arXiv subjects

Morgan Le Delliou

Publications and source records attributed to Morgan Le Delliou.

At least 19 recordsLinked to original sources

arXiv2026

Surface Density of Disk Galaxies in MOND

In this paper, we extend a paper by Milgrom (2009, MNRAS 398, 1023) dealing with the existence of a quasi-universal surface density for object of all mass and structure, if they are in the Newtonian regime, i.e., that their mean acceleration is larger than MOND typical acceleration $a_0$. This result is in agreement with Donato et al. (2009)'s results, claiming the existence of a quasi-universal surface density in all masses in galaxies. The Milgrom paper also predicts that objects with mean inner acceleration smaller than the values discussed do %es not show the quasi-universal behavior of the surface density discussed. In the present paper, we extend the result of Milgrom's paper, based on a point mass model, considering spiral galaxies, modelled with a double exponential disk. Similar to Milgrom's results, we find the existence of a universal surface density for galaxies with large surface density, and a different behavior for galaxies having small surface density.

astro-ph.CO↗

arXiv2025

Dynamical friction shear and rotation in Chaplygin cosmology

In this study, we build upon the findings of Del Popolo et al. (2013) by further analyzing the influence of dynamical friction on the evolution of cosmological perturbations within the framework of the spherical collapse model (SCM) in a Universe dominated by generalized Chaplygin gas (GCG). Specifically, we investigate how dynamical friction alters the growth rate of density perturbations, the effective sound speed, the equation-of-state parameter www, and the evolution of the cosmic expansion rate. Our results demonstrate that dynamical friction significantly delays the collapse process compared to the standard SCM. Accurate computation of these parameters is crucial for obtaining consistent results and reliable physical interpretations when employing the GCG model. Furthermore, our analysis confirms that the suppression of perturbation growth due to dynamical friction is considerably more pronounced than that caused by shear and rotation, as previously indicated by Del Popolo et al. (2013). This enhanced suppression effectively addresses the instability issues, such as oscillations or exponential divergences in the dark-matter power spectrum, highlighted in linear perturbation studies, such as those by Sandvik et al. (2004).

astro-ph.CO↗

arXiv2025

Hubble tension in an anisotropic Universe

We explore the Hubble tension within an anisotropic cosmological framework by revisiting the Bianchi type-I model introduced in Le Delliou et al. 2020. Motivated by ongoing debates surrounding back-reaction effects and observed anomalies in the cosmic microwave background (CMB), we investigate whether a departure from isotropy in the late Universe could reconcile the observed discrepancies in Hubble constant measurements. Using a Bayesian inference framework, we constrain the model parameters employing multiple nested sampling algorithms: bilby, PyMultiNest, and nessai. We perform the analysis under both uniform and Gaussian priors, allowing us to systematically assess the sensitivity of the inferred cosmological parameters to different prior assumptions. This dual-prior strategy balances agnostic parameter exploration with constraints informed by theory and observation. Our findings demonstrate the reliability of our inference pipeline across different samplers and emphasize the crucial role of prior selection in non-standard cosmological model testing. The results suggest that anisotropic models remain viable contenders in addressing current cosmological tensions: even though the present model does not show alleviation of the Hubble tension, the data points towards anisotropies. Future work may extend this methodology to more complex anisotropic scenarios and incorporate additional cosmological probes such as CMB polarization and gravitational wave standard sirens.

astro-ph.CO↗

arXiv2024

Neutron star mass in dark matter clumps

This paper investigates a hypothesis proposed in previous research relating neutron star (NS) mass and its dark matter (DM) accumulation. As DM accumulates, NS mass decreases, predicting lower NS masses toward the Galactic center. Due to limited NSs data near the galactic center, we examine NSs located within DM clumps. Using the CLUMPY code simulations, we determine the DM clumps distribution, with masses from 10 to $10^{8}$ $M_{\odot}$ and scales from $10^{-3}$ to 10 kpc. These clumps' DM exhibit a peak at the center, tapering toward the outskirts, resembling our Galaxy's DM distribution. We analyse these DM clumps' NS mass variations, considering diverse DM particle masses and galaxy types. We find relatively stable NS mass within 0.01 to 5 kpc from the clump center. This stability supports the initial hypothesis, particularly for NSs located beyond 0.01 kpc from the clump center, where NS mass reaches a plateau around 0.1 kpc. Nevertheless, NS mass near the clump's periphery reveals spatial dependence: NS position within DM clumps influences its mass in Milky Way-type galaxies. Moreover, this dependence varies with the DM model considered. In summary, our study investigates the proposed link between NS mass and DM accumulation by examining NSs within DM clumps. While NS mass remains stable at certain distances from the clump center, spatial dependencies arise near the clump's outer regions, contingent on the specific DM model.

astro-ph.GA↗

arXiv2024

On the dwarf galaxies rotation curves diversity problem

In this paper, we show how baryonic physics can solve the problem of the striking diversity in dwarf galaxies rotation curves shapes. To this aim, we compare the distribution of galaxies of the SPARC sample, in the plane $V_{\rm 2 kpc}$-$V_{\rm Rlast }$ (being $V_{\rm 2kpc}$ the galaxy rotation velocity at $2$ kpc, and $V_{\rm Rlast}$ that outermost one) with that of galaxies that we simulated taking account of baryonic effects. The scatter in the rotation curves in the $V_{\rm 2 kpc}$-$V_{\rm Rlast }$ plane, and the trend of the SPARC sample's, and our simulated galaxies', distribution is in good agreement. The solution of the "diversity" problem lies in the ability of baryonic process to produce non self-similar haloes, contrary to DM-only simulations. We show also that baryonic effects can reproduce the rotation curves of galaxies like IC2574 characterized by a slow rising with radius. A solution to the diversity problem can be obtained taking appropriately into account the baryon physics effects.

astro-ph.GA↗

arXiv2024

An Anisotropic Model for the Universe

Motivated by the back-reaction debate, and~some unexplained characteristics of the CMB, we investigate the possibility of some anisotropy in the universe observed around us. To this aim, we build up a novel prediction for the Hubble law for the late universe from a Bianchi type I model, {taken as proof of concept,} transcribing the departure of such model from a $Λ$CDM model. We~dicussed the redshift measurement in this universe, and~finally formalized the Hubble diagram.

gr-qc↗

arXiv2024

Neutron Stars and Dark Matter

Neutron stars change their structure with accumulation of dark matter. We study how their mass is influenced from the environment. Close to the sun, the dark matter accretion from the neutron star does not have any effect on it. Moving towards the galactic center, the density increase in dark matter results in increased accretion. At distances of some fraction of a parsec, the neutron star acquire enough dark matter to have its structure changed. We show that the neutron star mass decreases going towards the galactic centre, and that dark matter accumulation beyond a critical value collapses the neutron star into a black hole. Calculations cover cases varying the dark matter particle mass, self-interaction strength, and ratio between the pressure of dark matter and ordinary matter. This allow us to constrain the interaction cross section, $σ_{\rm dm}$, between nucleons and dark matter particles, as well as the dark matter self-interaction cross section.

astro-ph.CO↗

arXiv2024

New insights on null and timelike warped symmetric spacetime splittings

We explore in detail the 2+2 and 1+1+2 formalism in spherically symmetric spacetimes, spanning from deducing the dynamical equations to relating them to the well-known generalised Painlevé-Gullstrand (GPG) coordinate system. The evolution equations are the Raychaudhuri equations for null rays, including those also known as cross-focusing equations whose derivation, to the best of our knowledge, we present for the first time. We physically interpret the scalars that arise in this scenario, namely the flow 2-expansion $Θ_{n}$, the flow acceleration $\mathcal{A}$, and the radial extrinsic curvature $\mathcal{B}$. We derive a coordinate independent formula for the redshift which shows that $\mathcal{B}$ is the sole source for the redshift in spherically symmetric spacetimes. We also establish the correspondence between the 1+1+2 scalars and the 1+3 splitting scalars, expansion and shear. We further make a comparison with the Newman-Penrose formalism, in order to clarify the context where each formalism is more useful, and finally, we extend our results to planar and hyperbolic symmetric warped spacetimes as well, in particular, the relationship between $\mathcal{B}$ and the redshift.

gr-qc↗

arXiv2024

Bounds from multi-messenger astronomy on the Super Heavy Dark Matter

The purely gravitational evidence supporting the need for dark matter (DM) particles is compelling and based on Galactic to cosmological scale observations. Thus far, the promising weakly interacting massive particles scenarios have eluded detection, motivating alternative models for DM. We consider the scenarios involving the superheavy dark matter (SHDM) that potentially can be emitted by primordial black holes (PBHs) and can decay or annihilate into ultrahigh-energy (UHE) neutrinos and photons. The observation of a population of photons with energies $E\ge 10^{11}$ GeV would imply the existence of completely new physical phenomena, or shed some light on DM models. Only the ultra-high energy cosmic ray observatories have the capabilities to detect such UHE decay products via the measurements of UHE photon induced extensive air showers. Using the upper bound on the flux of UHE cosmic rays beyond $10^{11.3}$ GeV implying $J(>10^{11.3}~{\rm{GeV}})< 3.6\times 10^{-5}$ km$^{-2}$sr$^{-1}$y$^{-1}$, at the $90\%$ C.L. reported by the Pierre Auger Observatory, we obtain global limits on the lifetime of the DM particles with masses $10^{15}\le M_{X} \le 10^{17}$ GeV. The constraints derived here are new and cover a region of the parameter space not yet explored. We compare our results with the projected constraints from future POEMMA and JEM-EUSO experiments, in order to quantify the improvement that will be obtained by these missions. Moreover, assuming that an epoch of early PBHs domination introduces a unique spectral break, $f_{\ast}$, in the gravitational wave spectrum, the frequency of which is related to the SHDM mass, we map potential probes and limits of the DM particles masses on the $f_{\ast}-M_{X}$ parameter space.

astro-ph.HE↗

arXiv2023

Constraints on the Local Cosmic Void from the Pantheon Supernovae Data

In principle, the local cosmic void can be simply modeled by the spherically symmetric Lemaitre-Tolman-Bondi (LTB) metric. In practice, the real local cosmic void is probably not spherically symmetric. In this paper, to reconstruct a more realistic profile of the local cosmic void, we divide it into several segments. Each segment with certain solid angle is modeled by its own LTB metric. Meanwhile, we divide the 1048 type Ia supernovae (SNIa) of the Pantheon Survey into corresponding subsets according to their distribution in the galactic coordinate system. Obviously, each SNIa subset can only be used to reconstruct the profile of one segment. Finally, we can patch together an irregular profile for the local cosmic void with the whole Pantheon sample. Note that, the paucity of each data subset lead us to focus on the inner part of each void segment and assume that the half radii of the void segments are sufficient to constrain the whole segment. We find that, despite $2σ$ signals of anisotropy limited to the depth of the void segments, the constraints on every void segment are consistent with $Λ$CDM model at $95\%$ CL. Moreover, our constraints are too weak to challenge the cosmic homogeneity and isotropy.

astro-ph.CO↗

arXiv2023

Solution to the hyperon puzzle using dark matter

In this paper, we studied the ``hyperon puzzle", a problem that nevertheless the large number of studies is still an open problem. The solution of this issue requires one or more mechanisms that could eventually provide the additional repulsion needed to make the EoS stiffer and, therefore, the value of $M_{\rm{max}, T}$ compatible with the current observational limits. In this paper we proposed that including dark matter (DM) admixed with ordinary matter in neutron stars (NSs), change the hydrostatic equilibrium and may explain the observed discrepancies, regardless to hyperon multi-body interactions, which seem to be unavoidable. We have studied how non-self-annihilating, and self-interacting, DM admixed with ordinary matter in NSs changes their inner structure, and discussed the mass-radius relations of such NSs. We considered DM particle masses of 1, 10, and 100 GeV, while taking into account a rich list of the DM interacting strengths, $y$. By analyzing the multidimensional parameter space, including several quantities like: a. the DM interacting strength, b. the DM particle mass as well as the quantity of DM in its interior, and c. the DM fraction, ${\rm f}_{DM}$, we put constraints in the parameter space ${\rm f}_{DM} - p^{\prime}_{\rm DM}/p^{\prime}_{\rm OM}$. Our bounds are sensitive to the recently observed NSs total masses.

gr-qc↗

arXiv2022

Non self-similar Luminosity-temperature relation and dynamical friction

Extending the results of a previous paper \citep{DelPopolo2005}, by taking into account the role of dynamical friction, we recovered the luminosity-temperature relation (LTR). While by assuming self-similarity, a scaling law in which $L\propto T^2$ is obtained, observations show that the relation between luminosity and temperature is steeper, ${L \propto T^ {\simeq 3}}$. This difference can be explained in terms of energy input by non-gravitational processes, like pre-heating, supernovae feedback, and heating from AGN. In this paper, we studied the LTR by means of a modified version of the punctuated equilibria model \citep{Cavaliere1999}, taking into account in addition dynamical friction, thus extending the approach found in \citep{DelPopolo2005}. The result is a non-self-similar LTR with a bend at $\simeq 2$ keV, with a slope $2.76 \pm 0.18$ at larger energies and $3.4 \pm 0.18$ at energies smaller than 2 keV. This result is in agreement with the XXL survey \citep{Giles2016}. Moreover the steeper slopes at smaller energies is in agreement with some studies claiming a further steepening of the LTR at the low mass end. We also compared the results of our model with the 400d groups sample, finding that in groups the slope is slightly steeper than in clusters, %and namely $3.35 \pm 0.3$, in agreement with the \citep{Zou2016} study for the 400d groups sample, that gives a slope $3.29 \pm 0.33$.

astro-ph.CO↗

arXiv2022

Splashback radius in a spherical collapse model

It has been shown some years ago that dark matter haloes outskirts are characterized by very steep density profiles in a very small radial range. This feature has been interpreted as a pile up of at a similar location of different particle orbits, namely splashback material at half an orbit after collapse. Adhikari et al. (2014), obtained the location of the splashback radius through a very simple model, namely calculating a dark matter shell trajectory in the secondary infall model while it crosses a growing, NFW profile shaped, dark matter halo. Since they imposed a halo profile instead of calculating it from the trajectories of the shells of dark matter, they were not able to find the dark matter profile around the splashback radius. In the present paper, we use an improved spherical infall model taking into shell crossing, and several physical effects like ordered, and random angular momentum, dynamical friction, adiabatic contraction, etc. This allow us to determine the density profile from the inner to outer region, and study the behavior of the outer density profile. We will compare the density profiles, and the logarithmic slope of the density profile with the results of Diemer \& Kravtsov (2014) simulations, finding a good agreement between the prediction of the model and the simulations.

astro-ph.CO↗

arXiv2022

Review of solutions to the Cusp-core problem of the $Λ$CDM Model

This review aims at proposing to the field an overview of the Cusp-core problem, including a discussion of its advocated solutions, assessing how each can satisfactorily provide a description of central densities. Whether the Cusp-core problem reflects our insufficient grasp on the nature of dark matter, of gravity, on the impact of baryonic interactions with dark matter at those scales, as included in semi-analytical models or fully numerical codes, the solutions to it can point either to the need for a paradigm change in cosmology, or to to our lack of success in ironing out the finer details of the $Λ$CDM paradigm.

astro-ph.CO↗

arXiv2022

A Mass Dependent Density Profile from Dwarfs to Clusters

In this paper, we extend the work of Freundlich et al. 2020 who showed how to obtain a Dekel-Zhao density profile with mass dependent shape parameters in the case of galaxies. In the case of Freundlich et al. 2020, the baryonic dependence was obtained using the NIHAO set of simulations. In our case, we used simulations based on a model of ours. Following Freundlich et al. 2020, we obtained the dependence from baryon physics of the two shape parameters, obtaining in this way a mass dependent Dekel-Zhao profile describing the dark matter profiles from galaxies to clusters of galaxies. The extension to the Dekel-Zhao mass dependent profile to clusters of galaxies is the main result of the paper. In the paper, we show how the Dekel-Zhao mass dependent profile gives a good description of the density profiles of galaxies, already shown by Freundlich et al. 2020, but also to a set of clusters of galaxies.

astro-ph.CO↗

arXiv2021

Cluster density slopes from Dark Matter-Baryons Energy Transfer

In this paper, we extend previous works on the relation between mass and the inner slope in dark matter density profiles. We calculate that relation in the mass range going from dwarf galaxies to cluster of galaxies. This was done thanks to a modeling of energy transfer via SN and AGN feedback, as well as via dynamical friction of baryon clumps. We show that, in the mass range above galaxy masses (Groups and clusters), the inner slope-mass relation changes its trend. It flattens (towards less cuspy profile) around masses corresponding to groups of galaxies and steepens again for large galaxy cluster masses. The flattening is produced by the AGN outflows (AGN feedback). The one-$ σ$ scatter on $α$ is approximately constant in all the mass range ($Δα\simeq 0.3$). This is the first paper extending the inner density profile slope-mass relationship to clusters of galaxies, accounting for the role of baryons. The result can be used to obtain a complete density profile, also taking baryons into account. Such kind of density profile was previously only available for galaxies.

astro-ph.GA↗

arXiv2020

Can Pulsars in the inner parsecs from Galactic Centre probe the existence of Dark Matter?

We discuss the formation of dark compact objects in a dark matter environment in view of the possible mass dependence of pulsars on the distribution of dark matter in the Galaxy. Our results indicate that the pulsar masses should decrease going towards the center of the Milky Way due to dark matter capture, thus becoming a probe for the existence and nature of dark matter. We thus propose that the evolution of the pulsar mass in a dark matter rich environment can be used to put constraints, when combined with future experiments, on the characteristics of our Galaxy halo dark matter profile, on the dark matter particle mass and on the dark matter self-interaction strength.

astro-ph.HE↗

arXiv2020

On the change of old neutron star masses with galactocentric distance

We show that the pulsar mass depends on the environment, and that it decreases going towards the center of the Milky Way. This is due to two combined effects, the capture and accumulation of self-interacting, non-annihilating dark matter by pulsars, and the increase of the dark matter density going towards the galactic center. We show that mass decrease depends both on the density profile of dark matter, steeper profiles producing a faster and larger decrease of the pulsar mass, and on the strength of self-interaction. Once future observations will provide the pulsar mass in a dark matter rich environment, close to the galactic center, the present result will be able to put constraints on the characteristics of our Galaxy halo dark matter profile, on the nature of dark matter, namely on its annihilating or non-annihilating nature, on its strength of self-interaction, and on the particle mass.

gr-qc↗