arXiv subjects

Milad Noorikuhani

Publications and source records attributed to Milad Noorikuhani.

4 recordsLinked to original sources

arXiv2026

A machine learning-based method for populating dark matter halos in N-body simulations with substructure

Dark matter N-body simulations that resolve halo substructure (subhalos) with high accuracy can be used for generating reliable catalogs of central and satellite galaxies via galaxy-halo connection models. However, such simulations are computationally expensive, especially when large numbers of realizations are required for statistical analyses such as robust estimations of covariance matrices for (statistical) cosmological quantities. In this work, we present a fast method for populating dark matter halos in a given simulation box with subhalos extracted from a high-resolution simulation box with the same cosmology but arbitrary initial conditions. For each halo in a given test set, the method first predicts whether it hosts at least one subhalo above a given mass threshold using a classifier built on a decision tree regressor. Then, for each predicted host, the method finds another host halo in a given high-resolution box, using a nearest neighbor search in the space of selected halo properties, and appropriately maps subhalos from that halo to the test one. By applying the method to different test sets, we make predictions for abundances, distributions and three-dimensional and projected (two-dimensional) two-point correlation functions of various subhalo populations and sub-populations. In most cases, the percent errors of our predictions are well below 5%. In general, the method can be used to populate halos in low-resolution simulation boxes with subhalos from a high-resolution simulation and also to investigate which halo properties are (more strongly) correlated with subhalo abundance and clustering in a given dark matter simulation.

astro-ph.CO↗

arXiv2023

Wide-angle and relativistic effects in Fourier-space clustering statistics

Galaxy power spectrum and bispectrum signals are distorted by peculiar velocities and other relativistic effects arising from a perturbed spacetime background. In addition, study of correlation functions of tracers in Fourier space is often done in the plane-parallel approximation under which it is assumed that line-of-sight (LOS) vectors are parallel. In this work we show that a simple perturbative procedure can be employed for a fast evaluation of beyond plane-parallel (wide-angle) corrections to the power spectrum and bispectrum. We also show that evolution of linear matter density fluctuations in a relativistic context can be found from a simple method. For the power spectrum at linear level, we compare leading order wide-angle contributions to multipoles of the galaxy power spectrum with those from non-integrated and integrated relativistic corrections and estimate their possible contamination on local fNL measurements to be of order a few. We also compute wide-angle corrections in the presence of nonlinear terms at one-loop order. For the bispectrum, we show that wide-angle effects alone, even with fully symmetric choices of LOS, give rise to imaginary, odd-parity multipoles of the galaxy bispectrum (dipole, octupole, etc.) which are in many cases larger than previously known ones of relativistic origin. We calculate these contributions and provide an estimator for measuring the leading order bispectrum dipole from data, using a symmetric LOS definition. Finally, we calculate the leading order corrections to multipoles of real plane-parallel bispectrum multipoles and estimate the apparent local fNL induced to be of order unity.

astro-ph.CO↗

arXiv2017

Anomaly induced quantum correction to charged black holes; geometry and thermodynamics

We consider the corrections due to quantum fluctuations of fields on charged black holes induced from the energy-momentum trace anomaly. Although the number of horizons stays unchanged and their positions receive only finite corrections, the geometry, thermodynamics and formation of RN black holes change seriously in particular for small ones. The entropy receives a logarithmic correction. The line $Q=M$, separating naked singularities from physical solutions is corrected, putting a lower limit on the mass and an upper limit on the temperature of the black hole as a function of its charge. The modifications are highly significant in the cases of near-extremal and small black holes. We also show that for black holes with small mass can stay in thermal equilibrium without any constraint on the volume of the container. This result is in contrast to the large black holes that need a finite volume container for thermal equilibrium. The minimum of the mass lower limits occurs at zero charge, resulting in the extremal Schwarzschild black hole with a specific mass of the order of $M_{p}$ and zero temperature. This state which has only gravitational interaction will be the final stage of Hawking radiation. Stability and lack of any interaction but gravitational, makes the extremal Schwarzschild black hole a serious candidate for dark matter particle.

hep-th↗

arXiv2017

Quantum vacuum effects on the final fate of a collapsing ball of dust

We consider the quantum vacuum effects of the massless scalar fields that are non-minimally coupled to the background geometry of a collapsing homogeneous ball of dust. It is shown that for a definite range of coupling constants, there are repulsive quantum vacuum effects, capable of stopping the collapse process inside the black hole and precluding the formation of singularity. The final fate of the collapse will be a black hole with no singularity, inside which the matter stays balanced. The density of the final static matter will be close to the Planck density. We show that the largest possible radius of the stable static ball inside a black hole with Schwarzschild mass $M$ is given by ${{\left( \frac{1}{90π}\frac{M}{{{m}_{p}}} \right)}^{\frac{1}{3}}}{{\ell }_{p}}$. If the black hole undergoes Hawking radiation, the final state will be an extremal quantum-corrected black hole, with zero temperature, with a remnant of matter inside. We show that the resolution of singularity is not disrupted under Hawking radiation.

hep-th↗