arXiv subjects

Marios Kalomenopoulos

Publications and source records attributed to Marios Kalomenopoulos.

11 recordsLinked to original sources

arXiv2026

Not all those missing are lost: leveraging galaxy clustering in incomplete catalogs to unleash dark sirens cosmology

Gravitational waves offer a unique opportunity to solve the Hubble tension. In order to do so, we have to extract as much information as possible from cross correlating gravitational wave events (used as "dark sirens") with incomplete galaxy catalogs. Traditional methods assume a uniform in comoving volume distribution for galaxies missing from the catalogs, which neglects the fact that galaxies tend to cluster, leading to less precise and possibly biased posteriors. In this paper, we introduce a new method for accounting for galaxy clustering when dealing with an incomplete galaxy catalog, by adding back galaxies to the incomplete catalog and distributing them in pixels according to the correlation function $\xi(r)$. We find that our method drastically improves on traditional ones when the galaxy catalog is only $1\%-10\%$ complete, leading to posteriors that are between 2 and 4 times as precise, depending on the completeness fraction, without introducing any biases. Our method produces results comparable to traditional methods at extremely low catalog completeness fractions (<0.5\%) or for very high uncertainties in the recovered luminosity distance and sky localization of the gravitational wave events.

astro-ph.CO↗

arXiv2026

Cosmological Constraints from Gravitational Wave-Fast Radio Burst Associations without Redshift Measurements for LIGO-Virgo and Cosmic Explorer

The potential association between gravitational waves (GWs) and fast radio bursts (FRBs) offers a unique multi-messenger probe for cosmology. In this paper, we develop a redshift-independent framework to constrain cosmological parameters using the luminosity distance - dispersion measure relation, accounting for realistic astrophysical uncertainties. We perform a comprehensive comparative analysis across different GWs detector sensitivities and modeling assumptions. Specifically, we investigate the performance of the current LIGO-Virgo (LV) network (at $z < 0.2$) versus the future Cosmic Explorer (CE). Our study further evaluates the impact of different dispersion measure (DM) distributions---specifically the corrected Macquart's PDF (Zhuge+2025) and the log-normal distribution---and explores the influence of including or excluding host galaxy DM contributions. Using realistic simulated observations, we find that while the current LV network lacks the precision to provide meaningful constraints, CE will enable high-precision cosmology. Even without spectroscopic redshifts, CE observations can effectively break parameter degeneracies and robustly constrain both cosmology and host galaxy parameters. These results highlight the necessity of next-generation detectors.

astro-ph.CO↗

arXiv2025

Pad\'e Approximants for cosmic Dispersion Measures

Fast Radio Bursts (FRBs) have become an indispensable tool for studying the ``missing baryons'', the Universe's ionisation properties, as well as the cosmological parameters. This is achieved by analysing the diffuse dispersion measure (${\rm DM}_{\rm diff}$) of FRBs as a function of redshift. However, the rapidly increasing data size requests more and more computational resources. In this work, we first develop an accelerated method for any cosmic dispersion measure by deriving an analytical approximation formula for flat, $\Lambda$CDM and $w$CDM universes. Focusing on FRBs, we show that our approximation works well for the ranges $0.01 \leq z \leq 2$, $0.2 \leq \Omega_m \leq 1.0$ and $-3.0 \leq w \leq -0.5$, with relative error to a numerically evaluated ${\rm DM}_{\rm diff}$ always smaller than $3.5 \%$ (in the worst case scenario). This error remains below observationally relevant ${\rm DM}$ scatter and is especially small near the concordance $\Lambda$CDM cosmology. Additionally, we perform a cosmological analysis of simulated FRB data and show that our approximation gives robust and unbiased results, even when applied in regions of parameter space where its relative error becomes larger than $3\%$. Finally, the approximation is more than $15$ ($2$) times faster than the numerical solution of $\Lambda$CDM ($w$CDM), and can reach a timing improvement of a factor of $25$ when used in an MCMC cosmological inference.

astro-ph.HE↗

arXiv2025

Clustering effects on the Dark Siren determination of $H_0$: A simulation study

Gravitational waves (GWs) offer an alternative way to measure the Hubble parameter. The optimal technique, the ``bright siren'' approach, requires the identification of an electromagnetic counterpart. However, a significant fraction of gravitational waves signals will not have counterparts. Such events can still constrain the Hubble parameter $H_0$ via statistical methods, exploiting galaxy information from the GWs sky localisation volume. In this work, we investigate the power of this method using high-resolution, cosmological simulations that include realistic clustering. We find that clustering leads to increased convergence of the $H_0$ posteriors, with clear recovery of the input value as early as $N_{\rm gw}=40$ events, compared to uniform catalogues, where the posterior remains largely unconstrained, even with $N_{\rm gw}=100$ events. In addition, we quantify the role of catalogue incompleteness. We show that catalogues with completeness levels as low as $f=25\%$ can be competitive with fully complete catalogues, confirming the impact of clustering. Completeness levels of $f=50\%$ perform statistically similar to complete catalogues with as few as $N_{\rm gw}=40$ events. This indicates the need to focus on improving gravitational waves detection capabilities, rather than obtaining more complete galaxy catalogues. Finally, we investigate additional properties of the method by taking into consideration physical weights, different observational errors, potential biases from the $H_0$ priors, a variety of detectors' horizon distances, and different methods of catalogue completion and statistical analysis.

astro-ph.CO↗

arXiv2025

An analysis and cosmological study of the propagation of gravitational radiation under a graviton of nonzero mass

Under the assumptions of General Relativity (GR), gravitational waves propagate at the speed of light and their mediation can be represented as a particle through a massless graviton. We investigate the impact and observability of the presence of a massive graviton, how such a modification to GR would also modify the propagation of observed gravitational waves from astrophysical sources, and how this effect can be used as an independent measurement of cosmological parameters, focusing on the Hubble parameter $H_0$ and matter energy $\Omega_m$. We simulate the impact of a massive graviton on compact binary coalescence observations in a near-future LIGO-Virgo-KAGRA interferometer network through a modification to the gravitational wave phase in the post-Newtonian framework. Our analysis finds that if we assume the presence of a graviton with a Compton wavelength of $\lambda_G \approx 5 \times 10^{16}$m, corresponding to a mass $m_G \leq 2.3 \times 10^{-23}$eV/c$^2$, we can utilize a simulated population of 60 binary black hole observations to constrain $H_0$ to a similar precision as current gravitational wave constraints without electromagnetic counterparts (at $90\%$ credible intervals): $H_0 = 58^{+34}_{-19}\,\mathrm{km\; s^{-1}\; Mpc^{-1}}$ and $\Omega_m=0.29^{+0.10}_{-0.08}$. More sensitive observatories will be necessary to probe lower values in the graviton mass range and fully exploit this method.

astro-ph.CO↗

arXiv2025

Evidence of the pair instability gap from black hole masses

Stellar theory predicts a forbidden range of black-hole masses between ${\sim}50$--$130\,M_\odot$ due to pair-instability supernovae, but evidence for such a gap in the mass distribution from gravitational-wave astronomy has proved elusive. Early hints of a cutoff in black-hole masses at ${\sim} 45\,M_\odot$ disappeared with the subsequent discovery of more massive binary black holes. Here, we report evidence of the pair-instability gap in LIGO--Virgo--KAGRA's fourth gravitational wave transient catalog (GWTC-4), with a lower boundary of $44_{-4}^{+5} M_\odot$ (90\% credibility). While the gap is not present in the distribution of \textit{primary} masses $m_1$ (the bigger of the two black holes in a binary system), it appears unambiguously in the distribution of \textit{secondary} masses $m_2$, where $m_2 \leq m_1$. The location of the gap lines up well with a previously identified transition in the binary black-hole spin distribution; binaries with primary components in the gap tend to spin more rapidly than those below the gap. We interpret these findings as evidence for a subpopulation of hierarchical mergers: binaries where the primary component is the product of a previous black-hole merger and thus populates the gap. Our measurement of the location of the pair-instability gap constrains the $S$-factor for $^{12}\rm{C}(\alpha,\gamma)^{16}\rm{O}$ at 300keV to $260_{-108}^{+190}$ keV barns.

astro-ph.HE↗

arXiv2025

Hubble constant constraint using 117 FRBs with a more accurate probability density function for ${\rm DM}_{\rm diff}$

Fast radio bursts (FRBs) are among the most mysterious astronomical transients. Due to their short durations and cosmological distances, their dispersion measure (DM) - redshift ($z$) relation is useful for constraining cosmological parameters and detecting the baryons in the Universe. The increasing number of localized FRBs in recent years has provided more precise constraints on these parameters. However, the larger dataset reveals limitations in the widely used probability density function ($p_{\rm diff}$) for ${\rm DM}_{\rm diff}$, which refers to the diffuse electron term of FRB DM. In this project, we collect 117 of the latest, localized FRBs, discuss the effect of a more accurate $\sigma_{\rm diff}$, which is a parameter in $p_{\rm diff}$ and once thoughts as ``effective standard deviation'', and more clearly rewrite their likelihood to better constrain the parameters above. We find that the widely used approximation $\sigma_{\rm diff} \sim F/\sqrt{z}$ only works under contrived assumptions and shows the greatest deviation from the true standard deviation in low redshift. In general, one should use an accurate method to derive this parameter from $p_{\rm diff}$. Our method yields better constraints on $H_0\Omega_b f_{\rm diff} = 2.813_{-0.258}^{+0.250}\;{\rm km/s/Mpc}$ or $H_0 = 66.889_{-5.459}^{+6.754} \;{\rm km/s/Mpc}$ when combining the FRB data with CMB measurements and taking $f_{\rm diff} = 0.84$. This fully analytical correction helps us better constrain cosmological parameters with the increasing number of localized FRBs available today.

astro-ph.CO↗

arXiv2025

Hawking Radiation from non-evaporating primordial black holes cannot enable the formation of direct collapse black holes

The formation of supermassive black holes (SMBHs) in the early Universe is a subject of significant debate. In this study, we examine whether non-evaporating primordial black holes (PBHs) can offer a solution. We establish initial constraints on the range of PBH masses that correspond to Hawking radiation (HR) effective temperatures in the range needed to suppress $H_2$ cooling, which would facilitate the formation of massive black hole seeds in atomic cooling halos. We also investigate the specific intensity of the HR from non-evaporating PBHs and compare it with the critical radiation needed for direct collapse black holes (DCBHs). We show that HR from non-evaporating PBHs cannot serve as an irradiating mechanism to facilitate the formation of the seeds for the SMBHs we observe in the high-redshift Universe unless, perhaps, the PBHs within the relevant mass range comprise a significant fraction of dark matter and are significantly clustered towards the center of the primordial halo.

astro-ph.GA↗

arXiv2021

2-minute science: A Science Communication Project for Astrophysics

Two-minute science (2-minute science) is a science communication project initiated and supported by early-career Greek astrophysicists. With this endeavor, which started in December 2020, we try to bridge the gap between the scientific community and the public. This project is based on the simple idea of writing short articles with an approximate reading time of two minutes. These articles cover several topics and their difficulty scales to cover a broad audience range, from young students to experienced adults. We support the idea of < > in Astrophysics in Greece, where any reader can pose a question. We offer the appropriate answer either by writing it ourselves or by contacting the field experts from the Greek astronomical society. Furthermore, our previous science communication experience leads us to design educational activities for students and/or adults based on pedagogical means. A successful one was an < > titled < >, a digital version of an escape room. Further activities are Astronomy workshops for teenagers, online talks to schools, and our participation in a scientific podcast to trigger the public interest in Astrophysics. We communicate this work through social media, where several thousands of people already follow our work.

physics.ed-ph↗

arXiv2021

Mapping the inhomogeneous Universe with Standard Sirens: Degeneracy between inhomogeneity and modified gravity theories

The detection of gravitational waves (GWs) and an accompanying electromagnetic (E/M) counterpart have been suggested as a future probe for cosmology and theories of gravity. In this paper, we present calculations of the luminosity distance of sources taking into account inhomogeneities in the matter distribution that are predicted in numerical simulations of structure formation. In addition, we show that inhomogeneities resulting from clustering of matter can mimic certain classes of modified gravity theories, or other effects that dampen GW amplitudes, and deviations larger than $δν\sim \mathcal{O}(0.1)\ (99\%\ \rm{C.L.})$ to the extra friction term $ν$, from zero, would be necessary to distinguish them. For these, we assume mock GWs sources, with known redshift, based on binary population synthesis models, between redshifts $z=0$ and $z=5$. We show that future GW detectors, like Einstein Telescope or Cosmic Explorer, will be needed for strong constraints on the inhomogeneity parameters and breaking the degeneracy between modified gravity effects and matter anisotropies by measuring $ν$ at $5 \%$ and $1 \%$ level with $100$ and $350$ events respectively.

astro-ph.CO↗

arXiv2016

Distribution of periodic trajectories of Anosov C-system

The hyperbolic Anosov C-systems have a countable set of everywhere dense periodic trajectories which have been recently used to generate pseudorandom numbers. The asymptotic distribution of periodic trajectories of C-systems with periods less than a given number is well known, but a deviation of this distribution from its asymptotic behaviour is less known. Using fast algorithms, we are studying the exact distribution of periodic trajectories and their deviation from asymptotic behaviour for hyperbolic C-systems which are defined on high dimensional tori and are used for Monte-Carlo simulations. A particular C-system which we consider in this article is the one which was implemented in the MIXMAX generator of pseudorandom numbers. The generator has the best combination of speed, reasonable size of the state, and availability for implementing the parallelization and is currently available generator in the ROOT and CLHEP software packages at CERN.

nlin.CD↗