arXiv subjects

Mehraveh Nikjoo

Publications and source records attributed to Mehraveh Nikjoo.

4 recordsLinked to original sources

arXiv2026

Weak Lensing by Photometric Density Ridges

Ridges in galaxy density fields measured by photometric surveys are 2D projections of filaments in the cosmic web, and so should lens light from background galaxies. We report on a detection of this effect in Dark Energy Survey Year 3 data at high significance, though not independently of galaxy-galaxy lensing. We describe improvements to the existing subspace-constrained mean shift algorithm to locate these ridges efficiently at scale, and examine the dependence of the signal in simulations on cosmological and algorithmic parameters. We find that it depends primarily on $S_8=σ_8 \left( Ω_m / 0.3 \right)^{1/2}$, and discuss improvements to our methodology that would be needed to allow precision parameter estimation.

astro-ph.CO↗

arXiv2024

Tomographic constraints on the production rate of gravitational waves from astrophysical sources

Using an optimal quadratic estimator, we measure the large-scale cross-correlation between maps of the stochastic gravitational-wave intensity, constructed from the first three LIGO-Virgo observing runs, and a suite of tomographic samples of galaxies covering the redshift range $z\lesssim 2$. We do not detect any statistically significant cross-correlation, but the tomographic nature of the data allows us to place constraints on the (bias-weighted) production rate density of gravitational waves by astrophysical sources as a function of cosmic time. Our constraints range from $\langle b\dotΩ_{\rm GW}\rangle<3.0\times10^{-9}\,{\rm Gyr}^{-1}$ at $z\sim0.06$ to $\langle b\dotΩ_{\rm GW}\rangle<2.7\times10^{-7}\,{\rm Gyr}^{-1}$ at $z\sim1.5$ (95\% C.L.), assuming a frequency spectrum of the form $f^{2/3}$ (corresponding to an astrophysical background of binary mergers), and a reference frequency $f_{\rm ref}=25\,{\rm Hz}$. Although these constraints are $\sim2$ orders of magnitude higher than the expected signal, we show that a detection may be possible with future experiments.

astro-ph.CO↗

arXiv2024

Hamiltonian formulation of gravity as a spontaneously-broken gauge theory of the Lorentz group

A number of approaches to gravitation have much in common with the gauge theories of the standard model of particle physics. In this paper, we develop the Hamiltonian formulation of a class of gravitational theories that may be regarded as spontaneously-broken gauge theories of the complexified Lorentz group $SO(1,3)_C$ with the gravitational field described entirely by a gauge field valued in the Lie algebra of $SO(1,3)_C$ and a `Higgs field' valued in the group's fundamental representation. The theories have one free parameter $β$ which appears in a similar role to the inverse of the Barbero-Immirzi parameter of Einstein-Cartan theory. However, contrary to that parameter, it is shown that the number of degrees of freedom crucially depends on the value of $β$. For non-zero values of $β$, it is shown that three complex degrees of freedom propagate on general backgrounds, and for the specific values $β=\pm i$ an extension to General Relativity is recovered in a symmetry-broken regime. For the value $β=0$, the theory propagates no local degrees of freedom. A non-zero value of $β$ corresponds to the self-dual and anti-self-dual gauge fields appearing asymmetrically in the action, therefore in these models, the existence of gravitational degrees of freedom is tied to chiral asymmetry in the gravitational sector.

gr-qc↗

arXiv2022

Exploring Parameter Spaces with Artificial Intelligence and Machine Learning Black-Box Optimisation Algorithms

Constraining Beyond the Standard Model theories usually involves scanning highly multi-dimensional parameter spaces and check observable predictions against experimental bounds and theoretical constraints. Such task is often timely and computationally expensive, especially when the model is severely constrained and thus leading to very low random sampling efficiency. In this work we tackled this challenge using Artificial Intelligence and Machine Learning search algorithms used for Black-Box optimisation problems. Using the cMSSM and the pMSSM parameter spaces, we consider both the Higgs mass and the Dark Matter Relic Density constraints to study their sampling efficiency and parameter space coverage. We find our methodology to produce orders of magnitude improvement of sampling efficiency whilst reasonably covering the parameter space.

hep-ph↗