrections of the spacetime curvature, we study the gravitational-wave (GW) signal induced by the nearly-scale invariant inflationary adiabatic curvature perturbations during the transition from the eMD era driven by the axion to the late radiation-dominated era. Remarkably, by accounting for the extra GW scalaron polarization present within $f(R)$ gravity theories, we find regions in the parameter space of the theory where one is met with a distinctive induced GW signal with a universal $f^6$ high-frequency scaling compared to the $f^7$ scaling present in general relativity (GR). Interestingly enough, for axion masses $m_a$ higher than 1 GeV and axion gauge couplings $f_a$ above $10^{-3}$ Planck mass, one can produce induced GW spectra within the sensitivity bands of future GW observatories such as the Einstein Telescope (ET), the Laser Interferometer Space Antenna (LISA), the Big Bang Observer (BBO) and the Square Kilometer Arrays (SKA).

gr-qc↗

arXiv2024

New probe of non-Gaussianities with primordial black hole induced gravitational waves

We propose a new probe of primordial non-Gaussianities (NGs) through the observation of gravitational waves (GWs) induced by ultra-light ($M_{\text{PBH}}< 10^{9}\rm{g}$) primordial black holes (PBHs). Interestingly enough, the existence of primordial NG can leave imprints on the clustering properties of PBHs and the spectral shape of induced GW signals. Focusing on a scale-dependent local-type NG, we identify a distinct double-peaked GW energy spectrum that, contingent upon $M_{\text{PBH}}$ and the abundance of PBHs at the time of formation, denoted as $Ω_\mathrm{PBH,f}$, may fall into the frequency bands of upcoming GW observatories, including LISA, ET, SKA, and BBO. Thus, such a signal can serve as a novel portal for probing primordial NGs. Intriguingly, combining BBN bounds on the GW amplitude, we find for the first time the joint limit on the product of the effective non-linearity parameter for the primordial tri-spectrum, denoted by $\barτ_\mathrm{NL}$, and the primordial curvature perturbation power spectrum $\mathcal{P}_{\cal R}(k)$, which reads as $\barτ_\mathrm{NL} \mathcal{P}_{\cal R}(k) < 4\times 10^{-20} Ω^{-17/9}_\mathrm{PBH,f} \left( \frac{M_{\rm PBH}}{10^4\mathrm{g}} \right)^{-17/9}$.

astro-ph.CO↗

arXiv2024

Gravitational-wave signatures of gravito-electromagnetic couplings

Gravitational waves (GWs) can undoubtedly serve as a messenger from the early Universe acting as well as a novel probe of the underlying gravity theory. In this work, motivated by one-loop vacuum-polarization effects on curved spacetime, we investigate a gravitational theory with non-minimal curvature-electromagnetic coupling terms of the form $ξR F_{μν}F^{μν}$, where $R$ is the scalar curvature and $F_{μν}$ the Faraday tensor, which can be responsible for the generation of primordial electromagnetic fields. We study then the GW signatures of such coupling terms deriving in particular for the first time to the best of our knowledge the modified tensor modes equation of motion. Remarkably, we find a universal infrared (IR) frequency scaling $f^5$ of the electromagnetically induced GW (EMIGW) signal, which, depending on the energy scale of inflation, the duration of inflation and reheating as well as the dynamical behaviour of the gauge coupling function $ξ$, can be well within the detection sensitivity bands of GW experiments such as SKA, LISA, ET and BBO, being thus potentially detectable in the future by GW observatories.

astro-ph.CO↗

arXiv2024

Observable Signatures of No-Scale Supergravity in NANOGrav

In light of NANOGrav data we provide for the first time possible observational signatures of Superstring theory. Firstly, we work with inflection-point inflationary potentials naturally realised within Wess-Zumino type no-scale Supergravity, which give rise to the formation of microscopic primordial black holes (PBHs) triggering an early matter-dominated era (eMD) and evaporating before Big Bang Nucleosythesis (BBN). Remarkably, we obtain an abundant production of primordial gravitational waves (PGW) at the frequency ranges of nHz, Hz and kHz and in strong agreement with Pulsar Time Array (PTA) GW data. This PGW background could serve as a compelling observational signature for the presence of quantum gravity via no-scale Supergravity.

hep-th↗