arXiv subjects

V. K Oikonomou

Publications and source records attributed to V. K Oikonomou.

3 recordsLinked to original sources

arXiv2026

Apparent Phantom Crossing in Gauss-Bonnet Gravity

The recent observations of the Dark Energy Spectroscopic Instrument (DESI) indicated the possibility that the dark energy equation of state parameter $w$ might change from $w<-1$ to $w>-1$ when the redshift $z\sim 0.5$, which is called the inverse phantom crossing. In this paper, we investigate the possibility of the phantom crossing, and we construct realistic models realizing the crossing in the framework of the scalar--Einstein--Gauss-Bonnet gravity and ghost-free $f(\mathcal{G})$ gravity. We also investigate the scenario of the apparent phantom crossing, where dark matter energy density decreases more slowly than usually expected, which might explain the DESI observations. In the scenarios developed, the energy conditions are not violated by any component of the cosmic fluid. In the framework of the apparent phantom crossing, we also propose a new scenario, where the particle corresponding to the scalar field in the scalar--Einstein--Gauss-Bonnet gravity is dark matter. The mass of the particle might increase due to the coupling with the Gauss-Bonnet invariant, which makes the decrease of the dark matter energy density slower. This last scenario may suggest that the inverse phantom crossing might be related to the transition from the decelerating expansion of the Universe to the accelerating expansion.

gr-qc↗

arXiv2024

Low-scale Mirror Standard Model Dark Matter and its Detection via Gravitational Waves and the Guitar Nebula

What if the dark matter Sector is truly dark, self-interacting and unreachable by terrestrial experiments? How could we find hints of such dark sector if it is experimentally unreachable by any terrestrial experiment? We study a low-scale mirror Standard Model which can act as a model for dark matter, which interacts only gravitationally with the Standard Model particles. The mirror Standard Model sector particles are stable particles can comprise a measurable part of the dark matter of the Universe. These mirror Standard Model particles acquire mass through a low-temperature dark first order phase transition. We examine in detail this dark phase transition and we indicate how stochastic gravitational waves can be generated through this transition. For the model we use, the produced energy spectrum of the gravitational waves can be detected by the Square Kilometer Array. Moreover, we propose a possible way to detect effects of the particle nature of dark matter, using observational data coming from the guitar nebula, which can work if dark matter is collisional, so interacting dark matter. Without specifying a model for interacting dark matter, thus choosing an agnostic approach for interacting dark matter, we assume that the guitar nebula bow shock is generated by the interaction of the high speed neutron star that passes through the interstellar medium, which is assumed to be comprised by interacting dark matter and hydrogen. Our main proposal is that the opening angle of the bow shock can be directly related to the speed of sound of the dark matter particles, and a large angle of the bow shock could be a strong indicator that the interstellar medium is comprised by collisional dark matter and hydrogen gas. This is motivated by the Bosma effect.

gr-qc↗

arXiv2020

Unification of a Bounce with a Viable Dark Energy Era in Gauss-Bonnet Gravity

In this work we shall demonstrate that it is possible to describe in a unified way a primordial bounce with the dark energy era, in the context of Gauss-Bonnet modified gravity. Particularly, the early time bounce has a nearly scale invariant power spectrum of primordial scalar curvature perturbations, while the dark energy era is a viable one, meaning that it mimics the $Λ$-Cold-Dark-Matter model and also is compatible with the Planck 2018 data on cosmological parameters. In addition, our analysis indicates that the dark energy era is free from dark energy oscillations, which occur in the context of $f(R)$ gravity. We further addressed the later issue by examining $f(R)$ extensions of Gauss-Bonnet models, and we showed that the $f(R)$ gravity part of the action actually produces the dark energy oscillations at redshifts $z\sim 4$.

gr-qc↗