arXiv subjects

Hamed Kameli

Publications and source records attributed to Hamed Kameli.

6 recordsLinked to original sources

arXiv2025

Clustering of Primordial Black Holes in Excursion Set Theory

We investigate the clustering of Primordial Black Holes (PBHs) within the framework of Excursion Set Theory (EST). The EST formalism is extended to compute the joint probability of forming PBH pairs within a clustering distance, based on two stochastic trajectories with a shared history. Our results show that an enhanced power spectrum not only increases the formation of PBHs in specific mass ranges but also enhances their clustering probability. We find a one-to-one correspondence between the blue-tilted spectral index and the mass ranges in which PBHs form and cluster. Additionally, we demonstrate that the clustering probability decreases asymptotically with increasing clustering distance, while a higher critical density threshold (barrier) leads to a suppression of clustering abundance.

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↗

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↗

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↗

arXiv2021

Primordial Black Holes in the Excursion Set Theory

We study primordial black holes (PBHs) formation in the excursion set theory (EST) in a vast range of PBHs masses with and without confirmed constraints on their abundance. In this work, we introduce a new concept of the first touch in the context of EST for PBHs formation. This new framework takes into account the earlier horizon reentry of smaller masses. Our study shows that in the EST, it is possible to produce PBHs in different mass range, with enhanced power spectrum, which could make up all dark matter. We also show that in a broad blue-tilted power spectrum, the production of PBHs is dominated by smaller masses. Our analysis put an upper limit $\sim\,$0.1 on the amplitude of the curvature power spectrum at length scales relevant for PBHs formation.

astro-ph.CO↗

arXiv2020

Modified initial power spectrum and too big to fail problem

The galactic scale challenges of dark matter such as "missing satellite" problem and "too big to fail" problem are the main caveats of standard model of cosmology. These challenges could be solved either by implementing the complicated baryonic physics or it could be considered as an indication to a new physics beyond the standard model of cosmology. The modification of collisionless dark matter models or the standard initial conditions are two promising venues for study. In this work, we investigate the effects of the deviations from scale invariant initial curvature power spectrum on number density of dark matter halos. We develop the non-Markov extension of the excursion set theory to calculate the number density of dark matter substructures and dark matter halo progenitor mass distribution. We show that the plausible solution to "too big to fail" problem could be obtained by a Gaussian excess in initial power in the scales of $k_* \sim 3 \text{h/Mpc}$ that is related to the mass scale of $M_* \sim 10^{11} M_{\odot}$. We show that this deviation leads to the decrement of dark matter sub-halos in galactic scale, which is consistent with the current status of the non-linear power spectrum. Our proposal also has a prediction that the number density of Milky way type galaxies must be higher than the standard case.

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