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Sameie, Omid

Omid Sameie

Postdoctoral Fellow
Department of Astronomy

Interests: Theoretical Astrophysics, Galaxy Formation models, Dark Matter.

Office Location
PMA 17.226

I grew up in eastern part of Iran where I received my B.Sc. degree from Ferdowsi University in 2012. Then, I moved to the Capital, Tehran, for my M.Sc. to study Early Universe Cosmology and Inflationary Models at Shahid Beheshti University and Institute for Research in Fundamental Sciences (IPM). In 2014, I moved to USA and started my PhD at University of California Riverside. During my PhD, I employed a combination of analytical models and computer simulations to study the impact of alternative Dark Matter models on the growth and evolution of structures in the Universe. I earned my PhD degree in June 2019.

I am a theoretical astrophysicist. By using state-of-the-art supercomputer facilities in the US, I develop and perform numerical simulations to study formation of the structures and galaxies in a cosmological context. 

In the standard model of cosmology, ΛCDM, where Λ is a cosmological constant and CDM is cold dark matter, dark matter (DM) constitutes ~20% of the total energy budget of the universe and is five times more prevalent than ordinary stuff called baryons (electrons, protons etc.). Yet, we poorly understand its nature and properties. The CDM model is extremely successful on explaining numerous independent observations on scales larger than galaxies, and it also predicts accurately many observations on the galactic scales. However there are hints that this may not be the ultimate cosmological theory (for a review on the challenges to the ΛCDM read this article, co-written by my PI, Mike Boylan-Kolchin). 

There are two general approaches for reconciling our theoretical models with the observations.
1) A large group of scientists argue that the relatively poor implementation of complex baryonic processes (including feedback mechanisms, Black Hole and AGN formation, physics of reionizations, cosmic rays and so on) in numerical simulations mislead us in interpreting the observational data. Hence, a huge chunck of resources is devoted to push forward our understanding of how baryonic processes contribute to formation of the structures.
2) A second approach would be to modify the physics of the dark matter. The mass scale relevant to many of those observations, which contradict the predictions of numerical simulations, is such that the mentioned baryonic processes are either not important or play little in the formation of the structures. Most of these "alternative dark matter" models have strong motivations from Particle Physics perspective. 

I spend most of my research time on simulating a class of these non-CDM dark matter models, called self-interacting dark matter (SIDM). Since its introduction in a classical paper by Spergel&Steinhardt in 2000, the SIDM models have gained a lot of attention. Within the past few years, it has been shown that not only SIDM preserves all successes of CDM, but also it can explain some of the observations that CDM hasn't yet  been successful in reproducing (for a review on SIDM models and their astrophysical implications read here). 

So far, most of cosmological simulation with SIDM have neglected the baryonic physics. Includig hydrodynamical processses in these simulations seems vital for assessing the success and failure of SIDM and other DM models. In collaboration with FIRE community, I am running and analyzing some of the most advanced numerical simulations of dwarf- and Milky Way-sized galaxies in the CDM and SIDM models in order to better understand the interplay between dark matter self-interactions and baryons. We ultimately compare the outcome of these simulations to the observational data.
Stay Tuned!

1-“Evolution of Self-Interacting Dark Matter Subhalos in the Milky Way’s Tides” Sameie, Omid; Yu, Hai-Bo; Sales, Laura; Vogelsberger, Mark; Zavala, Jesu ́s, Phys.Rev.Lett. 124 (2020) 141102

2- “The effect of dark matter interactions on halo abundance- a Press-Schechter approach” Sameie, Omid; Benson, Andrew; Sales, Laura V.; Yu, Hai-Bo; Moustakas, Leonidas A.; Creasey, Peter, 2019, ApJ, 874,101S

3- “Globular clusters formed within dark aloes I: Present-day abundance, distribution and kinematics” Creasey, Peter; Sales, Laura V.; Peng, Eric W.; Sameie, Omid, 2018, MNRAS, 482, 219

4- “The impact of baryonic disks on the shapes and profiles of the self-interacting dark matter haloes”Sameie, Omid; Creasey, Peter; Yu, Hai-Bo; Sales, Laura V.; Vogelsberger, Mark; Zavala, Jesu ́s, 2018, MNRAS, 479, 359

5- “Spreading out and staying sharp - creating diverse rotation curves via baryonic and self-interaction effects” Creasey, Peter; Sameie, Omid; Sales, Laura V.; Yu, Hai-Bo; Vogelsberger, Mark; Zavala, Jesu ́s, 2017, MNRAS, 468, 2283

In 2017, I was awarded NASA MIRO FIELDS Fellowship.