arXiv subjects

Shant Baghram

Publications and source records attributed to Shant Baghram.

At least 19 recordsLinked to original sources

arXiv2025

Microlensing Signatures of Dyson Sphere-like Structures around Primordial Black Holes as Technosignatures of Extraterrestrial Advanced Civilizations

We investigate the microlensing detectability of extraterrestrial technosignatures originating from Dyson sphere \textendash like structures, such as Dyson Swarms surrounding primordial black holes (PBHs). These hypothetical swarms consist of stochastically varying, partially opaque structures that could modulate standard microlensing light curves through time-dependent transmission effects. We introduce a probabilistic framework that includes a stochastic transmission model governed by variable optical depth and random gap distributions. We perform a parameter scan and generate heatmaps of the optical transit duration. We study the infrared excess radiation and peak emission wavelength as complementary observational signatures. Additionally, we define and analyze the effective optical depth and the anomalous microlensing event rate for these stochastic structures. Our findings provide a new avenue for searching for extraterrestrial advanced civilizations by extending microlensing studies to include artificial, dynamic modulation signatures.

astro-ph.IM↗

arXiv2025

Schwarzschild Spacetime and the Local Limit of Nonlocal Gravity

We investigate static spherically symmetric solutions within the framework of the local limit of nonlocal gravity. This theory departs from Einstein's general relativity (GR) through the introduction of a scalar gravitational susceptibility function $S(x)$, $1+S > 0$, which is a new feature of the spacetime that vanishes in the GR limit. It is shown that the Schwarzschild spacetime constitutes an exact solution of the modified source-free gravitational field equations provided $S$ is a constant. The mass of the corresponding Schwarzschild solution is given by $M/(1+S)$, where $M$ denotes the mass of the solution in the GR limit. The interpretation of the solution in terms of a black hole is ruled out due to the divergence of the algebraic invariants of the Weitzenb\"ock torsion at the Schwarzschild horizon.

gr-qc↗

arXiv2025

Addressing Dipole Tension via Clustering in $\Lambda$CDM and beyond

The dipole in the angular distribution of the cosmic microwave background (CMB) is attributed to the Doppler effect and our motion relative to the CMB rest frame. It is expected that observations of large-scale structures (LSSs) would also exhibit a related kinematic dipole. However, numerous studies of the LSS dipole have shown significant discrepancies with predictions based on the CMB. In this work, we investigate how considering the clustering dipole affects the LSSs distribution dipole using the National Radio Astronomy Observatory (NRAO) Very Large Array (VLA) Sky Survey (NVSS) and the Wide-field Infrared Survey Explorer (WISE), and examine the nonlinear regime to calculate the correlation between the clustering and the kinematic dipole. Our results show up to $\lesssim28\%$ enhancement in the clustering dipole amplitude compared to previous studies, with increases of up to $\lesssim 22\%$ in $\Lambda$CDM and $\lesssim 28\%$ in modified gravity scenarios. Additionally, we explore a model in which the distribution of matter on LSS could be intrinsically anisotropic by a long-mode modulation. Using the remnant discrepancy between the observed and predicted dipole, we derive an upper limit for the amplitude of intrinsic dimensionless anisotropy $\lesssim 0.22$. Furthermore, we investigate these results within the framework of the $f(R)$ modified gravity model. We conclude that nonlinear clustering and local structure correlations partially alleviate the dipole tension within $\Lambda$CDM, yet, this anomaly remains a challenge. Two alternative models are in the direction of relaxing the tension. However, further investigation and more accurate data are needed to support a stronger argument.

astro-ph.CO↗

arXiv2025

Luminosity Function of Galaxies in Voids: A Modification Inspired by Excursion Set Theory

In the standard picture of cosmology, the galaxies reside in dark matter (DM) halos. DM halos are distributed in the cosmic web in different environments. The luminosity of the galaxies in different environments can be used as a probe to assess a cosmological model. This study focuses on the properties of galaxies in void regions, where halos typically do not experience extreme conditions. By examining the galaxy luminosity function, we aim to understand the dependence of galaxy properties on their environment and redshift so that later, we can use this as a tool to evaluate cosmological models. We employ the excursion set theory to incorporate parameters related to the number density of DM halos into the luminosity function. Using the Galaxy and Mass Assembly (GAMA) survey and 2dFGRS datasets, we fit our theoretical models to observational data, examining the environmental and redshift dependence of the galaxy luminosity function. Our results indicate that we model the galaxy luminosity function in voids effectively by considering the linear density contrast of the environment and the growth function $D(z)$ for redshift dependence. This study provides a model for the environmental dependence of galaxy luminosity function that offers an improvement in the $\chi ^2$ parameter compared to the previously proposed model in \cite{mcnaught2014galaxy}. Both Bayesian information criterion (BIC) and Akaike information criterion (AIC) tests support the superiority of this model for the void region.

astro-ph.CO↗

arXiv2025

Addressing the too-big-to-fail problem and the void phenomenon through a modified initial power spectrum

We investigate the impact of early-time initial conditions on nonlinear structure formation and evolution within the framework of the semi-analytical Excursion Set Theory (EST). Our analysis reveals that adding a Gaussian bump to the initial curvature power spectrum at small scales enhances the abundance of massive halos while sharply reducing the number of small-mass halos, and consequently, satellite galaxies. Moreover, this modification increases the frequency of major mergers while suppressing high-mass-ratio minor mergers. These features may offer resolutions to the missing satellite and Too Big to Fail (TBTF) problems. In underdense regions -- voids -- the same modifications increase the likelihood of finding massive halos embedded in voids while similarly decreasing the small-halo population, and consequently, faint galaxies. This behavior suggests a potential solution to the void phenomenon, in which embedded halos, despite being too massive, were too rare to be noticed. More precisely, our results indicate that an excess of massive structures emerges at mass scales near the center of the Gaussian bump: $k_* = 1.85 \,\rm{h/Mpc}$ and $k_* = 3.95 \,\rm{h/Mpc}$. These scales correspond to mass scales of $M_* = 10^{11}$ and $M_* = 10^{10}$, respectively. This modification extends up to two orders of magnitude in higher mass scales, while reducing the abundance of halos below $M_*$ by two to three orders of magnitude. Additionally, we find that evolutionary conditions, halo-in-halo, and particularly halo-in-void statistics serve as more sensitive and complementary probes for differentiating among cosmological models.

astro-ph.CO↗

arXiv2025

Elliptically Polarized Plane Gravitational Waves

Exact plane gravitational radiation fields are presented within the framework of general relativity and their properties are described. The physics of nonlinear elliptically polarized plane gravitational waves is developed in close analogy with electromagnetic waves. The motion of free test particles in the dynamic gravitational fields of elliptically polarized plane waves is investigated. In particular, we demonstrate the cosmic jet property of these spacetimes, namely, most timelike geodesics tend to line up in the direction of wave propagation and produce a cosmic jet whose speed asymptotically approaches the speed of light.

gr-qc↗

arXiv2024

In Search of Extraterrestrial Artificial Intelligence Through Dyson Sphere-like structures around Primordial Black Holes

Are we alone? It is a compelling question that human beings have confronted for centuries. The search for extraterrestrial life is a broad range of quests for finding simple forms of life up to intelligent beings in the Universe. The plausible assumption is that there is a chance that intelligent life will be followed by advanced civilization equipped or even dominated by artificial intelligence (AI). In this work, we categorize advanced civilizations (on an equal footing, an AI-dominated civilization) on the Kardashev scale. We propose a new scale known as the space exploration distance to measure civilization advancement. We propose a relation between this length and the Kardashev scale. Then, we suggest the idea that advanced civilizations will use primordial black holes (PBHs) as sources of harvesting energy. We calculate the energy harvested by calculating the space exploration distance. Finally, we propose an observational method to detect the possibility of extraterrestrial AI using Dyson sphere-like structures around PBHs in the Milky Way and other galaxies.

astro-ph.GA↗

arXiv2024

Local Peculiar Motions in McVittie and LTB Spacetimes

We consider two inhomogeneous cosmological models, namely, the flat McVittie spacetime and a simple specific LTB spacetime. Relative to the world line of a reference comoving observer that remains spatially at rest, we study the local deviations of the world lines of free test particles. These local peculiar motions can be invariantly characterized within the framework of a quasi-inertial Fermi normal coordinate system established along the world line of the reference comoving observer. Tidal dynamics in the McVittie model involves the sum of the curvature due to the inhomogeneity, the curvature due to the background FLRW spacetime and a mixed term, while tidal dynamics in the particular LTB model turns out to be qualitatively the same as in the Einstein-de Sitter universe. Peculiar motions in the two cosmological models are briefly compared and contrasted.

gr-qc↗

arXiv2024

One-point Statistics in various cosmic environments in the presence of massive neutrinos

Studying the structures (halos and galaxies) within the cosmic environments (void, sheet, filament, and node) where they reside is an ongoing attempt in cosmological studies. The link between the properties of structures and the cosmic environments may help to unravel the nature of the dark sector of the Universe. In this paper, we study the cosmic web environments from the spatial pattern perspective in the context of $ \Lambda $CDM and $ \nu \Lambda $CDM as an example of an extension to the vanilla model. To do this, we use the T-web classification method and classify the cosmic environments for the catalogues from the gevolution N-body simulations for $ \Lambda $CDM and $ \nu \Lambda $CDM cosmology. Then, we compute the first nearest neighbour cumulative distribution function, spherical contact cumulative distribution function, and $ J$-function for every cosmic environment. In the context of the standard model, the results indicate that these functions can differentiate the various cosmic environments. In association with distinguishing between extensions of the standard model of cosmologies, these functions within the cosmic environment seem beneficial as a complementary probe.

astro-ph.CO↗

arXiv2024

McVittie-Plummer Spacetime: Plummer Sphere Immersed in the FLRW Universe

The McVittie-Plummer spacetime is a spherically symmetric inhomogeneous cosmological model that represents a spherical star system embedded in a standard FLRW cosmological model. We study the main physical properties of this gravitational field. Regarding the interplay between the physics of the local system and the expanding background, we employ the Misner-Sharp mass-energy function to show that there is a relatively weak time-dependent general relativistic coupling between the astrophysical system and the background FLRW cosmological model. The coupling term is proportional to the inverse of the scale factor and decreases as the universe expands.

gr-qc↗

arXiv2024

Local Limit of Nonlocal Gravity: A Teleparallel Extension of General Relativity

We describe a general constitutive framework for a teleparallel extension of the general theory of relativity. This approach goes beyond the teleparallel equivalent of general relativity (TEGR) by broadening the analogy with the electrodynamics of media. In particular, the main purpose of this paper is to investigate in detail a local constitutive extension of TEGR that is the local limit of nonlocal gravity (NLG). Within this framework, we study the modified FLRW cosmological models. Of these, the most cogent turns out to be the modified Cartesian flat model which is shown to be inconsistent with the existence of a positive cosmological constant. Moreover, dynamic dark energy and other components of the modified Cartesian flat model evolve differently with the expansion of the universe as compared to the standard flat cosmological model. The observational consequences of the modified Cartesian flat model are briefly explored and it is shown that the model is capable of resolving the H_0 tension.

gr-qc↗

arXiv2024

Dynamic Dark Energy from the Local Limit of Nonlocal Gravity

Nonlocal gravity (NLG), a classical extension of Einstein's theory of gravitation, has been studied mainly in linearized form. In particular, nonlinearities have thus far prevented the treatment of cosmological models in NLG. In this essay, we discuss the local limit of NLG and apply this limit to the expanding homogenous and isotropic universe. The theory only allows spatially flat cosmological models; furthermore, de Sitter spacetime is forbidden. The components of the model will have different dynamics with respect to cosmic time as compared to the standard $Λ$CDM model; specifically, instead of the cosmological constant, the modified flat model of cosmology involves a dynamic dark energy component in order to account for the accelerated phase of the expansion of the universe.

gr-qc↗

arXiv2024

Local Limit of Nonlocal Gravity: Cosmological Perturbations

We explore the cosmological implications of the local limit of nonlocal gravity, which is a classical generalization of Einstein's theory of gravitation within the framework of teleparallelism. An appropriate solution of this theory is the modified Cartesian flat cosmological model. The main purpose of this paper is to study linear perturbations about the orthonormal tetrad frame field adapted to the standard comoving observers in this model. The observational viability of the perturbed model is examined using all available data regarding the cosmic microwave background. The implications of the linearly perturbed modified Cartesian flat model are examined and it is shown that the model is capable of alleviating the $H_0$ tension.

gr-qc↗

arXiv2023

Clustering of dark matter in the cosmic web as a probe of massive neutrinos

The large-scale structure of the Universe is distributed in a cosmic web. Studying the distribution and clustering of dark matter particles and halos may open up a new horizon for studying the physics of the dark Universe. In this work, we investigate the nearest neighbour statistics and spherical contact function in cosmological models with massive neutrinos. For this task, we use the relativistic N-body code, gevolution and study particle snapshots at three different redshifts. In each snapshot, we find the halos and evaluate the letter functions for them. We show that a generic behaviour can be found in the nearest neighbour, $G(r)$, and spherical contact functions, $F(r)$, which makes these statistics promising tools to constrain the total neutrino mass.

astro-ph.CO↗

arXiv2023

Voids and Halos in Voids statistics as a probe of the Expansion History of the Universe

Structures in the Universe are arranged into the cosmic web. Distributions, statistics, and evolutions of the structures can be used as probes for cosmological models. We investigate the number density of voids and dark matter halos-in-voids in the Excursion Set Theory (EST). We study the Markov and non-Markov frameworks of EST in both spherical and ellipsoidal collapse models. Afterward, we compare the number density of voids and halos-in-voids in the standard $Λ$CDM and the reconstructed model. The reconstructed model is a model-independent reconstruction based on background observations. This work explores the effects of the collapse model barrier in the different EST frameworks on the statistics of voids and the statistics of halos-in-voids. Finally, we find the hint that cosmological models can be distinguished by the number density of halos-in-voids in the $1.0-2.5$ redshift range. The maximum difference is observed in $z\sim1.9$.

astro-ph.CO↗

arXiv2023

Anisotropic Cosmology in the Local Limit of Nonlocal Gravity

Within the framework of the local limit of nonlocal gravity (NLG), we investigate a class of Bianchi type I spatially homogeneous but anisotropic cosmological models. The modified field equations are presented in this case and some special solutions are discussed in detail. This modified gravity theory contains a susceptibility function S(x) such that general relativity (GR) is recovered for S = 0. In the modified anisotropic cosmological models, we explore the contribution of S(t) and its temporal derivative to the local anisotropic cosmic acceleration. The implications of our results for observational cosmology are briefly discussed.

gr-qc↗

arXiv2022

Structure of cosmic web in non-linear regime: the nearest neighbour and spherical contact distributions

In non-linear scales, the matter density distribution is not Gaussian. Consequently, the widely used two-point correlation function is not adequate anymore to capture the matter density field's entire behaviour. Among all statistics beyond correlation functions, the spherical contact (or equivalently void function), and nearest neighbour distribution function seem promising tools to probe matter distribution in non-linear regime. In this work, we use halos from cosmological N-body simulations, galaxy groups from the volume-limited galaxy group and central galaxies from mock galaxy catalogues, to compare the spherical contact with the nearest neighbour distribution functions. We also calculate the J-function (or equivalently the first conditional correlation function), for different samples. Moreover, we consider the redshift evolution and mass-scale dependence of statistics in the simulations and dependence on the magnitude of volume-limited samples in group catalogues as well as the mock central galaxies. The shape of the spherical contact probability distribution function is nearly skew-normal, with skewness and kurtosis being approximately 0.5 and 3, respectively. On the other hand, the nearest neighbour probability distribution function is nearly log-normal, with logarithmic skewness and kurtosis being approximately 0.1 and 2.5, respectively. Accordingly, the spherical contact distribution function probes larger scales compared to the nearest neighbour distribution function, which is influenced by details of structures. We also find a linear relation between the mean and variance of the spherical contact probability distribution function in simulations and mock galaxies, which could be used as a distinguishing probe of cosmological models.

astro-ph.CO↗

arXiv2022

Mass assembly history of dark matter halos in the light of $H_0$ tension

The Hubble tension may introduce a new course of action to revise the standard $Λ$CDM model to unravel dark energy and dark matter physics. The Hubble parameter can be reconstructed by late-time observations of the background evolution model independently. We relate the reconstructed Hubble parameter to the structure formation and large scale structure observables in this work. We use the excursion set theory to calculate the number density of dark matter halos and the mass function of progenitors. We obtain the results for both the Markov and non-Markov extension of the excursion set theory in the context of spherical and ellipsoidal collapse. We show that the number density of dark matter halos in the reconstructed model has approximately $\sim2σ$ difference in comparison to the Planck-2018 $Λ$CDM in the mass range of $M\gtrsim10^{12}M_{\odot}$. We also compare the dark matter halo progenitor mass function with the pair-galaxy statistics and their mass assembly history from observational data of the HST, CANDEL survey. Due to complications to distinguish the ratio of accretion and merger in mass assembly, our result on pair fraction is for illustration only. However, a $\sim5$ times more accurate observations will be promising to distinguish the reconstructed model and the Planck-2018 $Λ$CDM.

astro-ph.CO↗