arXiv subjects

Stylianos A. Tsilioukas

Publications and source records attributed to Stylianos A. Tsilioukas.

7 recordsLinked to original sources

arXiv2026

Topological dark energy from spacetime foam: A challenge for $Λ$CDM

Using only the standard considerations of spacetime foam and the Euclidean Quantum Gravity techniques known long ago, we result to a model of Topological Dark Energy (TDE) that competes the standard ΛCDM paradigm with regard to data fitting efficiency, while providing a microphysical mechanism for Dark Energy. Specifically, it is known that at the foam level, topologically non-trivial solutions such as instantons appear. In the particular case of Einstein-Gauss-Bonnet gravity, we obtain an effective dynamical Dark Energy term proportional to the instanton density, and the latter can be easily calculated through standard techniques. Hence, we can immediately extract the differential equation that determines the evolution of the topologically induced effective dark energy density. Significantly, this TDE scenario allows for changing sign of Dark Energy during the cosmic evolution and also exhibits Dark Energy interaction with Dark Matter. We confront the TDE scenario, in both flat and non-flat cases, with Pantheon+ Supernovae Type Ia (SNIa), Baryonic Acoustic Oscillations (BAO), and Cosmic Chronometers (CC) datasets. By applying standard model selection methods, we find the TDE scenario to be statistically equivalent with ΛCDM. Finally, we show that the TDE scenario passes constraints from Big Bang Nucleosynthesis (BBN) and thus does not spoil the thermal history of the Universe.

gr-qc↗

arXiv2026

Reconstructing the generalized Barrow holographic dark energy with physics-informed neural networks

Barrow holographic dark energy connects cosmic acceleration with possible quantum-gravitational deformations of horizon entropy, encoded in the Barrow exponent $Δ$. If such effects are scale dependent, however, there is no fundamental reason for $Δ$ to remain constant throughout cosmic history. In this work we reconstruct $Δ(z)$ directly from observations, without assuming any particular functional form, using the Cosmo-PINN physics-informed neural-network framework. The generalized Barrow holographic evolution equation is incorporated into the training, while PantheonPlus supernovae, DESI DR2 baryon acoustic oscillations and cosmic chronometers constrain the reconstruction. We find a mild and smooth redshift evolution, with the posterior mean favoring negative $Δ$ and this tendency becoming stronger when the Cepheid calibration is included. Nevertheless, $Δ=0$ and constant negative values remain compatible with current uncertainties. The reconstructed cosmology yields a viable late-time evolution, with $w_{\rm DE}$ close to $-1$ and the expected transition to accelerated expansion. Our results demonstrate that cosmological observations can directly probe the functional behavior of a quantity entering the underlying entropy law itself.

physics.gen-ph↗

arXiv2026

Early- and late-time constraints on Wald-Gauss-Bonnet topological dark energy and implications for the $H_0$ and $S_8$ tensions

The persistent $H_0$ and $S_8$ tensions motivate the search for new dark-energy mechanisms capable of modifying the late-time expansion history while preserving the successful early-Universe predictions of $Λ$CDM scenario. Wald-Gauss-Bonnet (WGB) topological dark energy provides a physically motivated realization of this possibility, where the effective dark-energy sector emerges from cosmic horizon thermodynamics and the black-hole formation and merger history. We present the first early- and late-Universe analysis of WGB cosmology, implementing the model as an effective fluid in a modified \texttt{CLASS} solver and constraining it against CMB data from \textit{Planck}, ACT~DR6 and SPT-3G, DESI~DR2 BAO, and Pantheon+ supernovae. While late-time data alone are consistent with $Λ$CDM, the full dataset prefers a non-zero WGB contribution, $\Cn=0.435^{+0.150}_{-0.132}$, corresponding to a $\sim3σ$ phantom-like deviation and an improved fit. The preferred solution raises $H_0$ from $68.5$ to $69.8~\kmsmpc$, reducing the Hubble tension by $\sim0.9σ$, at the cost of a mild increase in $S_8$. The reconstructed cosmological observables show that WGB leaves the primary CMB almost unchanged while enhancing lensing and small-scale clustering, revealing a characteristic $H_0$-$S_8$ trade-off. WGB dark energy therefore emerges as a physically motivated and observationally viable late-time mechanism for partially alleviating the Hubble tension without introducing new early-Universe physics.

gr-qc↗

arXiv2026

Observational implications of Wald-Gauss-Bonnet topological dark energy

We investigate the observational implications of Wald--Gauss--Bonnet (WGB) topological dark energy, a modified cosmological framework derived from the gravity-thermodynamics conjecture applied to the Universe's apparent horizon, with the Wald--Gauss--Bonnet entropy replacing the standard Bekenstein--Hawking one. Assuming a topological connection between the apparent horizon and interior black hole (BH) horizons, we derive modified Friedmann equations where the evolution of dark energy depends on BH formation and merger rates, which are approximated by the cosmic star formation rate. These equations introduce an additional, astrophysics--dependent contribution to the cosmological constant. We test two scenarios, one with a vanishing cosmological constant ($Λ= 0$) and another with a modified $Λ$ against late--Universe data (SNIa, BAO, Cosmic Chronometers) via a Bayesian analysis. Although the WGB framework is consistent with observations, information criteria statistically favor the standard $Λ$CDM model. An analysis of linear perturbations shows that the growth of cosmic structures is nearly indistinguishable from that of $Λ$CDM, with negligible dark energy clustering and minimal deviation in the effective Newton's constant. The standard thermal history is also preserved. In conclusion, WGB cosmology presents a phenomenologically rich alternative that connects dark energy to black hole astrophysics while remaining compatible with current cosmological data.

gr-qc↗

arXiv2026

Building an inertia dynamometer with vocational students: a low-budget apparatus for teaching rotational dynamics

We report the design, construction, and classroom use of a low-cost inertia dynamometer, built as a year-long project-based learning (PBL) activity with adult students at a Greek Evening Vocational High School (EPAL). The apparatus consists of a machined steel drum of calculated moment of inertia $I = 0.6507~\mathrm{kg\,m^2}$, mounted on a student-welded frame and instrumented with a green-laser / light-dependent resistor (LDR) optical interrupter. The analogue output is sampled at 44.1\,kHz by the microphone input of a laptop computer, which is used as an opportunistic analogue-to-digital converter; torque and power curves are then reconstructed in software from the inter-pulse intervals via $τ= Iα$ and $P = τω$. The drum's moment of inertia is cross-checked by an inclined-plane rolling experiment. A wide-open-throttle test with a 50\,cc scooter reproduces the expected flat-power / falling-torque signature of a continuously variable transmission in the low-to-moderate RPM range; the LDR's millisecond-scale recovery time imposes an upper bandwidth limit that provides an unplanned but pedagogically rich lesson in sensor physics. The project integrated industrial-lathe fabrication, arc welding, analogue electronics, and numerical differentiation into a single coherent workflow. We describe the apparatus, the physics, the signal-processing pipeline (for which MATLAB and Python/Octave code are provided as supplementary material), and reflect on the pedagogical outcomes for a student population traditionally disengaged from abstract physics.

physics.ed-ph↗

arXiv2025

Topological dark energy from black-hole formations and mergers through the gravity-thermodynamics approach

We apply the gravity-thermodynamics approach in the case of Einstein-Gauss-Bonnet theory, and its corresponding Wald-Gauss-Bonnet entropy, which due to the Chern-Gauss-Bonnet theorem it is related to the Euler characteristic of the Universe topology. However, we consider the realistic scenario where we have the formation and merger of black holes that lead to topology changes, which induce entropy changes in the Universe horizon. We extract the modified Friedmann equations and we obtain an effective dark energy sector of topological origin. We estimate the black-hole formation and merger rates starting from the observed star formation rate per redshift, which is parametrized very efficiently by the Madau-Dickinson form, and finally we result to a dark-energy energy density that depends on the cosmic star formation rate density, on the fraction $f_{\text{BH}}$ of stars forming black holes, on the fraction of black holes $f_\text{merge}$ that eventually merge, on the fraction $ f_{\text{bin}}$ of massive stars that are in binaries, on the average mass of progenitor stars that will evolve to form black holes $ \langle m_{\text{prog}} \rangle $, as well as on the Gauss-Bonnet coupling constant. We investigate in detail the cosmological evolution, obtaining the usual thermal history. Concerning the dark-energy equation-of-state parameter, we show that at intermediate redshifts it exhibits phantom-like or quintessence-like behavior according to the sign of the Gauss-Bonnet coupling, while at early and late times it tends to the cosmological constant value. Finally, we study the effect of the other model parameters, showing that for the whole allowed observationally estimated ranges, the topological dark-energy equation-of-state parameter remains within its observational bounds.

gr-qc↗

arXiv2024

Dark energy from topology change induced by microscopic Gauss-Bonnet wormholes

It is known that the appearance of microscopic objects with distinct topologies and different Euler characteristics, such as instatons and wormholes, at the spacetime-foam level in Euclidean quantum gravity approaches, leads to spacetime topology changes. Such changes, in principle, may affect the field equations that arise through the semiclassical variation procedure of gravitational actions. Although in the case of Einstein-Hilbert action the presence of microscopic wormholes does not lead to any non-trivial result, when the Gauss-Bonnet term is added in the gravitational action, the above effective topological variation procedure induces an effective cosmological constant that depends on the Gauss-Bonnet coupling and the wormhole density. Since the later in a dynamical spacetime is in general time-dependent, one obtains an effective dark energy sector of topological origin.

gr-qc↗